Among the most fundamental questions one can ask about sexual behavior are of the nature of attraction and arousal. Why is someone attracted to another? Why are some attracted to males, others to females, and yet others primarily to children? Why is obesity a “turn on” for Mary but not for Sally, and why are large breasts a “turn off” for Bill yet a stimulant for Bob? While such questions might be conscientiously asked, the meaningfulness of the answers is fleeting.

Fads may certainly be involved. At any particular time the relative value of each specific feature or trait varies. Looks, intellect, wealth, family, religion, sense of humor, independence, and fertility, just to name a few, have all had their time in the limelight as salient in partner selection. Among the matters that seem to be taken for granted, however, is sexual orientation: one’s erotic attraction to those of the same or opposite sex, or both. Although there may be flexibility in some desires, only for a minority is there flexibility in sexual orientation. For most individuals it is a fundamental prerequisite in choosing a partner. In wanting an adult sexual encounter, an individual might act upon a concept like, “I want to bed someone five feet to five feet six inches, with blue eyes, blond hair, a long red dress, and high heels.” Only rarely would this occur without an understanding that the individual would be a female or a male in drag. And this prime concern toward the sex of the intended partner is more than on the other features.

Where does this emphasis on sexual orientation come from? The simple answer is, “From an interaction of nature and nurture.” Each of us has a biological predisposition to orient in a certain way—heterosexual, bisexual, or homosexual—and this bias, organized prior to birth, is then subsequently influenced by social and cultural forces.1, 2, 3


To ensure the topic under discussion is clear, some definitions are in order. These are particularly important to provide a framework within which the evidencefor biological influences can be evaluated. These definitions are concomitant with the proposition that five main levels of sexual expression must l appreciated to get a proper sexual profile of any individual: sexual identity, sexual orientation, gender patterns, mechanisms, and reproduction. The acronym PRIMO keeps all of these in mind (P = gender Patterns; R = Reproduction; I = sexual Identity; M = sexual Mechanisms; O = sexual Orientation; Diamond 1976).

Sexual orientation refers to the sex, male or female, of the erotic-love-affectional partners a person prefers. The terms heterosexual, homosexual, and bisexual should best be adjectives, not nouns, and best applied to behaviors, not people. Thus, a description might be, “Engages in homosexual activities.” In casual usage, however, one often speaks of a person as a homosexual or a heterosexual; indeed people often refer to themselves the same way. Such usage does not direct itself to the frequency or motive for any particular behavior, only its focus. Use of such expressions as “is a homosexual,” often links together those whose regular sexual partners are of the same sex with those whose same-sex encounters are rare in comparison with heterosexual contacts. The term homosexual is best reserved for those whose sexual activities are exclusively or almost exclusively with members of the same sex, the term heterosexual for those whose erotic companions are always or almost always with the opposite sex, and the term bisexual or ambisexual for those with sexual activities with members of either sex (Diamond 1993a, 1993b, 1994a, 1994b). This is the way the terms will be used in this chapter.

Sexual identity speaks to the way one views one’s self as male or female. Usually this inner conviction of identification coincides with society’s and parental impressions and mirrors the outward physical appearance. It also typically follows with the gender role or behavior patterns society imposes or the individual prefers. But this is not always so. These distinctions are crucial, particularly in regard to transsexualism or other identity-gender pattern interactions. In the real world, the transsexual, as are others, is typically labeled in terms of his or her sexual anatomy. Initially reared as society views them, transsexuals eventually plot their own sex-reversed life course. The self-image of transsexuals is of the opposite sex, and their mirror images are in conflict with their mind’s image (Benjamin 1966; Bolin 1987; Diamond 1992; Doctor 1988; Green & Money 1969). This aspect of life is separate from their sexual orientation since a transsexual may be homosexually or heterosexually inclined. In everyday terms, people may “identify” as homosexual or see their “identity” as heterosexual. This use of the term is in an affiliative sense. It is as if one might identify as an American Indian or a Unitarian. The term core identity, coined by Stoller (1968), is used by some to indicate the central nature of this conviction.4

One’s gender patterns, gender identity, and gender role are different from, although related to, the concept of orientation and sexual identity. Gender refers to society’s idea of how boys or girls or men and women are expected to behave and should be treated. A sex role is the acting out of one’s biological predisposition, while a gender role is the acting out of social norms or expectations, whether or not they coincide with personal urges or preferences. Gender identity refers to how an individual sees himself or herself as fulfilling these societal expectations. The descriptive terms often used in this regard are relative to masculinity and femininity. Patterns of behavior are phenotypical expressions. The terms boys and girls and men and women are social terms; the terms male and female are biological terms. It is obvious that a male can act as a girl or woman and a female can act as a boy or man.5

Patterns, gender, and roles have everything to do with the society in which one lives and may or may not have much to do with biology (Gagnon & Simon 1973). Males, for instance, can live as women, and females can live as men; a male may be reared as a boy but grow to live as a woman. It is common but often misleading to use sex and gender as synonyms. One’s sex is biological, and one’s gender is socially taught, imposed, or chosen. True, sex and gender are most often in concert, but they are not necessarily so.6

Sexual mechanisms are inherent physiological differences between males and females that structure significant features of erotic life. In one of the most obvious distinctions, the counterpart to male penis erection as a feature of erotic arousal is female lubrication, not clitoral erection (Masters & Johnson 1966). Ejaculation is another obvious distinction; it is a common feature of male orgasm but not of female orgasm. Yet another well-known difference is the wide range of orgasmic types among females, ranging from the frequent finding of women for whom orgasm is rare or nonexistent to those for whom multiple orgasm is common. Among males either extreme is rare (Kinsey et al. 1948, 1953). These differences obviously pertain to copulation, but many other mechanisms exist that are less related to coitus but very much related to sexual expression and childbearing. Muscle mass, body and skeletal articulation, enzyme and endocrine production, and other physiological features and reflexes are additional distinctions. The endocrine system of males is relatively tonic in operation, while the female system is basically cyclic during the reproductive years (see Rowland, this volume).

Reproduction, the fifth level of analysis, is relatively evident. Males have testicles, produce sperm, and have a penis and associated organs for the delivery of their gametes. Females have ovaries, which produce ova, which can be fertilized; a uterus, within which an embryo can mature and be delivered through a vagina; and breasts, which can be stimulated by pregnancy to function. The neuroendocrine system of the male is tonic and programmed for the consistent production and delivery of these gametes, while the neuroendocrine system of the female is cyclic, paced so that ovulation is best timed to coincide with receptive uterine conditions.

For most people, all five levels of identity, orientation, gender patterns. mechanisms, and reproduction are in concert. The typical male sees himself as such, acts in a masculine manner—a combination of biologically and socially determined behaviors—is treated as a male by society, and prefers to have erotic interactions with females. The typical female sees herself as such, acts in a feminine manner—also a combination of biologically and socially determined behaviors—is treated as a female by society, and prefers to have erotic interactions with males. Variations are not uncommon: an individual can prefer erotic relations with one of the same sex (male or female homosexual), see himself as a female (male transsexual), see herself as a male (female transsexual), or prefer to adopt the clothing and lifestyle of the opposite gender while maintaining a heterosexual life (a transvestite) or a homosexual one (a drag queen). Knowing one’s sexual orientation may allow predictions as to manifestations of other levels, but the permutations among the five levels are many.

Reproductive capacity and mechanisms are fixed, identity and orientation somewhat less so, and gender patterns most flexible. Thus, individuals can be heterosexual, ambisexual, or homosexual and yet be quite different in how their orientation is manifest to the outside world. Superimposed on all of this is one’s personality, which often makes it seem impossible to unravel the mysteries of orientation and identity. I hold that those characters, orientation and identity, are central features of one’s existence and thus structure manifestations of gender patterns more than vice versa; they determine how the individual will interact with the environment.


Before proceeding further, a comment about the nature of evidence is warranted. Each scholarly discipline has developed certain standards by which presented arguments or data are usually evaluated. These tests of validity mightinvolve double-blind studies, the use of suitable controls, statistical analysis, peer review and critique, suitable comparison with available models, consistency with established theories within the particular field as well as other disciplines, and so on. Anecdotal findings are not evidence; they may, however, be presented.

In addition, the law of parsimony usually governs which of competing theories should reign; the explanation that best links the majority of findings and depends on the fewest assumptions is to be most credited. This chapter will attempt to hold to these standards in proving a biological predisposition for sexual orientation. Gladue (1993) uses a legal metaphor to suggest that perhaps instead of looking for “proof beyond a reasonable doubt, ” we accept “the preponderance of evidence.”


Twins and Families Reared Together

The strongest evidence that human sexual orientation has a biological bias comes from genetic studies of families and twins. The classical twin studies in this area were done in the 1950s by Franz J. Kallmann (Kallmann 1952a, 1952b, 1963).This researcher worked with monozygotic and dizygotic male twin pairs in which at least one of the co-twins at the onset of the study admitted to homosexual behavior. Among these twins Kallmann found that if one of the identical (monozygotic) twins was homosexually oriented, so was his brother. Among the nonidentical (dizygotic) brothers, on the other hand, the twins were not too different from the general male population relative to sexual preference. Kallmann also reported that if one member of a monozygotic twin pair of brothers rated a 5 or 6 on the Kinsey scale, the chance that his brother also rated 5 or 6 was better than 90 percent; if the brothers differed in rating, it was usually only within one or two points on the Kinsey scale.7

Kallmann’s work was not easily accepted (e.g., Julian 1973). The mood of the 1950s through the 1970s preferred to have human behavior a matter of social construction or free will rather than biological predisposition. Further, the timing of Kallmann’s work competed with that of Kinsey, who thought sexual orientation a product of upbringing or social situation.8 The fact that Kallmann’s numbers seemed to come out so cleanly also encouraged skepticism. A slew of studies soon followed that reported monozygotic twins not concordant for homosexuality (Davison, Brierley & Smith 1971; Friedman, Wollesen & Tendler 1976; Green & Stoller 1971 ; Heston & Shields 1968; Klintworth 1962; McConaghy & Blaszczynski 1980; Meskinoff et al. 1963; Parker 1964; Perkins 1973; Rainer et al. 1960; Wollesen & Tendler 1976; Zuger 1976). Thus, theories that held to a genetic component to homosexuality lost support. For twenty years (1955-1975) John Money argued that almost all aspects of orientation and identity were products of upbringing (Diamond 1995b), and Masters and Johnson (1979) also argued that homosexuality was of social rather than biologic origin.

This situation essentially held until the 1980s when newer studies emerged supporting a major biological component to sexual orientation. Richard Pillard and colleagues (Pillard, Poumadere & Carretta 1982; Pillard & Weinrich 1986) examined families in which at least one member was openly homosexual. As controls, a set of families from heterosexual “index” individuals were also studied. The investigators inquired of the sexual orientation of all siblings and found that if a family contained one son who was homosexual, 18 to 25 percent of his brothers would also be homosexual. If an index individual was heterosexual, the chance of other brothers being homosexual was only about 4 percent. The former figure is significantly higher than expected by chance and the latter figure in the range to be expected (Diamond 1993b).

Michael Bailey and colleagues (Bailey & Bell 1993; Bailey & Benishay 1993; Bailey et al. 1993), as done for males, looked for familial factors in female homosexuality. Similar to the findings with males, female homosexuality appeared to be familial. Depending on how homosexuality was defined within the study, it was essentially found that 12 to 20 percent of the homosexual probands had homosexual siblings in comparison with some 2 to 5 percent for the heterosexual probands. From their data, Bailey and Bell (1993) noted that the strongest links to homosexual orientation were associated with genetic closeness rather than parental influences.

Recently, Blanchard and Sheridan (1992) and Blanchard, Zucker, Bradley, and Hume (1994) followed the early work of Lang (1940, 1960), Jensch (1941), Kallmann (1952a), Suarez and Przybek (1980), and others in comparing the sex, not the orientation, of siblings of a group of matched homosexual or effeminate (prehomosexual) boys with a control group of presumably heterosexual boys. The results of all these investigators are consistent: among siblings of such homosexual males, the sex ratio of brothers to sisters was significantly higher than the expected typical 106 males to 100 females. Among “homosexual families” the ratio was more like 126-131 brothers to 100 sisters (significantly more males than females). These high ratio differences were not seen among samples of homosexuals who were not feminine as boys (Zucker & Blanchard 1994). These data suggest an excess of brothers is seen more often among the most effeminate of homosexual males.

Blanchard and colleagues in these same and newer (Blanchard & Zucker 1994; Zucker & Blanchard 1994) studies looked at the loci of these probands in birth order. In keeping with the findings of Slater (1962) and Tsoi, Kock, and Long (1977), all studies found that males destined to be homosexual or who already were homosexual tended to be born significantly later than other siblings. Several theoretical hypotheses were postulated to link sex ratio and birth order effects biologically to homosexuality. One, put forth by MacCulloch and Waddington (1981) and reasserted by Ellis and Ames (1987), speculates that maternal immunization against male fetal tissues or products, specifically testosterone, brought about by previous pregnancies with males might reduce the mother’s ability to carry male fetuses to term or the androgen’s activity and ability to masculinize the fetal brain fully.9 Another theory posited that stress to the mother during pregnancy might be involved (Dörner et al. 1980, 1983; Ellis & Ames 1987; Ellis et at. 1988).

Regardless of the mechanism involved in this sex ratio disparity, these findings essentially eliminate the theory that homosexuality results from the over-abundance or presence of females (sisters) in families from which homosexuals originate, these sisters supposedly imparting feminine attributes and interests to their younger brothers (West 1977).

Family sibling studies and twin studies are significant in bolstering each other. In recent investigations of’ twins, as with the most noted work of Kallmann and other earlier workers, the concordance for homosexuality is strong. Bailey and Pillard (1991) with a study of more than 100 twins found that 52 percent of identical brothers of self-identified homosexual men were also homosexual, compared with 22 percent of fraternal twins and 11 percent of unrelated (adoptive) brothers. Frederick Whitam and colleagues found even stronger concordances (Whitam, Diamond & Martin 1993): approximately 65 percent for identical male twins and 30 percent for fraternal twins. Among their small sample of females, they found that concordance for homosexuality was 75 percent for one-egg twins and 0 percent for male-female twins; among four male-female twin pairs the males all had Kinsey scores of 4, 5, or 6 while the females were exclusively heterosexual. Three sets of triplets also appeared in this sample. One set consisted of three females with a pair of monozygotic twins who are both lesbian and a third heterosexual sister. A second pair of triplets consisted of a monozygotic male pair, both homosexual, with a heterosexual sister. A third monozygotic male triplet set reported not only the same sexual orientation but similar lifestyle patterns.

Bailey et al. (1993) specifically looked for factors that influence sexual orientation in women. They found 48 percent concordance for homosexuality among monozygotic twins, 16 percent for dizygotic twins, and only 6 percent among adopted sisters. Probands also reported 14 percent of the non-twin biologic sisters to be homosexual.

Twins Reared Apart

Twins reared apart are especially useful for evaluating the relative roles of genetics and environment in behavior. In most modern societies, however, it is increasingly difficult to find such individuals. Elke Eckert and colleagues (Eckert, Bouchard, Bohlen & Heston 1986) reported findings from six pairs of monozygotic twins reared apart since infancy in which at least one member of each pair was homosexually active. Among the two male pairs, one pair was concordant for homosexuality, and one pair was partially concordant. Among the four female pairs, in contrast, none was concordant in sexual orientation. This, as in the other studies reported, implicates genetics as being a stronger factor in male than in female sexual orientation. Two such reared-apart male pairs appeared in the Whitam, Diamond, and Martin (1993) sample. Both pairs met only after they were adults. One set was concordant for homosexual orientation, and one was not.

Obviously simple genetics does not paint the total picture. An interesting factor exists that is not yet fully understood. Among both monozygotic and dizygotic twins, male and female, when the twins are discordant, the divergence is usually large. One might be a Kinsey 6 and the other a Kinsey 0. Why even identical twins who share the same set of genes and upbringing should differ so markedly is not yet understood. Unless some environmental feature acts as an on-off switch, which is certainly possible, more ambisexual siblings with a Kinsey 2, 3, or 4 would be predicted.10 Other interacting factors must be involved.

Chromosome Studies

A second most significant area of genetic evidence supporting a biological basis for sexual orientation comes from chromosome analysis and related types of pedigree studies.

At a basic level, it must be recognized that any behavior is dependent on some encoding within the nervous system. Since the work of Murry Barr (1966) it is known that mammalian neurons can be sexually distinguished by the presence or absence of so-called sex chromatin (Barr body). This is a special staining bit lodged on the inner surface of the nuclear membrane of nerve cells in females. It is the remnant of the second X chromosome in females; only one is typically functional. No being females, neither male heterosexuals or male homosexuals show such chromatin.

Recently the work of Dean Hamer and colleagues went further. Using family tree studies, they determined that not only did homosexuality seem to run in families, but it was more significantly influenced by the mother’s lineage than the father’s. Hamer and colleagues (Hamer et al. 1993; LaVay & Hamer 1994) found practically no homosexuals in paternal lines and “compared with randomly chosen families, rates of homosexuality in maternal uncles increased from 7 to 10 percent and in maternal cousins from 8 to 13 percent. This familial clustering, even in relatives outside the nuclear family, presents an additional argument for a genetic root to sexual orientation.” Were a trait to be passed along on the X chromosome, which can only come from mothers (fathers contributing the Y), it would be preferentially manifest in brothers, maternal uncles, and maternal male cousins. This is precisely what was found.

Hamer and colleagues then sought physical evidence of such a trait that might be manifest by some chromosomal pattern. They found, in homosexual brothers, a remarkable and significantly similar marker pattern of genes on the Xq28 region of the X chromosome.11 Of forty pairs of gay brothers investigated, thirty-three pairs shared the same marker. Among a randomly selected control group of brothers, Xq28 markers were randomly distributed. Although it is still curious that seven pairs of brothers did not show similarity in gene marking, the overall results are highly significant in indicating there is probably a gene or genes within this region that strongly influences sexual orientation.

Bolstering this conclusion is a new major study by William Turner (1995) investigating more than 200 families of homosexual male and female probands. Unlike the earlier studies, which looked at the sex ratio and heterosexual-homosexual composition of siblings, Turner compared the male-female parental sibling ratios in families. Instead of an anticipated 106 to 100, he found a very large unbalanced secondary sex ratio in the maternal generation of male, but not female, homosexuals, a highly significant ratio of twice (or more) as many aunts as uncles. The maternal uncle to aunt ratio for female homosexuals was about one to one. In his study, Turner also found 65 percent of the mothers of homosexuals had no or only one live-born brother. Fathers of homosexua1s did not have sex ratio anomalies among their siblings. According to Turner’s analysis, some 35 percent of the males conceived in the maternal generation were miscarried - a 50 percent excess of deaths of male concepti compared with female concepti. Other aspects of familiarity Turner found are multiple instances of infertility and suicides, almost exclusively among maternal rather than paternal relatives.

Turner did more: he compared these statistics with findings from other medical conditions with known genetic transmission rates, such as Addison’s disease (adrenoleukodystrophy), color blindness, and hemophilia A. Male fetal wastage similar to that of offspring of fathers in families known to have male semilethal Xq28 disorders and children born to the mothers of these Xq28 problems show about the same ratio of those children born to mothers of homosexuals. Hemophilia A and color blindness were relatively rare in the control population. From his pedigree studies, Turner concludes that homosexuality is a semidominant genetic phenomenon, semilethal to males, with gene(s) in the pseudoautosomal region of Xq28.12 Moreover, hypothesizes Turner, this region is very vulnerable to variable posimitotic cytosine methylation to produce the elongated trinucleotide repeats in the Xq28 region that Hamer et al. (1993) find a marker of homosexuality. All of these findings, regardless of rearing practices, support a genetic factor operating in sexual orientation.

Turner also contributes to this argument in another way. He cites numerous medical, biological, and social conditions in which monozygotic twins have been shown to differ although their parentage and environment are shared. But he makes the point that neither genetically nor environmentally does this mean their heritage is identical. While the environment may intuitively be accepted as variable between each member of a monozygotic twin set. it is also possible their genetics also differs. For instance, Kastern and Krypsin-Sorensen (1988) documented a set of monozygotic twins where one is male and the other female. They state: “While the role of chromosomal fragility in rearrangements is not certain, it is clear that somatic variations, such as translocations, deletions and so forth, in the genome are quite common. ... The genome of mammals is a very plastic entity, capable of frequent changes and indicating subtle, but perhaps very important, differences from the genetic structure of the germ line.” It has been estimated that approximately 1 in 20,000 males has two X chromosomes and no Y and about 1 in 20,000 females is XY (Kelly 1991).

Buhrich, Bailey, and Marlin (1991) studied twins specifically in an attempt to identify how much genetics or the environment might contribute to sexual preference. While they conservatively call their results tentative, they conclude “restrictive multivariate models yielded a significant genetic influence on sexual orientation.”

Before leaving this section we must deal with the question: If sexual orientation is genetic, why don’t all monozygotic twins behave similarly? Genetic transmission may follow patterns of other behaviors assumed to have a strong genetic component and yet not display 100 percent concordance in monozygotic pairs. Kaij (1960) found, for example, the rate of concordance for alcoholism to be 54 percent in monozygotic pairs and 24 percent for dizygotic pairs. Most significant, Nagylaki and Levy (1973) found monozygotic twins have more reversed asymmetries than dizygotic pairs: there is a larger proportion of discordance of handedness in monozygotic pairs. Also Bouchard and colleagues (1990), among the twins they studied, many reared apart since birth, found the traits most fixed by heredity were those intuitively thought to be most influenced by training. The closest links between twins were: 1) traditionalism or obedience to authority (the tendency to follow rules and authority, to endorse high moral standards and strict discipline), 2) harm avoidance (the tendency to shun the excitement of risk and danger), and 3) aggression (to be physically aggressive and vindictive). But even in these features, not all twins showed these traits equally. Even conjoined wins have often been seen to be quite disparate in temperament and interests.

It also must be appreciated that while monozygotic twins may share the same genetics, they might not share the same prenatal environment and only partially share the same postnatal one. Sexual orientation might be seen as biologically related to different in utero conditions. Melnick, Myrianthopolos, and Christian (1978), for example, have suggested that monochorionic twins are more alike than dichorionic twins. However, the shared blood circulation of the common chorion may be more unfavorable for one twin than the other. There often are, for instance, significant differences in birth weight between monozygotic twins. Sexual orientation may be biologically determined in utero by biochemical mechanisms that remain to be identified, but these influences must be recognized as under genetic influence.

Lastly, in considering genetics and homosexuality one must mention the work of Hall and Kimura (1994). These investigators detected that male heterosexuals and homosexuals differ significantly in the type of fingerprint pattern (leftward asymmetry) they possess. This biological feature is present before birth Starting at about the seventh week post conception; about the same time sexual differentiation starts. Thus sexual orientation and dermatoglyphics must assort similarly or these two characteristics must be subject to the same prenatal processes.

Nervous System

It is true, but simplistic, to point out that males and females are anatomically significantly different in many more ways than in relation to genitals and the reproductive system. Mean height and weight differences and so forth are outward manifestations of other anatomical and physiological processes that differ over and above those mechanisms required for reproduction. Enzymatic processes of the brain and liver, workings of the kidney, and behavioral circadian rhythms, for instance, differ between men and women. Do these processes differ between heterosexuals and homosexuals? And what if they do? It is difficult for most laypeople and many scientists to see how physiological or biochemical differences could affect behavior even when they do.13

It is in the realm of the nervous system, however, that laypeople and scientists alike can accept that differences may have an influence on behavior. As the storehouse and mediator of instinct, reflex, and learning, the nervous system looks to be the ideal place to search for meaningful differences between males and females and those of different sexual orientations. In the past several years, this area of research has, like the classical sex chromatin work of Barr (1966), stimulated the imaginations of the general and gay public, as well as the scientific community.

Nervous System Structure

Research reports from the Netherlands were among the first to indicate that certain brain structures differ between males and females and heterosexuals and homosexuals. The Dutch researchers F. Swaab and M. A. Hofman (Swaab & Hofman 1990) found a brain region called the suprachiasmatic nucleus is much larger (approximately 1.7 times as large and containing 2.1 times as many cells) in homosexual males than in heterosexual males. Swaab and Fliers (1985), Swaab and Hofman (1988), and Swaab et al. (1993), found that the sexually dimorphic nucleus of the preoptic area (SDN-POA) of the human hypothalamus, as reported earlier for the rat by Gorski et al. (1978), also differed in size between men and women. Swaab and Hofman found the SDN-POA from brains of two male-to-female transsexuals fell within the female range, not the male range. Significantly, these researchers reported that this locus in the human, although first showing signs of development prenatally, seemed to be maximally differentiated between the ages of 2 and 4 years. Since this nucleus is involved in the regulation of sexual behavior in male and female rats and other species as well, this suggests human differences related to the organization and later activation of levels of human sexual activity may be more or less set from early on. Interestingly, Swaab and Hofman (1988) also found that the sex difference in this nucleus begins a sharp decline after the age of 50, about when the sex drive of men is said to diminish.

The comparison between heterosexual and homosexual brains was relatively new, but the findings of sex differences in the brain were not. Morel (1948) from France and Rabl (1958)from Germany had shown that the massa intermedia, a commissural fiber group linking thalamic lobes of the right and left, were different between the sexes. Newer studies in the United States by Allen and Gorski (1987) again found the massa intermedia 50 percent larger in females as in males and more often missing altogether in males (Allen & Gorski 1992). They found sex differences in the bed nucleus of the stria terminalis (Allen & Gorski 1990) and the anterior commissure. In animals the bed nucleus has been shown to be involved in sex-specific activities, aggression, sexual behavior, and neuroendocrine functioning. In all mammals, the anterior commissure is a major fiber bundle that links the right and left hemispheres.

These studies did not cause much of a stir until Simon LeVay, in the United States, found a region of the medial preoptic area of the hypothalamus (interstitial nucleus of the anterior hypothalamus, 3 or INAH3) smaller in homosexuals and women than in male heterosexuals (LeVay 1991). It is, on average, about two tunes larger in men than women and two to three times larger in heterosexual males than in homosexual males. And the INAH3 region of homosexuals is about the same size as that of the women’s (LeVay 1993).14 In some gay men the INAH3 was completely absent. The brains of lesbians have yet to be examined.

Others too reported that different areas of the human brain differ between males and females and that these differences are associated with nonreproductive as well as reproductive functions. But the work of LeVay, published in the widely read journal Science, stimulated review of and attention to brain differences that might be associated with sexual orientation. Allen et al. (1989), Allen and Gorski (1992), Hines et al. (1992). and Hines (1993) found that the anterior commissure is smallest in heterosexual males, larger in females, and largest in homosexual males. These differences held even after correction for overall brain size.

In 1982, de Lacoste-Utamsing and Holloway reported sexual dimorphism in the human corpus callosum. Demeter, Ringo, and Doty (1988) and Allen and Gorski (1992), while reporting some conflicting findings when looking at the corpus callosum (CC), the largest commissural body in the human brain, also report significant sex differences. After studying this structure, Hines reported (1993), “We have found that the size of the splenial [caudal] region of the corpus callosum ... reported to be larger in women than in men, may relate to sexual orientation in women. ... Women with a history of some homosexual experience had significantly smaller midsagittal splenial areas than those whose sexual experience involved men only.” Emory et al. (1991) looked for differences in the CC of transsexuals but found none; neither could they find sex differences. A recent study of the corpus callosum (Bosinski et al. 1994), however, reported the total size “relatively larger in females than in males and in female-to-male transsexuals. ... Relative size of CC in FMT [female to male transsexuals] is different from ratios in their biological sex (female controls) and resembles that in males.”

Most recently three new aspects of brain morphology were found to differ between homosexual and heterosexual males and between males and females. Scamvougeras et al. (1994) reported that the isthmus of the corpus callosum, associated with interhemispheric auditory transmissions, is on average 13 percent larger in gay men compared with straights, and Caillé and Lassonde (1994) reported these fibers differed between men and women. Also, patterns of neuronal density of cortical layers 2 and 6, the layers associated most with reception and distribution of intracortical information, were found to differ between males and females (Witelson, Glezer, & Kigar 1994). This finding is in keeping with many other sex differences reported by this investigator (see, e.g., Witelson 1991).

In addition to these findings from humans, much research on structural sex differences in the nervous system comes from animal experimentation studies. More than differences in the regions mentioned above, significant male-female differences have been found in the amygdala, an area associated with various emotions such as aggression and fear, and the spinal cord, associated with copulatory movements and functions (Clemens, Wagner & Ackerman 1993; for excellent reviews see Neumann & Steinbeck 1972 and Tobet & Fox 1992). Obviously, research in humans and animals focusing on brain differences between individuals with different sex orientation, and identity is an area that will continue to receive a great deal of additional attention.

Nervous System Function

The functioning of the brain and nervous system has, since the 1950s and 1960s, been known to differ between men and women. Lansdell (1961, 1962) reported significant findings between men and women in their verbal ability and design preference, which reacts differentially to temporal lobe surgery. After surgery to the dominant lobe, women maintain their previous artistic judgment, while men lose theirs. This effect suggested to Lansdell that “judgment and verbal ability may overlap in the female brain, but are in opposite hemispheres [lateralized] in the male.”15 Lipsitt and Levy (1959) showed that at least some sex differences are set by 3 days of age; females have a lower threshold to electric shock stimulation than do males.

In more recent times the number of studies that show inherent sex differences in brain functioning has increased markedly (see Hampson & Kimura 1992 and Levy & Helter 1992 for review). These studies seldom fail to arouse consternation among those who feel that either this should not be or that such differences are the result of social conditioning or learning. One area of functioning in particular that has aroused critique is related to differences in mathematical and spatial reasoning between males and females.

A recent review by Camilla Benbow and David Lubinski (1993) shows that despite national testing agencies’ continually trying to remove questions found to show the greatest gender disparities, males continue to outscore females in mathematical and spatial tasks at the highest levels. This holds even on tests, such as the SAT, where there is no sex difference in overall scores. The males score almost one-half standard deviation higher than do the females. And there is no evidence that these types of scoring differences have diminished among the highly gifted over the past decades.16 Benbow and Lubinski document that this math ability is associated with enhanced right hemisphere brain functioning found in males and not females. Benbow (1988) suggests this difference is due to prenatal exposure to high levels of testosterone.

Wilmott and Brierley (1984) found that homosexual men scored lower on the Wechsler Adult Intelligence Scale (WAIS) than did heterosexual men but comparable to heterosexual women. ThisIQ difference was thought related to significantdifferences in spatial ability. Such spatial ability differences were later specifically documented by Sanders and Ross-Field (1986) and confirmed by Gladue et al. (1990). Sanders and Ross-Field found that male homosexuals did less well than heterosexual males on two measures of spatial ability, the Vincent Mechanical Diagrams Test and the Water Level Task, a measure of horizontality. In 1993, Sanders and Wright reported that on performance tasks such as with targeted throwing and other manual dexterity tests, heterosexual men differed significantly from women and homosexual men. The heterosexual men were better at targeted throwing, while the women and homosexual men were better at pegboard tasks. Sanders (1994) recently also found the performance of homosexual men resembled that of heterosexual women more than heterosexual men in regard to different mechanical and visual tasks and cerebral lateralization less than heterosexual men. Others (Tkachuk & Zucker 1991; McCormick & Witelson 1991) also found similar differences related to sex and sexual orientation. This last team of investigators administered tests of verbal fluency. Here females typically show an advantage, and again the male homosexuals scored more like the high-scoring females than like the low-scoring heterosexual males. In essence, all of these studies show that heterosexual men were significantly better at tasks of spatial ability than were homosexual men or women and that male homosexuals either scored similarly to heterosexual women or intermediate to heterosexual men and women. Studies of the spatial ability or manual dexterity of lesbians have yet to be reported.17 Animal research on the spatial ability of males and females shows strong sex differences and the organizational influences of gonadal steroids (Williams & Meck 1991, 1993).

Prenatal Influences

Although some of the sex differences noted may be directly linked to genetics, other crucial factors have to be considered. Any complete discussion of the etiology of anatomical and physiological differences between the sexes needs to deal with prenatal endocrine influences that intercede between the genetic influences and resulting sex-linked behaviors (Diamond 1965, 1968, 1979; Reinisch & Sanders 1992).

In mammals, genetic forces initiated by fertilization lead to gonadal processes—the maturation of testes or ovaries and release of their hormones, or absence thereof—that effect structural changes in genitalia and other structures and also organize the developing nervous system in gross and microscopic ways).18, 19 This organization biases the individual toward male and or female behaviors. These influences are on reproductive and nonreproductive behaviors (Beatty 1979, 1992). This process is well documented for nonhuman mammalian species, and many reviews are available (Arnold & Gorski 1984; Diamond 1968, 1976; George & Wilson 1988; Gerall, Moltz & Ward 1992; Goy & McEwen 1980; Kelly 1992). By extension and evolutionary extrapolation, it is assumed that similar processes hold for humans, and indeed the references supply evidence for such. I have, for many years, proposed that organizational processes influence both sexual orientation and sexual identity (Diamond 1965, 1968, 1976, 1979, 1993a, 1994a, 1994b). Such has been adequately demonstrated in animal experimentation.

Starting in the late 1960s and into the 1980s much animal research, from many species, documented how hormones administered paranatally during critical periods could demasculinize and feminize male animals or masculinize and defeminize female ones?20 Basically it was found that females exposed to testosterone during pregnancy or soon after birth, when adult, exhibit malelike copulatory behaviors and a lowered tendency to lordose, and males castrated or givenantiandrogens during pregnancy or soon after birth, when adults, exhibit female copulatory behaviors and reduced male behaviors. The literature in this area is voluminous and has been well reviewed (Adler, Pfaff & Goy 1985; Arnold & Gorski 1984; Baum 1979; Döhler, Ganzemüller & Veit 1993; Gerall, Moltz & Ward 1992; Goy & McEwen 1980; Haug et al. 1993).

Work in the late 1980s and into the 1990s has greatly extended this area of research. Research, particularly in the Netherlands with the work of Slob and associates Bakker and Brand and in Sweden in the laboratories of Larsson and Vega-Matuszcyk (see References for the work of these authors), has shown that paranatal hormone manipulation during a critical period not only induces the changes noted but also significantly alters the development of sexual partner preference. Androgens given to a female rat during a critical period ofneonatal or prenatal life induce her, when adult and treated with male hormones, to prefer a female sexual partner to a male. Conversely, neonatal castration or the administration of an antiandrogen to a male rat induces him, when adult, to prefer a male sexual partner when treated with female hormones.21 Such behavior is not forthcoming from intact control animals. In essence, comparable influences are believed to intercede in human development.

Do these forces obtain for humans? Evidence is available to make it seem so. The strongest evidence comes from females that were subject to androgens prior to birth. This comes about from adrenal malfunctions of the fetus (congenital adrenal hyperplasia, CAH) or metabolic problems of the mother (e.g., adrenoblastoma). These individuals, depending on the amount of androgens to which they had been subjected and the timing of this exposure, vary in the demonstration of the effect. They might be only slightly affected or so affected they are misidentified as males at birth and reared accordingly. Overall, CAH females raised as women generally live and identify as women but show signs of defeminization and signs of physical and behavioral masculinization, and significantly, a higher-than-anticipated incidence of lesbianism (Ehrhardt, Epstein & Money 1968; Money, Schwartz & Lewis 1984). In one report by van Seters and Slob (1988) two CAH females not only adapted well to a crossed-sex assignment but married as men and, despite having only a micropenis (actually a hypertrophied clitoris) had “satisfactory” marital sexual relations with their wives.22 (See Berenbaum 1990 for a review of CAH and intellectual and psychosexual functioning.)

On the opposite side of the coin are males who are somehow demasculinized and feminized. This is most dramatically seen in cases of androgen insensitivity. In such instances the XY individual is unable to utilize testosterone normally and thus retains the physical form of the female and the accompanying bias to female thinking patterns.23 These persons are always raised as girls and seem to adapt well as women; albeit infertile ones. It might be argued that, raised as women, androgen-insensitive cases demonstrate the power of rearing. However, these individuals do not act as typical females but might he considered “hyperfemales.” They almost invariably want to marry, have children, stay at home, and care for them. Few want to conquer the business world. Finally, consider that boys born to mothers subject to high dosages of estrogen during pregnancy showed extremely feminine behaviors despite normal male upbringing (Yalom, Green & Fisk 1973).

Significantly, it is not only endogenous hormones effects per se that might influence partner preference. Ward and coworkers (Ward 1972, 1977; Ward & Ward 1985; Ward & Weisz 1980, 1984) have shown that pregnant rats subject to extreme stress deliver male pups that, as adults, exhibit female lordosis patterns and fail to mate as males. This work was followed by others who showed that the stressors might include overcrowding (Dahlöf, Hård & Larsson 1977; Harvey & Chevins 1984) and malnutrition (Reese & Fleming 1981). The effect of stress has since been extended to other species and other behaviors (see Ward 1992 for review). Of particular interest is that the stressors can be of diverse types and affect brain and spinal cord sexual dimorphism.

Although the experimental work with animals is clear, the evidence for humans is still being gathered. Dörner et al. (1980), Ellis and Ames (1987) and Ellis et al. (1988) present data linking prenatal stress in human mothers to homosexuality in their sons. Dörner et al. looked at the effect of trauma from war, and Ellis looked at a set of thirty-one potentially stressful events in everyday life. This work follows from the animal literature but is controversial relative to the methods and criteria used for evaluating stress and the long retrospective period involved (Gandelman 1992). Also, the work of Bailey, Willerman, and Parks (1991) did not find the effect, but they report, “Mothers of effeminate children reported more stress-proneness than other mothers” and “Male homosexuality nevertheless was strongly familial.” Further data are needed to substantiate or refute this thesis, but the door seems open to this influence.

It is also worthwhile mentioning the work of Reinisch and Karow (1977), who found significant personality differences among groups of children exposed prenatally to progestins or estrogens and their matched controls. Those exposed to progestins (with purported androgenic features) were “characterized as more independent, sensitive, self-assured, individualistic, and self-sufficient. In contrast, the subjects exposed to the estrogen regime were more group oriented and group dependent.” These are differences usually associated with maleness and femaleness, respectively.


Another line of thought seems to point to constitutional rather than environmental factors as directing sexual identity and orientation. These are instances where these vectors seem in conflict. Several types of studies are instructive here.

Upbringing and Culture

J. Imperato-McGinley (1983) and her colleagues (Imperato-McGinley et al. 1974, 1979; Imperato-McGinley & Peterson 1976) studied a group of indigenous persons in the Dominican Republic. These people were XY individuals who, due to a genetic quirk, were born without penises or a scrotum. During pregnancy these individuals had absent or reduced levels of 5a-reductase, which is needed to convert testosterone to dihydrotestosterone to differentiate the male genitals. Most subjects had separate urethral and vaginal openings within an uro-genital sinus. Their parents thought these offspring to be girls and raised them accordingly. At puberty, however, the penis and scrotum developed.24 Despite having been raised as females from birth, almost every one of these teenagers then switched to life as heterosexual males. Upbringing as girls destined to marry males had little influence on their adult orientation or sexual identity. Instead of being fixed in the sex of rearing from birth to puberty, these males readily adopted their sex-appropriate gender, orientation, and identity. Such cases, since they portray so little influence of upbringing, add grist to the argument that heterosexual orientation and masculine identity are more likely matters of genetic predisposition than social forces alone. Also reducing the factor of society is that similar findings were reported for indigenous populations in New Guinea, Turkey and among some Arabs in Israel. Here too, individuals reared as girls, on their own switched to living as males once past puberty.

Some argue (e.g., Gooren, Fliers & Courtney 1990) that the parents of these children knew in advance they would be switching their children’s gender so these subjects do not constitute a true test of the nature-nurture issue. This is certainly true for the later cases studied, but it does not hold for the early cases, before the natives’ association with modern medicine. Prior to contact, the children were reared as typical native girls until puberty and genital development, after which they switched to living as adolescent boys and married accordingly (Imperato-McGinley 1983). It also does not figure that if the parents knew the children would be boys after puberty, they would not raise them as boys from the start.

Work by Herdt (1981, 1984) and Stoller and Herdt (1985) offer examples where long-term exposure to childhood homosexual behaviors seems not to have induced postpubertal homosexual activity. These researchers document a New Guinea culture, the Sambia, where homosexual behavior is taught, encouraged, and institutionalized to transfer masculinity from adults to adolescents; the young boys are institutionalized to fellate the adult men to obtain their mana (spiritual power)-giving semen. Moreover, female bodies are presented as unattractive and poisonous, and to be avoided. Nevertheless, upon reaching adulthood, these boys choose females as regular sexual partners and are almost always heterosexual. Neither youths nor men report impulses to suck penises or engage in anal intercourse.

Schiefanhövel (1990) reports on a similar New Guinea culture. the Kaluli, who use anal intercourse to transmit the masculinity-inducing semen between older men and younger boys. He too stresses that heterosexual, not homosexual or bisexual, behavior is the preferred and exclusive outlet for these males when they mature. And this obtains despite a severe shortage of adult women due to female infanticide. Although adult-child same-sex activities are fostered in some societies, and this seems to have been part of the condition in ancient Greece (Cuillenain 1992), there is no known culture where adult-adult homosexual behavior is encouraged, is a preferred mode of behavior, or is a practice of other than a minority (Diamond 1993b; Ford & Beach 1951; Karlen 1971).

Sociologists Whitam and Mathy (1986) studied homosexuality in four different cultures: Brazil, Guatemala, the Philippines, and the United States. Across these quite diverse societies they found many similarities in how homosexual lifestyles were manifest—for example, preferences in occupational interests, involvement in entertainment and the arts, and cross-dressing. These researchers concluded that the similarities were not culturally instituted but were more likely the result of inherent biological tendencies manifest despite acceptance or rejection by the community. They conclude, “Sexual orientation is not highly subject to redefinition by any particular social structural arrangement” (p. 31).

There are societies in which homosexuality is not only illegal but subject to the death penalty (e.g., Iran) and societies in which the practice is tolerated or considered of little concern to the populace at large. And I have mentioned groups among which homosexual activities are encouraged as part of growing up. It is instructive to consider population figures to ascertain lithe prevalence of homosexual activity is correlated with sonic environmental factor we might call social tolerance or intolerance. Intuitively it seems reasonable to assume that if homosexuality was a practice readily molded by culture, such behavior would be more prevalent in societies that tolerate it most or punish it least. This hypothesis is not supported by the data (Diamond 1993b), available from Britain, Denmark, France, Japan, the Netherlands, Palau, the Philippines, Thailand, and the United States. In the relatively non-homophobic societies of Denmark, Palau, the Philippines, and Thailand, we find reported among the lowest rates of same-sex activity.25

Long-term longitudinal research studying the ontogenetic development of homosexuality is rare. Green (1987), in one of the few and perhaps best-known studies of this type, followed prepubertal boys with obvious effeminate behavior. These were matched with a control group of noneffeminate boys. Green’s study lasted fifteen years, to see how the boys would develop. In U.S. culture being a “sissy” is socially difficult and stigmatized. Of the families involved with the “sissy” boys, many tried on their own as well as with professional help to discourage the effeminate behavior. A group of the parents even entered their sons into formal treatment programs to change their mannerisms. When interviewed as adults, “two-thirds of the original group of ‘feminine’ boys reveal that three-fourths of them developed as homosexual or bisexual. By contrast, only one of the two-thirds of the previously control boys was homosexually or bisexually oriented.” The professional intervention did not alter the percentage of effeminate boys who, as adults, engaged in homosexual behavior.

Bell, Weinberg, and Hammersmith (1981) in the United States and Siegelman (1981a, 1981b) in Great Britain looked for features that might distinguish the family constellations and social backgrounds of adult heterosexuals, homosexuals, and bisexuals. Among other things, they were looking for evidence to support or refute hypotheses that associate homosexuality with a particular family constellation (e.g., close, binding and dominant mothers and weak, detached and rejecting fathers) as suggested by Bieber et al. (1962) and others. Their basic finding was that no common parameter of family or upbringing could be linked causally to sexual orientation, nor could any link be found between any aspect of an individual’s childhood or adolescent experiences and homosexual or bisexual activities. Most homosexuals are reared as heterosexuals in apparently conventional households. Green (1987) said similarly about his group of effeminate boys.

Bell, Weinberg, and Hammersmith (1981) cautiously conclude: “Exclusive homosexuality seemed to be something that was firmly established by the end of adolescence and relatively impervious to change or modification by outside influences” (p.211) and “our findings are not inconsistent with what one would expect to find if indeed, there were a biological basis for sexual preference” (p. 216, emphasis in original).


Transsexuals in many ways are the archetype to demonstrate that identity and orientation can be independent of rearing and environmental influences. Almost invariably these individuals are brought up in accordance with their bodily appearance. Then, against the wishes of their family and all social institutions, they refuse to continue in the life to which they feel they were wrongly born. If an XY individual is brought up as boy, the transsexual feels to be a girl and wishes to develop into a woman. If an XX individual is brought up as a girl, the opposite is true: this individual feels to be a boy and develop into a man (Benjamin 1966; Doctor 1988; Green & Money 1969). Often this occurs despite much evidence of success in the original gender (Diamond, 1995a).

This “fixedness” of heterosexuality, homosexuality, and transsexuality, despite accidental, family, or professional efforts to change these features, is note-worthy. Children brought up by transsexual parents do not develop as transsexuals, nor do those raised by homosexuals develop homosexually (Green 1978). Mandel, Hotvedt, and Green (1979) and Mandel and Hotvedt (1980) followed the development of boys raised in households where the parental influence was openly lesbian. They concluded, “Analysis of the children’s data has not revealed any sexual identity conflict or homosexual interest. Relationships with fathers and other males do not differ significantly [from that of boys reared in heterosexually parented families].” There is “no evidence of gender conflict or poor peer relations” for samples of children reared by lesbian mothers (Hotvedt & Mandel 1982) and no significant differences between households run by lesbian or heterosexual mothers (Green et al. 1986). Similar work has been done studying boys growing up in households parented by openly gay males. These boys, when adult, like those brought up in lesbian households, also were heterosexually oriented without conflict or homosexual interest (Green 1978). Also, neither homosexuals nor heterosexuals perceived their parents’ personalities differently (Newcomb 1985).

Cross-Sex Rearing

There are now known to me at least three instances, two sets of male twins and one singleton, in which an individual was reared as a female, with surgery and endocrine treatment to alter the biology to facilitate the transformation. The first case involved a set of twins extensively reported upon (Diamond 1982; Money & Ehrhardt 1972; Money & Tucker 1975).As a result of an accident during circumcision by cautery, one of the boys had his penis burned off. Believing that male identity requires a penis, it was decided to rear the individual as a female and augment the upbringing with surgical and endocrine treatment. The child was then subsequently reared as a girl. More than twenty years later, despite subsequent surgical orchiectomy, treatment with female hormones, rearing, and psychotherapy to facilitate a female psyche, this individual had never accepted the female status or role, despite earlier claims by Money and Ehrhardt (1972) and Money and Tucker (1975) (Diamond 1982, 1993a).From early on, the twin, without ever being told of his previous history, rebelled against the imposition of a female status. This individual, who was raised as a girl and given estrogen therapy, on his own elected to have a mastectomy and live as a male: he sought and had phalloplastic and scrotal reconstruction surgery. Now, as a mature adult, he lives as a married male. His adjustment is not without its difficulties but is preferable to imposed life as a female (Diamond & Sigmundson 1995).

The second set of identical twins involves two Samoan children who were brought to my attention when they were 6 years old. One was causing a great deal of disorder at school. The “female” of this twin set was acting rowdy and picking fights, not only with female classmates but also males. Case records revealed that ambiguous genitalia at birth had prompted the surgeons then in attendance to reassign him as a female. They convinced the parents to rear the child as a girl with appropriate castration and hormonal follow-up.

Despite the rearing as a female, even at this young age, the child rebelled against wearing girls’ clothes and the parents’ and teachers’ admonitions to “act like a girl. ” His typical play patterns and demeanor were those of a 6-year-old rambunctious boy. In discussion with me, the brother often slipped into using the male pronoun when referring to his twin, for example, “He, I mean she, swims better than me.” Asked to draw a child, the misassigned twin drew an ambiguous figure he identified as male. The child spontaneously expressed the desire to grow up as a boy.

The third case is similar. In 1990 I was called to review the behavior and condition of a 4-year-old child. Here again, the history revealed that due to the traumatic loss of a penis soon after birth, the decision was made to reassign the boy as a girl. The decision was followed with appropriate castration and therapy and the advice to rear the child as a girl. In consultation with Dr. Richard Green, then of UCLA, it became apparent that by the age of 4, this individual was exhibiting marked boyish behavior sufficient to disturb the parents and attending professionals; the child was not easily accepting the female role. The fixedness of behavior patterns along male lines, and aversion to the female role, was strong despite the contrary upbringing. This child, as far as is known, has not yet begun to show an erotic preference for males or females.

In these last two instances, the individuals were too young to express erotic interest in a sexual partner. I predict, as I did in regard to the first twin mentioned above (Diamond 1976, 1978, 1979), that despite being reared as girls, they will be gynophilic (heterosexual relative to biologic sex) (Diamond 1982). The post-natal removal of penis and testis in a human and imposition of a female rearing has never proved sufficient to overcome the inherent bias of the normally differentiated male nervous system.

A case I investigated with Dr. David Schnarch makes this point in a different way. A boy, at 3 years of age, was accidentally penectomized during a phimosis repair. The surgeons in residence wanted to “rectify” the situation by refashioning the genitals as a female and having the parents raise the child as a girl. It was determined, however, that the parents would not he sophisticated enough, motivated sufficiently, or otherwise capable of accepting and participating in this boy-to-girl transition. Also, experience with sex reassignment in a similar previous case had proved unsuccessful (Diamond 1982). The boy later had penile reconstruct ion, with favorable outcome. Now, at the age of 12, he has seemingly adjusted well, is socially accepted and identifies as a male. It is not known if he has had any sexual experiences.

In summary then, despite the supposed power of upbringing, role modeling, and learning, there is no known case anywhere in which an otherwise normal individual, even without suitable genitalia, has accepted rearing or life status in an imposed role of the sex opposite to that of his or her natural genetic and endocrine history or accepted an imposed homosexual sexual orientation. All cases of such supposedly happening have proved to be individuals genetically or endocrinologically influenced prior to birth.26


This chapter started with a comment on evidence. One is also appropriate here: those who argue against a biological basisfor sexual orientation often cite studies of negative findings that supposedly refute those of positive findings (e.g., Byne 1994; Byne & Parsons 1993). It is rare in biology or behavior to find all research to agree in every aspect of a long-running controversy. That is the nature of the controversy. If all evidence pointed in one direction, only the prejudiced would continue the debate. The behavioral and sexological sciences are not perfect. Argument by “objection” or “call for perfection” certainly might prompt additional research but will not yield solutions, nor do they really disprove a theory (Fearnside & Holther 1959).The opposition needs to do more:  it needs to present strong alternative hypotheses with controlled research and data sufficient to substantiate them.

Any genetic patterning is complex because most parents of homosexuals are heterosexual and the input to any individual’s development is numerous and both blunt and subtle. And there certainly are instances where one’s manifestation of desired behaviors is modified by learning and social mores. Preferred innate behaviors of all types are avoided and non-preferred ones adopted in order to comply with law or social pressures.

Looked at from another angle it can be asked, if sexual orientation is so easily modified by environmental forces, why don’t we readily see its effect? Why is not therapy to change homosexual to heterosexual behavior readily successful even when started early and when parents and the individual are eager for such change (Green 1987)? If the basis for homosexual behavior is some set of environmental forces, why are these factors not only so difficult to identify but impossible to bolster with reliable and consistent findings? Those environmental forces most under suspicion have been looked for and found wanting. Certainly particular stages or critical periods in development seem more significant than others in organizing and activating these behaviors. Perhaps we have to be more sophisticated in selecting time periods to investigate or developing techniques of investigation.

Does this mean I think all features of the biological contribution to sexual orientation are understood? Hardly. We still have much to learn. Biology sets the predisposition, the bias to an orientation. The social experiences of the individual will determine how this bias is manifest and orientation evolves. One might have a constitution eminently suitable for swimming but never develop this skill without exposure to a pool; one can have great mathematical reasoning ability but never develop that skill without proper schooling. An individual may be mediocre in ability but excel as a result of certain experiences. The exposures, schooling, and experiences that are needed to release or mold the sexual orientation predisposition are yet to be documented. It is my guess, however, that the features needed to release these constitutional biases are quite plastic and the individual flexible in responding to opportunities. Free choice is also needed. The absent or weak father situation, when it seems associated with homosexuality, may be due to the child’s being able to express himself or herself without excessive parental repression (Diamond 1979). (A model for how one’s biological bias and predisposition might interact with the environment, via symbolism or learning or scripting, is offered in Diamond 1979.)

It must also be recognized that many individuals repeatedly manifest ambisexual and homosexual activities even when opportunities seem absent or against social dictates to the extent they put their social life, if not their actual physical life, in jeopardy. These behaviors, as with heterosexual ones, are often expressed as compulsive and self-generated and arising from within.

For some persons, the idea that sexual orientation is biologically biased toward heterosexuality or homosexuality and predisposed is threatening. They would like to think the choice is open and always fresh. This is more often a political stance than a scientific one. Others have expressed concern that if the developmental biological forces for sexual orientation are revealed, some nefarious agency might use the knowledge to force conformity to a dictated ideal or otherwise modify a potential homosexual outcome (DeCecco 1987; Gagnon 1987; Schmidt 1984; Sigusch et al. 1982; see also Dörner 1983). Believing it is social construction that leads to homosexual behavior is probably more destructive since even despots know it is easier to modify the social arena than the biological. And it has been shown by Ernulf, Innala, and Whitam (1989) that more people are tolerant of homosexual behavior if they think it is biological (see also Rosenberg, 1994). Regardless of such claims, I believe knowledge of the actual forces behind sexual orientation, whether they be biological or social, more than ignorance of these factors is increasingly likely to solve problems of social discord and foster a mutual respect for diversity.



1 I first proposed and defended the interaction approach for humans in the 1960s. At this time it was generally believed that orientation and identity were solely products of the environment and upbringing (Diamond 1965, 1968, 1995b). Since then I have presented this view many times from different perspectives; see the References for details.

2 While this chapter assumes that most people accept that contemporary research holds that the expression of sexual orientation results from an interaction of biological and social forces, many persons still prefer to believe sexual orientation and partner preference are determined primarily or dominantly by social forces alone. See other chapters in this book for details.

3 The roots of bisexuality are similar to those for heterosexuality or homosexuality: for simplicity, however, discussion of bisexuality will be inferred from the basic overall discussion. A fuller exposition on bisexuality per se can be found in Diamond (1994b).

4 McConaghy and Armstrong (1983) cogently examine some aspects of the inter-relationship of orientation, identity, and gender and the way the terms are used. They went further: they found that subjects reporting a homosexual component were more consistent in their self-concept of identity than were those without homosexual components. This finding supports the “hypothesis that subjects who are unaware of deviant sexual feelings (normal controls) may not have a strong, consistent sense of sexual identity” dependent on standard environmental variables. Those whose characters are in question—homosexuals, transvestites, and transsexuals—may be more introspective about themselves and more consistent in their identity despite their usually typical rearing.

5 The distinctions of boy from man and girl from woman are typically those of age. In some cultures, however, they reflect parenthood, puberty, or accepted responsibility.

6 Since gender refers to human social views, it is confusing and obviously incorrect to use the term in regard to animals or when referring to biological differences.

7 The Kinsey scale is a seven-point ranking used as a shorthand measure of an individual’s sexual orientation. individuals whose sexual partners are all of the opposite sex are coded as K 0; those whose partners are all of the same sex are coded as K = 6; those with half and half are coded as K 3;and so on. This scale is used for both actual behavior and psychological desire or fantasy as well. Any individual might, for instance, be rated a 3 as a composite of a K = 0 in behavior with a K = 6 in fantasy.

8 The sexual behavior studies by Kinsey et al. (1948, 1953), in contrast to the personal view of Kinsey himself, hinted that the data they found pointed to biological factors as involved in sexual attraction.

9 Diamond and Young (1963) and Diamond, Westphal, and Rust (1969) showed that, in guinea pigs, there existed mechanisms that protect the mother during pregnancy from masculinization by androgens produced by male fetuses. These processes probably also occur in humans.

10 By no means do I think this would be a simple mechanism. Otherwise, it would certainly have been detected by now.

11 To systematize investigations of the human genome, all chromosome regions have been given identifying designations.

12 Why the loading of male offspring among male siblings of homosexuals but the loading in favor of females in the maternal generation is not clear.

13 Indeed, these biochemical differences may be quite important in engendering or signaling significant sex differences; they may even be more important than physical differences. However, their “distance” from any easy proximate behavior may shield them from easy discovery.

14 The range in size measured for this nucleus showed overlap in the sizes of the structure examined, so some females have nuclei larger than some males.

15 In concert with this finding, the size of the corpus callosum in women has been shown related positively to verbal fluency and negatively to language lateralization. See Hines et al. (1992). for an extended discussion of this cognitive functioning and brain differences.

16 There is a great sex difference in dispersion. Males are also overrepresented at the lower end of the scoring as well. Meta-analytic reviews usually tail to consider these range differences but concentrate on means.

17 Studies of lesbians are relatively rare due to their small proportion in the general population (Diamond 1993a) and their comparatively more frequent reluctance to volunteer for such studies.

18 Fish and other non-mammalian vertebrate species are. in contrast, more definitely and dramatically influenced by environmental factors in their sexual development. See Diamond (1993a, 1994b and Francis (1992).

19 Organization is a technical term used in embryology. It implies that the nervous system is programmed during pregnancy for activation later in life, usually starting at puberty.

20 Masculinization and feminization are not opposite ends of a continuum. They are two separate dimensions of gender patterns and mechanisms.

21 The mediation of these early endocrine influences is only briefly indicated here. The mode of action of different gonadal products. their metabolites or aromatized forms, in this process of sexual differentiation, development, or partner selection is noteworthy and should be appreciated. See, for example, Olsen (1993) and the other sources indicated in this section for an in-depth analysis.

22 It is true that these individuals, reared as males, as well as some of those from the earlier reports, can be said to be responding to environmental factors in “adopting” a homosexual orientation. However, the majority of such XX persons are not reared as males.

23 The inability to utilize testosterone is due to a genetically transmitted enzyme deficiency that prevents the body tissues from responding to the androgen.

24 The exact mechanism of this phenomenon is still being elucidated. It may be that at puberty the genital tissues become responsive to the circulating testosterone or levels of dihydrotestosterone increase. The full issue of in utero and pubertal sexual differentiation in these individuals is reviewed by Imperato-McGinley (1983).

25 Indeed, it may be because the incidence of homosexuality is so low in these societies that they are more tolerant of the practice. The chicken-and-egg question persists.

26 Unquestionably, fear of imprisonment or other severe punishment can induce an individual to avoid certain preferred behaviors or adopt non-preferred ones. Here, however, we are considering instances where open choice is available and extreme conditions are not at issue.


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***Not found***

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