Chicken and egg: nature and nurture
Mammals
It is well accepted that many aspects of mammalian sexual behavior are genetically controlled and transmitted from generation to generation. McGill and colleagues (e.g., 1963, 1969), for example, have shown that for male mice stable behavioral traits include almost every aspect of sexual behavior from mount latency, intromission latency, thrusts per intromission, to post castration retention of ejaculatory ability. Jakway (1959) has shown comparable strain differences in male guinea pigs and Goy and Jakway (1959) have shown strain differences in the sexual behavior of female guinea pigs. Such might be measured by latency to estrus, duration of estrus, likelihood of coming into estrus and so on. Other investigators have shown similarly in male and female rats, cattle (see Price 1987 for review) and other mammalian species. Commercial male sheep have been studied by Perkins and her colleagues (Perkins and Fitzgerald, 1992; Miller, 1990). Not only were high and low “drive found inheritable in the males of this species but so too was homosexual activity implicated as heritable1.
Those who study non-human mammalian development accept that sexual differentiation and attendant subsequent behaviors come about by a well known string of events. Genetic forces initiated by fertilization lead to gonadal processes, the maturation of testes or ovaries and release of their subsequent hormones (or their absence) that bias the developing nervous system organizing it to behavioral effects to be seen after puberty. Each step of this process is subject to genetic mediation (see George and Wilson, 1988; Olsen, 1992 and other chapters in this volume for reviews). All subsequent environmental and maturational events in the individuals development are superimposed on this bias.
Amphibia and teleosts
Looking at amphibia and teleosts offers a very different perspective. In these species the environmental influences are often the most important superimposition on the animals genetics to determine sexual differentiation. Among species of turtles, for instance, high incubation temperatures result in the hatching primarily of females while males are produced with low temperatures (Pieau, 1975; Bull, 1983). The reverse is typically true in alligators (Ferguson and Joanen, 1982) and lizards (Charnier, 1966). And in some turtles females develop at extreme hot and cold temperatures and males at intermediate temperatures (Yntema, 1976; Bull, 1983; Gutzke and Paukstis, 1984). These developments occur at temperature ranges normally found in the home territories of these species.
Among fish the environmental influences are even more “sophisticated”. For examples I will use the work of two of my students. Robert Ross (Ross et al., 1983) found that in the saddleback wrass Thalasoma duperrey individuals born as females and producing eggs will switch to being male, producing sperm and mating as males, if two conditions are met. There must not be any larger male in her visual vicinity and there must be at least one smaller conspecific, visually present. The work of Marvin Lutnesky (1992) found that among the angelfish Centropyge potteri, also a protogyneous species (individuals start life as females and later can become males) the number of social interactions and population density in which the female finds herself will determine if she changes sex. The female with the most encounters with other males and females, usually the largest female in the harem, will become a male. Individuals ofthese species become male after they have already matured and produced young as females. Then, if induced to change, they “father” subsequent generations. Among other species - such as fish of the genus Amphiprion - the animals are protoandrous and the individuals start out as males with the postmaturational environmental stimuli inducing switch to the female sex. These sorts of phenomena are termed “sequential hermaphroditism”.
“Prematurational sex change” also occur in certain species such as the minnows (family Cyprinidae). Here all individuals start out life as females. From the total population, a portion, for reasons yet undetermined, begin to develop testicular tissue and mature as males. The rest of the young mature as females (Takahashi, 1977; Takahashi and Shimizu, 1983).
To add interest to this non mammalian story, among other teleost species, the deep sea bass Serranidae Tigrinus, for instance, simultaneous hermaphroditism exists. These animals form stable pair bonds, defend territory together and mate each day, once as a male and once as a female (Pressley, 1981). Other aspects of the normal physical environment can influence sex determination in teleosts. For example pH can effect differentiation in cichlids and poeciliids (Rubin, 1985) and day length may effect differentiation in the Siamese fighting fish, Betta splendens (Forselius, 1957). A review of these fascinating developmental styles is given in Francis (1992).
The string of events in this second set of examples from amphibia and fish is thus quite different from that first seen with mammals. Among amphibia and fish we have species without sex chromosomes but with autosomes which contain genes of sex distinguishing behavioral characteristics that mesh with environmental events to differentiate the nervous system. This in turn organizes development of the gonads. Certainly, for these animals as well as mammals, genetics are part of every step; just in a different way to the observer. Genetics “primes” the animal to which environmental stimuli it must respond. In mammals, differentiation of the gonads, induced by internal genetic events, leads to brain differentiation and subsequent appropriate sexual behavior. In contrast, in fish and amphibia, brain differentiation, induced by external environmental events, leads to gonadal development and then appropriate sexual behavior.
Manipulating development
At first glance, in mammals constitutional factors seem to be most important for sexual development, and in amphibia and fish it seems environmental events are most significant. A moment’s reflection shows that both genetic and environmental factors are crucial for both. If the environment in which a mammal finds itself changes, for instance, it may show different strain typical behavior. The copulatory patterns observed under standard testing conditions would be markedly altered should there be a hungry fox in the vicinity. And obviously it is the genetic differences between the species that had the environment’s visual cues crucial for Thalasoma and social cues crucial for Centropyge. As biologists have known since the 1920s, for each species the heredity-environment interaction has to be studied separately for each trait 2,3.
Regrettably the links are not always simply revealed. Without testing there is no way to know how any trait will be transmitted. Commercial breeders often see this when they attempt to breed out male “duds” in favor of “studs” or do similarly for the female counterparts, without sacrificing the commercial nature of the product they are trying to obtain. The goal of these breeders is a better “crop” or yield. They are not always successful; the enhancement or manifestation of one trait often diminishes another.
Endocrine manipulation (changing the internal environment) has also been used commercially to modify behavioral development. The best known example here is the use of castration to pacify or stop the sexual activity of adult or preadult animals. The mirror to this is the administration of steroids to songbirds to stimulate singing or to fighting animals (e.g. game cocks, fighting dogs) to enhance pugnaciousness.
Laboratory experiments are also well known for manipulating the prenatal, neonatal or post-pubertal environment, particularly with hormones, drugs or other environmental challenges, to alter sexual development and subsequent behavior. Thus the issue can no longer logically be whether the environment or genes can influence sexual development. The goals are to unravel which genetic and environmental factors mold development and how do they do so.
Sexual orientation
Proponents of a strong environmental component to human sexual orientation propose that one is taught or reinforced to be heterosexually or homosexually oriented. Let us examine these sorts of evidence and compare them with more pointed genetic studies.
Environmental contributions
The fixedness of effeminacy in males: non reinforced western cultures: family influence
Green (1987) reported on boys prepubertally seen as obviously effeminate. He studied them for 15 years and compared their development with “control” boys Of the families involved with the “sissy” boys, most tried on their own to discourage the effeminate behavior and a minority of the parents even entered their sons into formal treatment programs to change their behavior. 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 (still in the study) of the previously ‘masculine’ (control) boys was homosexually or bisexually oriented”.
Bell et al. (1981) in the United States and Siegelman (1981) in Great Britain looked for features that might distinguish the family constellations and backgrounds of adult heterosexuals, homosexuals and bisexuals. 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 their homosexual or bisexual activities.
Bell et al. (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, transsexuals are the archetype to demonstrate that behavior can be independent of rearing and environmental influences. Despite being brought up in accordance with their bodily appearance, and against the wishes of their family and all social institutions, these individuals refuse to continue in the life to which they were assigned. If an XY individual is brought up as a 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 person feels to be a boy and develop into a man.
The individual’s feeling about himself or herself, how he or she identifies as male or female (not masculine or feminine which is different), is independent of his or her sexual orientation. An individual’s sexual orientation/partner preference can assort independently from his or her sexual identity4. One can be a transsexual who is heterosexual, homosexual or ambisexual. Also, children brought up by transsexual parents do not develop as transsexuals (Green, 1978). A more complete discussion of this is available (Diamond, 1976, 1979, 1980, 1992).
Cross-cultural studies
Whitam and Mathy (1986) studied homosexuality in Brazil, Guatemala, the Philippines and the United States. Across these diverse societies they found many similarities in how homosexual life styles were manifest. Similar behaviors included preferences in occupational interests, involvement in entertainment and the arts and cross-dressing. These researchers concluded that the similarities were not culturally instituted but 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).
The fixedness of heterosexuality western cultures: family influences
Mandel et al. (1979, 1980) followed the development of “role modeling” in 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.)”. They find: no evidence of gender conflict or poor peer relations” for children reared by lesbian mothers (Hotvedt and Mandel, 1982). Boys growing up in households parented by openly gay males were heterosexually oriented without conflict or homosexual interest (Green, 1978).
Cross-sex rearing
There are now at least three personally known 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 and Ehrhardt, 1972 ). As a result of a surgical accident during circumcision by cautery, one of the boys had his penis burned off. Believing that sexual identity would be based upon the sex of rearing, the decision was made to rear this individual as a female (Money and Ehrhardt, 1972)5. Now, more than 20 years later, it is known that despite pediatric orchidectomy, treatment with female hormones, and psychotherapy to facilitate a female psyche, this individual has never accepted the female status or role as claimed by the early investigators (Diamond, 1982; Diamond, 1993b). Prior to puberty the twin, without ever being told of his previous history, rebelled against the imposition of a female status. At 18 years of age, this individual who was raised as a girl sought and had phalloplastic and scrotal reconstruction surgery. Now, as a mature adult, he lives as a male and seeks females as sexual partners His adjustment is not without difficulties but, to him, seems preferable to imposed life as a female.
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 disruptive 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. With appropriate castration and hormonal follow-up, they convinced the parents to rear the child as a girl.
Despite the rearing as a female, even at this young age, the child rebelled against the parents’ and teachers’ admonitions to “act like a girl”. The twin’s typical play patterns and demeanor were those of a six year old rambunctious boy and such that the brother, often slipped into using the male pronoun when referring to his twin; e.g. “He, ugh, 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 four-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. Castration and therapy followed, with the advice to rear the child as a girl. In consultation with Dr. Richard Green of U.C.L.A. it became apparent that by the age of four this individual was exhibiting marked boyish behavior sufficient to disturb the parents and attending professionals. The child was not accepting the female role. The fixedness of behavior patterns along male lines, and aversion to the female role, was strong despite the contrary upbringing.
In these last two instances, the individuals were too young to express erotic interest in a sexual partner. I predict in these cases, as I did in regard to the first twin mentioned above (Diamond, 1976, 1978, 1979) that, despite being reared as girls, they will be gynephilic. The postnatal removal of penis and testis, in a human, and imposition of a female rearing has never proven sufficient to overcome the inherent bias of a normal male nervous system.
Cross-Cultural studies
The work by Herdt (1981) and Stoller and Herdt (1985) report examples of environmental forces having little success in inducing long term homosexual activity. These researchers document a New Guinea culture 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 semen. Moreover, female bodies are presented as unattractive, to be avoided and poisonous. Nevertheless, these boys, upon reaching adulthood, choose females as regular sexual partners, and are almost always heterosexual. Neither youths nor men report impulses to suck penises nor engage in anal intercourse.
Schiefanhovel (1990) reports on a similar New Guinea culture. Here anal intercourse is used 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.
A third type of cross-cultural evidence supports an inherent sexual orientation independent of upbringing. Reports by Imperato-McGinley and colleagues (Imperato-McGinley and Peterson, 1976; Imperato-McGinley et al., 1979) describe a population in which, due to a now understood genetic enzyme defect, males were born without penises. They were assumed to be girls and raised accordingly. At puberty, when the enzyme began to appear, the individuals started to develop penises and respond to their own endogenous male hormones. Then, despite their rearing, the boys assumed male identities and patterns. Since these original reports many subsequent similar situations have been reported. Occasionally these studies have been criticized (e.g. Gooren et al., 1990) by saying this phenomenon had been understood by the natives in the population and the children reared appropriately from birth. Indeed that is now true. However, prior to the 1950s this was not so. In those earlier days the boys were reared as girls and then, with varying degrees of ease, switched to live as males. The prepubertal environmental forces failed to imprint feminine development.
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 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 nor accepted an imposed sexual orientation.
Population data
There are societies in which homosexuality is not only illegal but subject to the death penalty (e.g. Iran). There are also societies in which the practice is tolerated or considered of no concern. And as we saw above, there are groups among which same-sex activities are encouraged as part of growing up. With these differences in mind it might be instructive to consider if the prevalence of homosexual activity is correlated with some environmental factor we might call “social tolerance”. These data below are more fully reported in Diamond (1993a).
United States of America
Data from eight studies in the United States between the years 1970 and 1991 indicate that approximately 4.8 percent of the adult male population engage in same-sex behavior (Table I). In comparison with these surveys, a 1984 survey found 9.9% of San Franciscans identified themselves as homosexual or bisexual (Schreiner, 1986). It might be hypothesized that the population of the city, reputed to have the largest and most powerful gay, lesbian and bisexual communities in the United States, would have a dramatically different proportion of individuals engaging in same sex activities. This, however, is probably a function of the city attracting homosexually oriented individuals rather than “producing” them.
| Table I: Reports of United States Population Surveys. | |
| STUDY | MALE HM. BEHAVIOR |
| Davis and Smith (1988) | 2.4 |
| Diamond, Ohye and Wells (1993) | 3.0 |
| Dixon et al. (1991) | 7.3 |
| Fey et al. (1989) | 6.7 |
| Michael et al. (1988) | 3.2 |
| Rogers and Turner (1991) | 6.1 |
| Schreiner (1986) | 4.0/9.9* |
| Smith (1991) | 5.6 |
| Mean | 4.8 |
| * This 9.9% figure is for the San Francisco area (see Diamond, 1993a for details) | |
Asia
Four national studies of Japanese students, conducted from 1974 to 1987, found that same-sex contact, which may include bisexual contact, was reported by an average of 5.8 % of the male sample and 4.0 % of the female respondents. In contemporary Japan there is a strong stigma against homosexuality.
Information from a total Philippine population is available. Anthropologist Donn Hart (1968) lived for years in the village of Caticugari on the island of Negros. In this village of 729 persons, Hart found six male homosexuals and no lesbians (<2%). In the province of Siaton, which Hart also studied, with a population of 2,862, there were 12 male homosexuals and “several” lesbians (<2%).
Hart (1968) writes “it is believed that the number of covert homosexuals is very small … the majority of residents of Siaton province and Caticugan are both lenient and indulgent of the local bayot [male homosexuals] and lakin-on [lesbians]”. Others too have reported on the tolerance of the Filipino people toward homosexual behaviors (Guthrie and Jacobs, 1966; Whitam and Mathy, 1986).
Research from Thailand by Sittitrai et al. (1992) reports same-sex contact by 3.6 % of the males interviewed but only 0.4% consider themselves homosexuals (K 4-6). Only 1.3 % of females had same-sex contact and 1.0% consider themselves as homosexual (K 4-6).
A 10 percent random sample of the total population from the Republic of Pilau asked of sexual behavior for the preceding 12 months. After the age of 20, exclusive homosexual activity was reported by 1.9% of the males and 2.8% of the females interviewed Bisexual activity was reported by 2.8 % of the males and 0.7 % of the females. (Morens and Polloi, personal communication). Pilau too is relatively tolerant of same-sex expression.
Table II: National population surveys of same-sex activity. |
|
STUDY |
MALE |
Great Britain |
5.0-9.0 |
Japan |
5.8 |
Netherlands |
7.8 |
Philippines* |
2.0 |
Pilau* |
4.7 |
Thailand* |
3.6 |
United States |
4.8 |
Mean |
4.8-5.4 |
* = the 3 most relatively tolerant of homosexuality. (see Diamond, 1993a for details) |
|
Europe and Great Britain
Research cited by Schover and Jensen (1988) for a randomly selected population of Danish women found only 2 of 625 women from 22 to 70 years of age reporting having had a homosexual experience.
A British national study (Wellings et al., 1990) found 9% of men and 4% of women having had homosexual experience. Only 5% of men and 1% of women reported ever having a homosexual partner (Table II).
A 1991 study from the Netherlands (van Zessen and Sandfort, 1992) found only about 13 in 100 males admitted to ever having a homosexual experience and only about 3.3% would consider themselves homosexual (K=5-6). Among females, only 10 in 100 reported ever having had homosexual activity. Only some 0.4% of the women considered themselves homosexual.
While we have no data on the incidence of homosexuality in strongly repressive societies, the findings we do have seem in the opposite direction from that which might be hypothesized on the basis of social tolerance. In the non-homophobic societies we find reported among the lowest rates of same sex activity. Also, while lesbianism can be said to carry less stigma than male homosexuality, considering world wide research, almost all studies have found the incidence of male homosexual activity exceeds that of females approximately by a factor of 26.
Bisexuality
Directing attention now specifically to studies of self identified male homosexual populations we gain relevant information about bisexuality which will be germane to later discussion.
Study of homosexual males in the United States, the Netherlands and Denmark by Weinberg and Williams (1974) showed about one in five American homosexuals were bisexually active as K=2, 3 or 4 and about one in ten Dutch or Danish gay men were similarly active. More recent studies by Bell and Weinberg (1978), McWhirter and Mattison (1984) and Higa (1988) find about three or more out of four claim to be exclusively homosexual and fewer than one in ten claim they ever had more than incidental sex with a female. Thus, considering Kinsey’s 7 point scale, distribution for same-sex or opposite-sex activity is definitely bimodal (see Diamond, 1993b, for details).
Genetic contributions
The literature on pedigrees and twin studies is large and a significant portion of it pertains to aspects of sexuality. This discussion will emphasize work related to sexual orientation. Details are available in Whitam et al. (1993).
Non twins
Pillard and colleagues studied families having a known heterosexual or homosexual adult (Pillard et al., 1982; Pillard and Weinrich, 1986; Weinrich, 1987). They found that among brothers, if a family contained one index son who was homosexual, between 20 and 25 per cent of the brothers would also be. If the index brother were heterosexual, the chance of other brothers being homosexual was only about 4 to 6 per cent.
Twins
Kallmann, in well known studies (1952a, b), reported nearly 100% concordance for homosexual orientation in monozygotic (MZ) and about 10% concordance in dizygotic (DZ) twins. Following these reports, however, MZ twins discordant for homosexual orientation were soon reported by many investigators (see Puterbaugh, 1990 for review).
Significantly, in 1991 Bailey and Pillard found a 52% concordance rate for homosexual orientation in MZ male pairs and a 22% concordance rate for DZ male pairs. King and McDonald in 1992 reported a 25% concordance rate among monozygotic twins and a 12% concordance rate among dizygotic twins, and Whitam, Diamond and Martin (1993) reported finding a 65% concordance among MZ male twins and 30% concordance among DZ male twins.
These observations of the last decade do not find the rates of concordance found by Kallmann (1952a, b) but taken together are significantly high that a strong genetic contribution for homosexuality can no longer seriously be contested (Table III).
| Table III: Concordance for male homosexuality among twins (* Percentages). | |||
| STUDY | N.PAIRS |
MONOZYGT* |
DIZYGOT* |
| Kallmann | 40/45 |
100 |
10 |
| Schiegal | 113 |
95 |
5 |
| Heston and Shields | 717 |
71 |
14 |
| Bailey and Pillard | 56/54 |
52 |
22 |
| King and McDonald | 20/25 |
25 |
13 |
| Whitam, Diamond and Martin | 34/27 |
65 |
30 |
| TOTALS | 68 |
16 |
|
Among our MZ male twins, 29.4% were discordant for sexual orientation; the most common pattern was the index twin K-6 and his brother K-0 (Whitam et al., 1993). These bimodal findings are consistent with those of Bailey and Pillard (1991).
The concordance rate for female MZ twins for homosexual orientation found was 75% (Whitam et al., 1993). Our sample size for female twins is quite small and a larger number might well produce a different rate of concordance. However, these findings question the notion that male homosexuality may be biologically derived while female homosexuality is learned (Eckert et al., 1986)
There are two types of male DZ twins: those whose co-twin is male and those whose co-twin is female. The concordance rate for homosexuality in the former is 28.6% and in the latter 33.3%. The combined concordance rate for male DZ twins with either a homosexual brother or sister is 30.4% (Whitam et al., 1993)
Triplets
Three sets of triplets appeared in our sample (Whitam et al., 1993). One set consisted of three females; a pair of MZ twins who are both lesbian and a third heterosexual sister. A second pair of triplets consisted of a MZ male pair, both homosexual, with a heterosexual sister. A third MZ male triplet set all reported same-sex orientation.
Other genetic considerations
Two more items to consider will be mentioned. Using data from different types of studies, they infer a biological component to homosexuality. Blanchard and Sheridan (1992) reported that:
“homosexual men had significantly more siblings than … homosexual women who in turn, had significantly more siblings than … nonhomosexual men. The sibling sex ratio of the homosexual men, 131 brothers per 100 sisters, was significantly higher than the sex ratio of live births for the population as a whole … [also] the homosexual men had a significantly later birth order than the nonhomosexual men”.
The second item for consideration stems from the recent report of Turner (1992). He finds, after studying family genealogies, that,
“in the absence of alcoholism … [homosexuality] may depend upon a gene in the pseudoautosomal region of the X and Y chromosomes”.
If alcoholism is present he believes homosexuality to have a different genetic origin. He considers these findings the result of differential fetal wastage. This can also account for the Blanchard and Sheridan findings.
Lastly, many critiques of the biological nature of homosexuality question why the phenomenon persists since there is no obvious evolutionary benefit to it. This has been answered in many ways (see e.g. Hutchinson, 1959; Kirsch, 1982; Trivers, 1974; Wilson, 1975). While interesting to consider, anything said here is speculative.
Obviously any genetic patterning is complex because most parents of homosexuals are heterosexual and rear their children accordingly. The concept of pleiotropy must obviously be considered with recognition that traits selected for by a mating couple are typically independent of the sexual orientation of future children. And if genetics are involved, why are not all MZ twins concordant?
In ways we do not yet understand, not all sibs of homosexual index individuals show homosexuality even among monozygotic twins. There is obviously more than one set of genes involved. And they apparently interact with genes of other traits and social forces to organize how the final behaviors will be manifest. And why, among MZ twin brothers that are not concordant for homosexuality, do only a minority show bisexual behavior and are exclusively heterosexual instead? Intuitively one might expect a higher ratio of bisexuals to heterosexuals. Sexual orientation is obviously manifest bimodally. Indeed, it may be that bisexuality is related to homosexuality and heterosexuality but quite different in its developmental pattern.
Discussion and conclusions
The material presented in this chapter demonstrates that both the environment and genetic programming interact in the development of sexual orientation, and for different species, the interaction occurs in different ways. Only some fish among all those with a certain genetic pool, for instance, change sex when the environment clues dictate. In humans, while certain twins have the genetic predisposition to manifest homosexual orientation, only certain individuals seem subject to unique, yet unknown, stimuli to show this phenotype.
Sexual orientation may follow patterns of other behaviors assumed to have a strong genetic component and yet not display 100% concordance in MZ pairs. Kaij (1960) found, for example, the rate of concordance for alcoholism to be 54% in MZ pairs and 24% for DZ pairs. Nagylaki and Levy (1973) found MZ twins have more reversed asymmetries than DZ pairs; there is a larger proportion of discordance of handedness in MZ pairs. Bouchard et al. (1990) found many traits fixed by heredity but not all twins showed these traits equally.
We have not seen sexual orientation noticeably influenced by events typically thought of as environmentally significant in development (e.g. role modeling, reinforcement, etc.). Other types of events might, nevertheless, be of consequence. For instance in utero chorion conditions may effect the nervous system. Melnick et al. (1978) 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. And certainly sexual orientation may be biologically determined in utero by biochemical mechanisms - under genetic mediation - which remain to be identified. And certainly the social-sexual environment in which discordant twins develop, rather than being alike, may actually be quite different. Even still-attached Siamese twins can have different personalities.
As proposed more than 25 years ago (Diamond, 1965) and many times since, sexual orientation is most probably the result of interacting inherent biological forces meshing with environmental pressures. The biology sets a predisposition, a bias, with which the individual interacts with his or her surround (Diamond 1965, 1968, 1976, 1978, 1979, 1980, 1982, 1992, 1993b; Diamond and Karlen, 1980; Whitam and Mathy, 1986). And particular stages or “critical periods” in development seem more significant than others in organizing these behaviors.
A last word. Legal restrictions and social taboos or other motives can move an individual to exhibit behaviors and form sexual and emotional attachments to partners he or she would not otherwise desire, or refrain from relationships that would be preferred. But given free opportunity, an individual’s true choice will be manifest (Diamond. 1978; Diamond, 1979). Our biological heritage has given most humans the flexibility to adjust. The more environmental freedom, the more genetics will be allowed to express itself.
For some persons the idea that sexual orientation is biologically biased is threatening. Some have expressed consternation that if the developmental biological forces for sexual orientation are known (or even suspected), governmental, religious, medical or social agencies might use the knowledge to force conformity to a dictated ideal or otherwise modify a potential homosexual outcome. Unfortunately, groups who have the end of homosexuality or ambisexuality as their goal, I fear need no scientific justification for their malignant ends. They are sufficient in their own ignorance and prejudice to further their aims. Indeed, the truth is more likely than not to benefit us all.
Acknowledgements
Thanks to Ronald Johnson, University of Hawai‘i, for his assistance and access to his behavior genetics notes.