Homosexuality in monozygotic twins reared apart

The methodological and ethical concerns of genetic studies of same-sex sexual behavior

Same-sex sexual behavior has long interested genetics researchers in part because, while there is evidence of heritability, the trait as typically defined is associated with fewer offspring. Investigations of this phenomenon began in the 1990s with linkage studies and continue today with the advent of genome-wide association studies. As this body of research grows, so does critical scientific and ethical review of it. Here, we provide a targeted overview of existing genetics studies on same-sex sexual behavior, highlight the ethical and scientific considerations of this nascent field, and provide recommendations developed by the authors to enhance social and ethical responsibility.

Probing the genomic landscape of human sexuality: a critical systematic review of the literature

Whether human sexuality is the result of nature or nurture (or their complex interplay) represents a hot, often ideologically driven, and highly polarized debate with political and social ramifications, and with varying, conflicting findings reported in the literature. A number of heritability and behavioral genetics studies, including pedigree-based investigations, have hypothesized inheritance patterns of human sexual behaviors. On the other hand, in most twin, adoption, and nuclear family studies, it was not possible to disentangle between underlying genetic and shared environmental sources. Furthermore, these studies were not able to estimate the precise extent of genetic loading and to shed light both on the number and nature of the putative inherited factors, which remained largely unknown. Molecular genetic studies offer an unprecedented opportunity to overcome these drawbacks, by dissecting the molecular basis of human sexuality and allowing a better understanding of its biological roots if any. However, there exists no systematic review of the molecular genetics of human sexuality. Therefore, we undertook this critical systematic review and appraisal of the literature, with the ambitious aims of filling in these gaps of knowledge, especially from the methodological standpoint, and providing guidance to future studies. Sixteen studies were finally retained and overviewed in the present systematic review study. Seven studies were linkage studies, four studies utilized the candidate gene approach, and five studies were GWAS investigations. Limitations of these studies and implications for further research are discussed.

Sexual partner preference in animals and humans

Sex differences in brain and behavior of animals including humans result from an interaction between biological and environmental influences. This is also true for the differences between men and women concerning sexual orientation. Sexual differentiation is mediated by three groups of biological mechanisms: early actions of sex steroids, more direct actions of sex-specific genes not mediated by gonadal sex steroids and epigenetic mechanisms. Differential interactions with parents and conspecifics have additionally long-term influences on behavior. This presentation reviews available evidence indicating that these different mechanisms play a significant role in the control of sexual partner preference in animals and humans, in other words the homosexual versus heterosexual orientation. Clinical and epidemiological studies of phenotypically selected populations indicate that early actions of hormones and genetic factors clearly contribute to the determination of sexual orientation. The maternal embryonic environment also modifies the incidence of male homosexuality via immunological mechanisms. The relative contribution of each of these mechanisms remains however to be determined.

Heterosexual Male Carriers Could Explain Persistence of Homosexuality in Men: Individual-Based Simulations of an X-Linked Inheritance Model

Homosexuality has been documented throughout history and is found in almost all human cultures. Twin studies suggest that homosexuality is to some extent heritable. However, from an evolutionary perspective, this poses a problem: Male homosexuals tend to have on average five times fewer children than heterosexual males, so how can a phenomenon associated with low reproductive success be maintained at relatively stable frequencies? Recent findings of increased maternal fecundity of male homosexuals suggest that the genes responsible for homosexuality in males increase fecundity in the females who carry them. Can an increase in maternal fecundity compensate for the fecundity reduction in homosexual men and produce a stable polymorphism? In the current study, this problem was addressed with an individual-based modeling (IBM) approach. IBM suggests that male homosexuality can be maintained in a population at low and stable frequencies if roughly more than half of the females and half of the males are carriers of genes that predispose the male to homosexuality.

An Ethological Perspective on Homosexuality during Adolescence

An ethological perspective is presented as an alternative to traditional developmental approaches to adolescence. From an evolutionary point of view, the onset of reproductive capability during adolescence marks the significance of this time during the life course. Thus, the apparent anomaly of male and female youth "preferring" not to engage in heterosexual activities that have the potential to maximize genetic fitness but to participate in homosexual activities that do not have the same adaptive repercussion poses a challenge for an ethological perspective to interpret. To better understand an ethological perspective, basic ethological principles are applied to the emergence and expression of homosexuality during adolescence. First, evidence bearing on the genetic etiological under-pinnings of homosexuality within humans is reviewed through an examination of (a) behavioral genetic studies and (b) hormonal research. Next, adaptive considerations are presented, in particular the prevalence of homosexual behavior among various animal species and human societies, as well as evolutionary pressures that maintain a stable level of homosexuality in the human population. Balanced polymorphism and kin selection are reviewed as answers to the ultimate or functional question. Finally, the "naturalness" of adolescent homosexuality as an outcome of an ethological perspective is proposed. Homosexuality is thus presented as a sexual orientation that is not so much a matter of choice but of biological necessity.

Sexual Orientation, Controversy, and Science

Summary

Ongoing political controversies around the world exemplify a long-standing and widespread preoccupation with the acceptability of homosexuality. Nonheterosexual people have seen dramatic surges both in their rights and in positive public opinion in many Western countries. In contrast, in much of Africa, the Middle East, the Caribbean, Oceania, and parts of Asia, homosexual behavior remains illegal and severely punishable, with some countries retaining the death penalty for it. Political controversies about sexual orientation have often overlapped with scientific controversies. That is, participants on both sides of the sociopolitical debates have tended to believe that scientific findings—and scientific truths—about sexual orientation matter a great deal in making political decisions. The most contentious scientific issues have concerned the causes of sexual orientation—that is, why are some people heterosexual, others bisexual, and others homosexual? The actual relevance of these issues to social, political, and ethical decisions is often poorly justified, however.

Human homosexuality: a paradigmatic arena for sexually antagonistic selection?

Sexual conflict likely plays a crucial role in the origin and maintenance of homosexuality in our species. Although environmental factors are known to affect human homosexual (HS) preference, sibling concordances and population patterns related to HS indicate that genetic components are also influencing this trait in humans. We argue that multilocus, partially X-linked genetic factors undergoing sexually antagonistic selection that promote maternal female fecundity at the cost of occasional male offspring homosexuality are the best candidates capable of explaining the frequency, familial clustering, and pedigree asymmetries observed in HS male proband families. This establishes male HS as a paradigmatic example of sexual conflict in human biology. HS in females, on the other hand, is currently a more elusive phenomenon from both the empirical and theoretical standpoints because of its fluidity and marked environmental influence. Genetic and epigenetic mechanisms, the latter involving sexually antagonistic components, have been hypothesized for the propagation and maintenance of female HS in the population. However, further data are needed to truly clarify the evolutionary dynamics of this trait.

An association between male homosexuality and reproductive success

The existence of homosexuality in humans poses a problem for evolutionary theory. Exclusive male homosexuality has a catastrophic effect on reproduction and yet inherited factors appear to contribute to it. Previous attempts to resolve this conundrum are inconsistent with aspects of evolutionary theory. Additional limitations are as follows. Until recently, accounts of homosexuality have paid little attention to the probable existence of adaptive bisexuality in ancestral populations, from which further variations in sexual orientation may have evolved. Secondly, previous explanations have concentrated on the ancestral environment of two to three million years ago as the determinant of modern sexuality, when more recent influences are likely to have had considerable impact. I argue in favour of a longitudinal rather than cross-sectional model of the ancestral environment. Thirdly, they have often ignored the possibility of variable phenotypic expression, whereby those individuals with a genetic propensity for homosexuality exhibit different and adaptive qualities on most other occasions. It has been demonstrated in previous studies that homosexual men have superior linguistic skills compared to heterosexual men. This may be the result of an adaptive feminising effect on the male brain and apply to many practising heterosexuals. Other adaptations to the recent ancestral environment may include enhanced empathy, fine motor skills and impulse control. By drawing together these contributing factors an evolutionary basis for homosexuality can be demonstrated.

Twin studies in behavioral research: a skeptical view

We review in detail two major ongoing research projects that employ samples of twins reared apart (and in one case, twins reared together). The studies attempt, via model fitting, to estimate proportions of genetic and environmental variance for many human traits. We discuss problems concerning the representativeness of samples, the accuracy and reliability of the data, the extent of contact of nominally separated twins, the measurement of selective placement effects, and the particular model-fitting procedures. The two studies agree in their conclusions, but we do not find the conclusions to be convincing. We suggest that no scientific purpose is served by the flood of heritability estimates generated by these studies.

The disparate maternal aunt-uncle ratio in male transsexuals: an explanation invoking genomic imprinting

A significant skewing in the sex ratio in favour of females has been reported for the families of homosexual men such that there are fewer maternal uncles than aunts. This finding is repeated for a large series of transsexual families in this study. Four hundred and seventeen male-to-female transsexuals and 96 female-to-male transsexuals were assessed. Male-to-female transsexuals have a significant excess of maternal aunts vs. uncles. No differences from the expected parity were found for female-to-male transsexuals or on the paternal side. A posited explanation for these findings invokes X inactivation and genes on the X chromosome that escape inactivation but may be imprinted. Our hypothesis incorporates the known familial traits in the families of homosexuals and transsexuals by way of retention of the grand parental epigenotype on the X chromosome. Generation one would be characterized by a failure to erase the paternal imprints on the paternal X chromosome. Daughters of this second generation would produce sons that are XpY and XmY. Since XpY expresses Xist, the X chromosome is silenced and half of the sons are lost at the earliest stages of pregnancy because of the normal requirement for paternal X expression in extra-embryonic tissues. Females survive by virtue of inheriting two X chromosomes, and therefore the possibility of X chromosome counting and choice during embryonic development. In generation three, sons inheriting the paternal X after its second passage through the female germline survive, but half would inherit the feminizing Xp imprinted genes. These genes could pre-dispose the sons to feminization and subsequent development of either homosexuality or transsexualism.

British psychiatry and homosexuality

BACKGROUND

Opposition to homosexuality in Europe reached a crescendo in the 19th century. What had earlier been regarded as a vice evolved as a perversion or psychological illness. Official reviews of homosexuality as both an illness and (for men) a crime led to discrimination, inhumane treatments and shame, guilt and fear for gay men and lesbians. Only recently has homosexuality been removed from all international diagnostic glossaries.

AIMS

To review how British psychiatry has regarded homosexuality over the past century.

METHOD

Review of key publications on homosexuality in British psychiatry.

RESULTS

The literature on homosexuality reflects evolving theories on sexuality over the past century. The assumptions in psychoanalysis and the behavioural sciences that sexuality could be altered led to unscientific theory and practice.

CONCLUSIONS

Mental health professionals in Britain should be aware of the mistakes of the past. Only in that way can we prevent future excesses and heal the gulf between gay and lesbian patients and their psychiatrists.

Biology and the future of psychoanalysis: a new intellectual framework for psychiatry revisited

The American Journal of Psychiatry has received a number of letters in response to my earlier "Framework" article (1). Some of these are reprinted elsewhere in this issue, and I have answered them briefly there. However, one issue raised by some letters deserves a more detailed answer, and that relates to whether biology is at all relevant to psychoanalysis. To my mind, this issue is so central to the future of psychoanalysis that it cannot be addressed with a brief comment. I therefore have written this article in an attempt to outline the importance of biology for the future of psychoanalysis.

Health and development of gay and lesbian youths: implications for HIV/AIDS

Health and behavioral issues of gay and lesbian adolescents have recently become a focus of research and interest. A well conceived framework within which to consider thoughtfully the uniqueness of problems faced by homosexual youths and the role of health-care providers is needed. The prevalence, sociocultural history, and theories of origin of the homosexual orientation (biologic, psychoanalytic, and social processes) as well as general issues in adolescent development (biologic, cognitive, and emotional) all contribute to the development of a comprehensive perspective through which better health care and education can be provided. Responsible sexual behavior, depression, and suicide are health-care issues that physicians must be sensitive to and address openly. Up to 50% of gay youths have "seriously contemplated," suicide and 25% are estimated to have attempted suicide, according to the literature. In one study, up to 38% of pediatricians were uncomfortable in caring for homosexual adolescents within their practice. Practical suggestions include reviewing the language in office information forms and brochures providing appropriate literature that demonstrates acceptance of homosexual and bisexual orientations, and avoiding heterosexist bias in questions inquiring about sexuality. Physicians have an opportunity to modify the health and psychosocial risks faced by gay and lesbian youths by restructuring professional settings and accepting broader responsibilities for raising community awareness.

Errors by the American Psychiatric Association, the American Psychological Association, and the National Educational, Association in representing homosexuality in amicus briefs about Amendment 2 to the U.S. Supreme Court

In October 1995, consortiums of psychiatric and educational profes sional organizations, including the American Psychological Association and the American Psychiatric Association and the National Educational Association, submitted amicus briefs to the U.S. Supreme Court asserting that the scientific literature unequivocally supports the following propositions (a) that homosexuals, including homosexual teachers, do not disproportionately molest children, (b) that children of homosexual patients are not more likely to become homosexuals, (c) that professionals agree that homosexuality is not a pathology, and (d) that homosexual attractions are biologically or genetically predetermined and are therefore beyond the control of the individual. The first two contentions are inconsistent with the scientific literature, and the second two grossly oversimplify a contentious and uncertain literature.

Development and familiality of sexual orientation in females

The development and familial clustering of sexual orientation were studied in 358 heterosexual, bisexual, and homosexual women. Sexual orientation, as measured by the Kinsey scales, was diverse yet showed statistical congruity and stability over a 1- to 1.5-year time span. Developmental patterns, as measured by retrospective reports on the ages of first sexual or romantic attraction and of self-acknowledgement of sexual orientation were very similar in the heterosexual and lesbian subjects except for the difference in object choice. The bisexual subjects displayed intermediate patterns that were more similar to the heterosexuals' on most facets yet closer to the lesbian subjects' on other dimensions. Familial clustering of nonheterosexual orientation was significant. Using two criteria, elevated rates of nonheterosexuality were found in four classes of relatives: sisters, daughters, nieces, and female cousins through a paternal uncle. The current data are not sufficient to distinguish between genetic and shared environmental sources of this familial aggregation. We discuss the possibility of using developmental criteria to differentiate between inherited and cultural sources of variation in female sexual orientation.

The sexoanalytical treatment of ego-dystonic homosexuality

After clarifying some definitions, the author attempts to identify homosexuality through the sexual individuation process and to trace its various ontogenetic inconsistencies. Two clinical case studies are provided to further illustrate the sexoanalytic treatment of ego-dystonic homosexuality.

Role of genetic factors in human sexual behavior based on studies of Tourette syndrome and ADHD probands and their relatives

Most significant variations in the expression of human sexuality are considered to be the result of learned behavior or psychological problems. Tourette syndrome (TS) is a common, hereditary tic and disinhibition disorder sometimes associated with compulsive use of obscene words (coprolalia) and previously reported to be occasionally associated with exhibitionism. To further explore the relationship between the Gts genes and sexual behavior, questions concerning a wide range of such behaviors were administered to 1,040 subjects, 14 years of age or older, consisting of 358 TS probands, 101 non-proband relatives with TS, 359 non-TS first degree relatives, 79 attention deficit hyperactivity disorder (ADHD) probands, 70 unaffected relatives of the ADHD probands, and 73 controls. The behaviors included magnitude of sex drive, sex orientation, exhibitionism, transvestitism, transsexualism, sadism, masochism, pedophilia, fetishism, aversion to being touched, and aversion to sex. While most of these behaviors occurred in a distinct minority of TS subjects, there was a significant positive correlation between each behavior examined and the degree of genetic loading for the Gts gene(s). The nature of these behaviors and their association with TS suggests many are variants of obsessive-compulsive disorder. Studies in animals indicate that changes in serotonin and dopamine play a significant role in the sexual behavior and many lines of evidence are consistent with the hypothesis that TS is due to genetic changes in serotonin and dopamine metabolism. These studies suggest that genetic factors play a much greater role in a wide range of forms of sexual expression than previously thought.

Homosexual orientation in twins: a report on 61 pairs and three triplet sets

Twin pairs in which at least one twin is homosexual were solicited through announcements in the gay press and personal referrals from 1980 to the present. An 18-page questionnaire on the "sexuality of twins" was filled out by one or both twins. Thirty-eight pairs of monozygotic twins (34 male pairs and 4 female pairs) were found to have a concordance rate of 65.8% for homosexual orientation. Twenty-three pairs of dizygotic twins were found to have a concordance rate of 30.4% for homosexual orientation. In addition, three sets of triplets were obtained. Two sets contained a pair of monozygotic twins concordant for sexual orientation with the third triplet dizygotic and discordant for homosexual orientation. A third triplet set was monozygotic with all three concordant for homosexual orientation. These findings are interpreted as supporting the argument for a biological basis in sexual orientation.

Psychoanalysis, psychobiology, and homosexuality

The potential role of biological influences in human sexual orientation was considered more seriously during the early phases of psychoanalysis than in the years since World War II. Recently studies of homosexuality and heterosexuality in the neurosciences have attracted widespread attention both in the scientific and lay communities. The salience of these new data for psychoanalytic theory and practice is just beginning to be explored. In this article, we review research on sexual orientation in the following areas: genetics, crosscultural studies, studies of development in individuals with abnormal prenatal hormone exposure, childhood play patterns, and brain studies in both nonhumans and humans. Differences between male and female homosexuality are explored. We propose that psychoanalytic theory can grow and profit from a careful consideration of new findings in the psychobiology of sexuality, and that the interaction between mind and body is the appropriate purview of psychoanalysis.

Sperm competition and the persistence of genes for male homosexuality

Homosexuality is increasingly recognized as having a genetic component. Why then does it persist, when common sense suggests that it should result in fewer offspring? Monozygotic-twin studies permit a rough estimate of the importance of genetics (70%) in the development of male homosexuality, and the proportion of homosexuals remains constant: Fisher's Theorem then tells us there is an advantage to the heterozygote, which we find need be no greater than 2%. Behavior and sperm competition suggest what this advantage might be.

Effect of reading a summary of research about biological bases of homosexual orientation on attitudes toward homosexuals

The effect of exposure to information regarding the development of homosexual orientation on attitudes toward homosexuals was investigated. Testing was conducted in five regularly scheduled undergraduate classes. 105 volunteer subjects from college classes were exposed to one of three treatment conditions. Subjects in the experimental group read a summary article of current research emphasizing a biological component of homosexual orientation. Subjects in one control group read a summary article of research focusing on the absence of hormonal differences between homosexual and heterosexual men. Subjects in another control group were not exposed to either article. All subjects completed the Index of Attitudes Toward Homosexuals. As predicted, subjects in the experimental group had significantly lower scores than subjects in the control groups. There was a significant interaction between treatment condition and sex of subject. Scores on the index were significantly lower for women, but only in the experimental group. These results show that exposure to a research summary can significantly affect immediately assessed scores on the Index of Attitudes Toward Homosexuals. Also, the pattern of effects of these experiences was different for women and men.

The genetic analysis of human behavior: a new era?

Recent developments in DNA-based techniques may revolutionize the study of human behavioral genetics. However, unless these methods are used with great care, many of the same mistakes which have plagued non-molecular genetic analyses of behavior will reoccur. Errors in the application of genetic approaches and in the interpretation of results have been a common feature of published studies in this field. We review studies in human behavioral genetics, focusing on those using identical twins and DNA-based linkage techniques in order to draw attention to recurrent problems in molecular and non-molecular studies. We suggest possible guidelines for future research in the area of the biological basis of human behavior.

Handedness, dyslexia and twinning in homosexual men

A study of handedness, dyslexia, stuttering and twinning, was included in a study of sexual habits of homosexual men. A questionnaire was mailed to homosexuals, and 394 forms suitable for data analysis were received. The results showed an increased rate of lefthand writing (17.5% compared to 8-8.4%), and a clear left shift. There were increased occurrence of both stuttering (7.1% compared to 1.6%) and reading difficulties (7.9% compared to 1-3%). The incidence of twins was lower than the population (1.3%). The results confirm earlier attempts to show a left shift in homosexuals, and support Geschwind's hypotheses about etiological factors for both lefthandedness and homosexuality.

Homosexuals who are twins. A study of 46 probands

Forty-six homosexual men and women who were twins took part in a study of their sexuality and that of their co-twin. Discordance for sexual orientation in the monozygotic pairs confirmed that genetic factors are insufficient explanation of the development of sexual orientation. There was a high level of shared knowledge of sexual orientation between members of twin pairs, and a relatively high likelihood of sexual relations occurring with same sex co-twins at some time, particularly in monozygotic pairs. The implications of these results for the study of the origins of sexual orientation and for twin research are discussed.

Sexual orientation, sexual identity, and sex-dimorphic behaviors in male twins

Sexual orientation, sexual identity, and sex-dimorphic behaviors were assessed concurrently and retrospectively, for childhood, in 95 pairs of male monozygotic (MZ) twins and 63 pairs of dizygotic (DZ) twins. There was a significantly higher rate of adult homosexuality among the MZ than among DZ twins. We employed a model-fitting approach using LISREL to test for genetic and environmental influences on variation for each trait singly and on the covariation among all six traits (three for childhood and three for adulthood). Univariate analyses confirmed the presence of familial factors for five of the six variables but were generally unable to distinguish shared environmental from genetic influences. Hierarchical tests of multivariate models supported the existence of an additive genetic factor contributing to the covariance among the variables. More restrictive multivariate models yielded a significant genetic influence on sexual orientation. Because of the different rates of orientation by zygosity and because of the restrictive nature of some of the multivariate models, our results are best considered tentative but do suggest that further biometrically oriented studies of sexual orientation and its correlates would be worthwhile.

Fundamental issues in the care of homosexual youth

The development of a homosexual identity spans the entire life cycle, but adolescence is a critical period in the lives of gay and lesbian persons. Various pre- and postnatal biologic and environmental factors are thought to determine sexual orientation early in life. An emerging homosexual identity may be reflected in youths' sexual attractions, fantasies, and cultural affiliations, as well as their behaviors. The adolescent experiences of homosexual persons profoundly affect health outcomes. The goals of care are to promote normal adolescent development, social and emotional well-being, and physical health. Comprehensive and coordinated educational, mental health, social, and medical services are recommended.

Theoretical perspectives accounting for adolescent homosexuality

Few topics in sexology elicit such a diversity of opinions and emotions as the question of etiology of homosexuality. Views frequently carry with them implicit or explicit messages concerning the psychologic health of this sexual orientation. Theories of sexual development usually portray adolescence as a critical time in the life course because of changes in: 1) anatomy and physiology; 2) psychologic functioning: the reawakening, renewal, and reliving of previously established sexual relations and drives; and/or 3) social conditions: an increased exposure and adherence to societal messages concerning appropriate and inappropriate social and sexual behaviors and relationships. This paper provides a brief overview of several major theories--evolutionary biology, psychoanalysis, and social processes--as they relate to the development of sexual orientation. In addition, an ethologic perspective that synthesizes various etiologic theories, as they relate to homosexuality during adolescence, is briefly reviewed. In these discussions, the issue of whether homosexuality is a normal or abnormal developmental state during adolescence is also addressed.