Common Genetic Factors among Sexual Orientation, Gender Nonconformity, and Number of Sex Partners in Female Twins: Implications for the Evolution of Homosexuality

Occupational Preferences, Childhood Behavior, and Openness: The Role of Sex, Sexual Orientation, and Gender Identity in Iran

Previous research suggests that both same-sex attraction and the personality trait "openness" are associated with sex-atypical preferences and behaviors. Here, we examined the links between adulthood occupational preferences, childhood play behavior, and openness among Iranian cisgender gynephilic males (n = 228), cisgender ambiphilic males (n = 48), cisgender androphilic males (n = 178), transgender androphilic males (n = 58), cisgender androphilic females (n = 226), cisgender ambiphilic females (n = 94), cisgender gynephilic females (n = 31), and transgender gynephilic females (n = 121) from Iran. Cisgender and transgender same-sex attracted males and females exhibited sex-atypical occupational preferences with the latter group showing even more sex-atypicality than the former. The personality trait openness did not differ between cisgender groups. Transgender androphilic males had a significantly higher mean score for openness compared to cisgender androphilic females and transgender gynephilic females, whereas transgender gynephilic females had a significantly lower mean score compared to cisgender androphilic males. In both males and females, childhood sex-atypicality, same-sex attraction, and openness were associated with sex-atypical occupational preferences. Our findings from Iran provides cross-cultural support for interconnectedness of childhood and adulthood sex-atypicality, openness, and same-sex attraction in males and females who are cisgender and transgender.

Self-Rated Masculinity and Femininity and Reported Number of Lifetime Sexual Partners Among Cisgender Heterosexual and LGBQ Men and Women

Prior research has quantitatively examined why heterosexual men and women report different numbers of lifetime different-sex sexual partners, whereas qualitative work has analyzed how men's masculinity-and to a lesser extent, women's femininity-is tied to gender norms about sexual activity. Less research, however, has quantitatively examined the associations between self-rated masculinity and femininity and reported number of lifetime sexual partners. This brief report uses a large sample of Canadians (n = 2117) to examine how self-rated masculinity and femininity relate to reported numbers of sexual partners among four groups of cisgender people: (1) heterosexual men (n = 972), (2) heterosexual women (n = 979), (3) gay and bisexual/pansexual men (n = 99), and (4) lesbian and bisexual/pansexual women (n = 67). Results demonstrate that self-rated femininity was negatively, and masculinity positively, associated with reported numbers of lifetime different-sex sexual partners among heterosexual women. No significant associations emerged for other groups. The lack of significant associations among heterosexual men may be attributable to the fact that most rated themselves as very masculine and not very feminine, whereas there was more variation among heterosexual women. In contrast, the non-significance among LGBQ women and men could reflect that subcultural norms and practices more strongly shape the number of sexual partners individuals report having in these communities. These results demonstrate that it would be beneficial for researchers to measure self-rated masculinity and femininity in future studies about sexual partnering practices, especially among heterosexual cisgender women.

Association of Recalled Childhood Sex-Typed Behavior with Sexual Orientation and Gender Identity in Iranian Adult Males and Females

Same-sex sexual attraction in both males and females has been associated with childhood sex-atypical behavior. Gynephilic females recall behaving in a manner that is less female-typical and more male-typical compared to cisgender androphilic females, whereas androphilic males recall behaving in a manner that is less male-typical and more female-typical compared to cisgender gynephilic males. In addition, male and female ambiphilic individuals exhibit intermediate levels of childhood sex-atypicality. In this study, we examined recalled childhood sex-typed behavior among Iranian cisgender gynephilic males (n = 236), cisgender ambiphilic males (n = 51), cisgender androphilic males (n = 191), transgender androphilic males (n = 60), cisgender androphilic females (n = 243), cisgender ambiphilic females (n = 96), cisgender gynephilic females (n = 32), and transgender gynephilic females (n = 122). Both cisgender androphilic males and cisgender gynephilic females recalled elevated childhood sex-atypicality, scoring intermediate between cisgender gynephilic males and cisgender androphilic females. Male and female ambiphilic participants scored intermediate between their other- and same-sex attracted cisgender counterparts. Transgender androphilic males exhibited hyper-feminized childhood behavior, scoring even more female-typical than cisgender androphilic females. Transgender gynephilic females recalled behaving as male-typical as cisgender gynephilic males. Consistent with previous research, our findings from Iran - a non-Western, Middle Eastern culture - provide cross-cultural support for the universality of childhood sex-atypicality as a normative developmental precursor of monosexual or ambisexual same-sex attraction in both males and females who are cisgender and transgender.

Genetic and Environmental Influences on Sexual Orientation: Moderation by Childhood Gender Nonconformity and Early-Life Adversity

Existing evidence indicates genetic and non-genetic influences on sexual orientation; however, the possibility of gene-environment interplay has not been previously formally tested despite theories indicating this. Using a Finnish twin cohort, this study investigated whether childhood gender nonconformity and early-life adversities independently moderated individual differences in sexual orientation and childhood gender nonconformity, the relationship between them, and the etiological bases of the proposed moderation effects. Sexual orientation, childhood gender nonconformity, and early-life adversities were assessed using standard questionnaires. Structural equation twin model fitting was carried out using OpenMx. Childhood gender nonconformity was significantly associated with reduced phenotypic variance in sexual orientation (β =  - 0.14, 95% CI - 0.27, - 0.01). A breakdown of the underlying influences of this moderation effect showed that this was mostly due to moderation of individual-specific environmental influences which significantly decreased as childhood gender nonconformity increased (βE =  - 0.38; 95% CI - 0.52, - 0.001) while additive genetic influences were not significantly moderated (βA = 0.05; 95% CI - 0.30, 0.27). We also observed that the relationship between sexual orientation and childhood gender nonconformity was stronger at higher levels of childhood gender nonconformity (β = 0.10, 95% CI 0.05, 0.14); however, significance of the underlying genetic and environmental influences on this relationship could not be established in this sample. The findings indicate that beyond a correlation of their genetic and individual-specific environmental influences, childhood gender nonconformity is further significantly associated with reduced individual-specific influences on sexual orientation.

Variations in gender identity and sexual orientation of university students

Background

Previous studies have shown that a small percentage of people in the general population have atypical gender identity and/or sexual orientation.

Aim

This study aimed to explore variations in gender identity and sexual orientation in university students and determine genetic factors associated with these variations.

Methods

Deviations from complete gender congruence and exclusive heterosexual orientation in 736 Japanese university students were quantitatively assessed with self-assessment questionnaires. Next, we conducted genetic tests for 80 participants who showed relatively low gender identity scores and/or atypical sexual orientation. These genetic tests consisted of repeat number analysis of the androgen receptor gene (AR) and a SKAT-O: an optimal unified sequence kernel association test, which is an exome-based rare variant association study. The results of the genetic tests were compared with the Japanese reference data and the results of our 637 control samples.

Outcomes

We calculated the gender identity and sexual orientation scores of all participants and analyzed the molecular data of 80 selected participants.

Results

The gender identity scores of 736 participants were broadly distributed: only ~15% of natal males and ~5% of natal females had the maximum score that corresponds to complete gender congruence. The sexual orientation scores also varied: ~80% of natal males and ~60% of natal females showed exclusive heterosexual orientation. We found no association between gender characteristics and AR repeat numbers. The SKAT-O showed that rare damaging variants of TDRP and 3 other genes were more common in the 80 participants than in the control group.

Clinical Implications

Our data support the view that gender is a phenotypic continuum rather than a binary trait.

Strength and Limitations

This study quantitatively assessed the gender characteristics of a large cohort of university students. Moreover, we conducted systematic screening for genetic factors associated with gender variations. The weaknesses of the study were the limited analytic power of the questionnaires, the relatively small sample for molecular analyses, and incomplete clinical information and relatively advanced ages of the control group.

Conclusion

This study revealed significant variations in gender identity and sexual orientation in university students, which may be partly associated with variants in TDRP or other genes.

Carving the Biodevelopment of Same-Sex Sexual Orientation at Its Joints

Sexual orientation is a core aspect of human experience and understanding its development is fundamental to psychology as a scientific discipline. Biological perspectives have played an important role in uncovering the processes that contribute to sexual orientation development. Research in this field has relied on a variety of populations, including community, clinical, and cross-cultural samples, and has commonly focused on female gynephilia (i.e., female sexual attraction to adult females) and male androphilia (i.e., male sexual attraction to adult males). Genetic, hormonal, and immunological processes all appear to influence sexual orientation. Consistent with biological perspectives, there are sexual orientation differences in brain development and evidence indicates that similar biological influences apply across cultures. An outstanding question in the field is whether the hypothesized biological influences are all part of the same process or represent different developmental pathways leading to same-sex sexual orientation. Some studies indicate that same-sex sexually oriented people can be divided into subgroups who likely experienced different biological influences. Consideration of gender expression in addition to sexual orientation might help delineate such subgroups. Thus, future research on the possible existence of such subgroups could prove to be valuable for uncovering the biological development of sexual orientation. Recommendations for such future research are discussed.

Same-sex sociosexual behaviour is widespread and heritable in male rhesus macaques

Numerous reports have documented the occurrence of same-sex sociosexual behaviour (SSB) across animal species. However, the distribution of the behaviour within a species needs to be studied to test hypotheses describing its evolution and maintenance, in particular whether the behaviour is heritable and can therefore evolve by natural selection. Here we collected detailed observations across 3 yr of social and mounting behaviour of 236 male semi-wild rhesus macaques, which we combined with a pedigree dating back to 1938, to show that SSB is both repeatable (19.35%) and heritable (6.4%). Demographic factors (age and group structure) explained SSB variation only marginally. Furthermore, we found a positive genetic correlation between same-sex mounter and mountee activities, indicating a common basis to different forms of SSB. Finally, we found no evidence of fitness costs to SSB, but show instead that the behaviour mediated coalitionary partnerships that have been linked to improved reproductive success. Together, our results demonstrate that SSB is frequent in rhesus macaques, can evolve, and is not costly, indicating that SSB may be a common feature of primate reproductive ecology.

Causal Influences of Same-Sex Attraction on Psychological Distress and Risky Sexual Behaviors: Evidence for Bidirectional Effects

Although health disparities among same-sex attracted compared to heterosexual individuals are typically explained by minority stress, there is limited evidence for a causal effect. This study investigated whether same-sex attraction was causally associated with psychological distress and risky sexual behavior using sociosexual behavior as a proxy. The sample comprised monozygotic and dizygotic twins and their non-twin siblings (n = 2036, 3780 and 2356, respectively) genotyped and assessed for same-sex attraction, psychological distress (anxiety and depressive symptoms), and risky sexual behavior. Causal influences were investigated with same-sex attraction as the predictor and psychological distress and risky sexual behavior as the outcomes in two separate Mendelian Randomization-Direction of Causation (MRDoC) models using OpenMx in R. The MRDoC model improves on the Mendelian Randomization and Direction of Causation twin models by allowing analyses of variables with similar genetic architectures, incorporating polygenic scores as instrumental variables and specifying pleiotropy and residual covariance. There were significant causal influences flowing from same-sex attraction to psychological distress and risky sexual behavior (standardized coefficients = 0.13 and 0.16; 95% CIs 0.03-0.23 and 0.08-0.25, respectively). Further analyses also demonstrated causal influences flowing from psychological distress and risky sexual behavior toward same-sex attraction. Causal influences from same-sex attraction to psychological distress and risky sexual behavior may reflect minority stress, which reinforces ongoing measures to minimize social disparities. Causal influences flowing in the opposite direction may reflect rejection sensitivity, stigma-inducing outcomes of risky sexual behavior, and recall bias; however, further research is required to specifically investigate these processes.

The New Genetic Evidence on Same-Gender Sexuality: Implications for Sexual Fluidity and Multiple Forms of Sexual Diversity

In September of 2019, the largest-ever (N = 477,522) genome-wide-association study of same-gender sexuality was published in Science. The primary finding was that multiple genes are significantly associated with ever engaging in same-gender sexual behavior, accounting for between 8-25% of variance in this outcome. Yet an additional finding of this study, which received less attention, has more potential to transform our current understanding of same-gender sexuality: Specifically, the genes associated with ever engaging in same-gender sexual behavior differed from the genes associated with one's relative proportion of same-gender to other-gender behavior. I review recent research on sexual orientation and sexual fluidity to illustrate how these findings speak to longstanding questions regarding distinctions among subtypes of same-gender sexuality (such as mostly-heterosexuality, bisexuality, and exclusive same-gender experience). I conclude by outlining directions for future research on the multiple causes and correlates of same-gender expression.

Cross-sex shifts in two brain imaging phenotypes and their relation to polygenic scores for same-sex sexual behavior: A study of 18,645 individuals from the UK Biobank

Genetic and hormonal factors have been suggested to influence human sexual orientation. Previous studied proposed brain differences related to sexual orientation and that these follow cross-sex shifted patterns. However, the neurobiological correlates of sexual orientation and how genetic factors relate to brain structural variation remains largely unexplored. Using the largest neuroimaging-genetics dataset available on same-sex sexual behavior (SSB) (n = 18,645), we employed a data-driven multivariate classification algorithm (PLS) on magnetic resonance imaging data from two imaging modalities to extract brain covariance patterns related to sex. Through analyses of latent variables, we tested for SSB-related cross-sex shifts in such patterns. Using genotype data, polygenic scores reflecting the genetic predisposition for SSB were computed and tested for associations with neuroimaging outcomes. Patterns important for classifying between males and females were less pronounced in non-heterosexuals. Predominantly in non-heterosexual females, multivariate brain patterns as represented by latent variables were shifted toward the opposite sex. Complementary univariate analyses revealed region specific SSB-related differences in both males and females. Polygenic scores for SSB were associated with volume of lateral occipital and temporo-occipital cortices. The present large-scale study demonstrates multivariate neuroanatomical correlates of SSB, and tentatively suggests that genetic factors related to SSB may contribute to structural variation in certain brain structures. These findings support a neurobiological basis to the differences in human sexuality.

Voice Pitch – A Valid Indicator of One’s Unfaithfulness in Committed Relationships?

Objectives

When judging a male speakers’ likelihood to act sexually unfaithful in a committed relationship, listeners rely on the speakers’ voice pitch such that lower voice pitch is perceived as indicating being more unfaithful. In line with this finding, a recent study (Schild et al. Behavioral Ecology, 2020) provided first evidence that voice pitch might indeed be a valid cue to sexual infidelity in men. In this study, male speakers with lower voice pitch, as indicated by lower mean fundamental frequency (mean F0), were actually more likely to report having been sexually unfaithful in the past. Although these results fit the literature on vocal perceptions in contexts of sexual selection, the study was, as stated by the authors, underpowered. Further, the study solely focused on male speakers, which leaves it open whether these findings are also transferable to female speakers.

Methods

We reanalyzed three datasets (Asendorpf et al. European Journal of Personality, 25, 16–30, 2011; Penke and Asendorpf Journal of Personality and Social Psychology, 95, 1113–1135, 2008; Stern et al. 2020) that include voice recordings and infidelity data of overall 865 individuals (63,36% female) in order to test the replicability of and further extend past research.

Results

A significant negative link between mean F0 and self-reported infidelity was found in only one out of two datasets for men and only one out of three datasets for women. Two meta-analyses (accounting for the sample sizes and including data of Schild et al. 2020), however, suggest that lower mean F0 might be a valid indicator of higher probability of self-reported infidelity in both men and women.

Conclusions

In line with prior research, higher masculinity, as indicated by lower mean F0, seems to be linked to self-reported infidelity in both men and women. However, given methodological shortcomings, future studies should set out to further delve into these findings.

Mental Health Disparities Mediating Increased Risky Sexual Behavior in Sexual Minorities: A Twin Approach

Increased risky sexual behavior in sexual minorities relative to heterosexual individuals may be partly explained by mental health disparities, and both factors may be further jointly influenced by common genetic and environmental factors. However, these relationships have not been previously investigated. The objectives of the present study were to investigate mental health disparities as a mediator of the relationship between sexual orientation and risky sexual behavior, controlling for genetic and environmental effects in this relationship and testing for sex differences. Participants included 5814 twins from a Finnish twin cohort. Specified latent factors included sexual orientation, mental health indicators, and risky sexual behavior. Twin models were fitted to the factor structure of the data whereby a Cholesky decomposition on the factors was compared to a mediation submodel using OpenMx. Sex differences were tested in the final model. Phenotypically, mental health disparities partially mediated the relationship between sexual orientation and increased risky sexual behavior, with comparable effects in males and females. However, while this indirect route from sexual orientation to risky sexual behavior mainly contained transmitted genetic effects in males, there was a significant proportion of transmitted shared environmental effects in females. This is the first study to demonstrate that the mediation relationships between sexual orientation, mental health disparities, and risky sexual behavior are not confounded by genetic and environmental factors. The significant sex differences need to be recognized in future research and intervention design to improve sexual health in sexual minorities.

On the Functional Relationships Among Sexual Orientation, Masculine and Feminine Gender Role Orientation, and Sociosexual Orientation in Young Heterosexual and Lesbian Women

The present study investigated the mutual interplay of sexual orientation, masculine and feminine gender role orientation, and sociosexual orientation in young women. To ensure cross-sample validity, 323 heterosexual women and 323 lesbian women, ranging in age from 18 to 29 years, were matched for possible confounding variables. Lesbian women scored significantly higher (d = 0.27) on the Desire subscale of the Revised Sociosexual Orientation Inventory, but not on the Behavior (d = 0.07) and Attitude subscales (d = 0.11). Concerning gender role orientation, heterosexual women were characterized by significantly higher scores on the Femininity scale (d = 0.20), whereas lesbian women showed more pronounced masculine gender role orientation (d = 0.16). Structural equation modeling revealed two functionally distinct relationships between sexual orientation and sociosexuality, on the one hand, and between gender role orientation and sociosexuality, on the other hand. Sociosexual desire was exclusively associated with a woman's sexual orientation with no indication of any mediating effect of gender role orientation. Sociosexual behavior was positively related to a woman's level of masculine gender role orientation, while sociosexual attitude was positively associated with masculine and negatively associated with feminine gender role orientation, irrespective of sexual orientation.

A Life History Approach to the Female Sexual Orientation Spectrum: Evolution, Development, Causal Mechanisms, and Health

Women's capacity for sexual fluidity is at least as interesting a phenomenon from the point of view of evolutionary biology and behavioral endocrinology as exclusively homosexual orientation. Evolutionary hypotheses for female nonheterosexuality have failed to fully account for the existence of these different categories of nonheterosexual women, while also overlooking broader data on the causal mechanisms, physiology, ontogeny, and phylogeny of female nonheterosexuality. We review the evolutionary-developmental origins of various phenotypes in the female sexual orientation spectrum using the synergistic approach of Tinbergen's four questions. We also present femme-specific and butch-specific hypotheses at proximate and ultimate levels of analysis. This review article indicates that various nonheterosexual female phenotypes emerge from and contribute to hormonally mediated fast life history strategies. Life history theory provides a biobehavioral explanatory framework for nonheterosexual women's masculinized body morphology, psychological dispositions, and their elevated likelihood of experiencing violence, substance use, obesity, teenage pregnancy, and lower general health. This pattern of life outcomes can create a feedback loop of environmental unpredictability and harshness which destabilizes intrauterine hormonal conditions in mothers, leading to a greater likelihood of fast life history strategies, global health problems, and nonheterosexual preferences in female offspring. We further explore the potential of female nonheterosexuality to function as an alloparental buffer that enables masculinizing alleles to execute their characteristic fast life history strategies as they appear in the female and the male phenotype. Synthesizing life history theory with the female sexual orientation spectrum enriches existing scientific knowledge on the evolutionary-developmental mechanisms of human sex differences.

Assessing the Relationship Between Sexual Concordance, Sexual Attractions, and Sexual Identity in Women

On average, there is a gender difference in sexual concordance, with men exhibiting greater agreement between genital and self-reported sexual arousal, relative to women. Much less is known about the substantial variation in women's sexual concordance; women's genital and self-reported sexual responses may correlate strongly and positively, not at all, or even strongly negatively. The within-gender variation in sexual concordance suggests that individual differences may be related to sexual concordance. We examined whether sexual concordance varies as a function of sexual orientation (based on self-reported sexual attractions and sexual identity labels) in a sample (N = 76) that included exclusively androphilic, predominantly androphilic, ambiphilic, and predominantly/exclusively gynephilic women. Participants viewed sexual and nonsexual stimuli that varied by actor gender while their vaginal vasocongestion and subjective sexual responses were measured. Women's sexual concordance varied as a function of their sexual attractions; women with any degree of gynephilia exhibited higher sexual concordance than exclusively androphilic women across a variety of sexual concordance measures, and these effects were demonstrated using correlation and multi-level modeling analyses. Only sexual concordance based on overall feelings of arousal varied by sexual identity, with heterosexual women exhibiting the lowest sexual concordance. Stimulus gender significantly influenced sexual concordance for most groups of women: Ambiphilic and predominantly/exclusively gynephilic women exhibited greater sexual concordance to female stimuli and exclusively androphilic women exhibited greater sexual concordance to male stimuli. These findings suggest that sexual orientation (particularly one's degree of gynephilia) may explain some of the within-gender variation seen in women's sexual concordance.

Straight but Not Narrow; Within-Gender Variation in the Gender-Specificity of Women's Sexual Response

Gender differences in the specificity of sexual response have been a primary focus in sexual psychophysiology research, however, within-gender variability suggests sexual orientation moderates category-specific responding among women; only heterosexual women show gender-nonspecific genital responses to sexual stimuli depicting men and women. But heterosexually-identified or "straight" women are heterogeneous in their sexual attractions and include women who are exclusively androphilic (sexually attracted to men) and women who are predominantly androphilic with concurrent gynephilia (sexually attracted to women). It is therefore unclear if gender-nonspecific responding is found in both exclusively and predominantly androphilic women. The current studies investigated within-gender variability in the gender-specificity of women's sexual response. Two samples of women reporting concurrent andro/gynephilia viewed (Study 1, n = 29) or listened (Study 2, n = 30) to erotic stimuli varying by gender of sexual partner depicted while their genital and subjective sexual responses were assessed. Data were combined with larger datasets of predominantly gyne- and androphilic women (total N = 78 for both studies). In both studies, women reporting any degree of gynephilia, including those who self-identified as heterosexual, showed significantly greater genital response to female stimuli, similar to predominantly gynephilic women; gender-nonspecific genital response was observed for exclusively androphilic women only. Subjective sexual arousal patterns were more variable with respect to sexual attractions, likely reflecting stimulus intensity effects. Heterosexually-identified women are therefore not a homogenous group with respect to sexual responses to gender cues. Implications for within-gender variation in women's sexual orientation and sexual responses are discussed.