Homosexuality as a Discrete Class

Genetic variants underlying human bisexual behavior are reproductively advantageous

Because human same-sex sexual behavior (SSB) is heritable and leads to fewer offspring, how SSB-associated alleles have persisted and whether they will remain in human populations are of interest. Using the UK Biobank, we address these questions separately for bisexual behavior (BSB) and exclusive SSB (eSSB) after confirming their genetic distinction. We discover that male BSB is genetically positively correlated with the number of offspring. This unexpected phenomenon is attributable to the horizontal pleiotropy of male risk-taking behavior-associated alleles because male risk-taking behavior is genetically positively correlated with both BSB and the number of offspring and because genetically controlling male risk-taking behavior abolishes the genetic correlation between male BSB and the number of offspring. By contrast, eSSB is genetically negatively correlated with the number of offspring. Our results suggest that male BSB-associated alleles are likely reproductively advantageous, which may explain their past persistence and predict their future maintenance, and that eSSB-associated alleles are likely being selected against at present.

Is everyone a mix of straight and gay? A social pressure theory of sexual orientation, with supporting data from a large global sample

Sigmund Freud, Alfred Kinsey, E.O. Wilson, and others have suggested that social pressure suppresses natural tendencies for humans to express bisexuality, the apparent norm for one of our two closest genetic relatives, the bonobo. An analysis of data obtained from a new online sample of 1,150,938 people in 215 countries and territories (63.9% from the United States, United Kingdom, and Canada) who completed the English version of a validated questionnaire of sexual orientation lends support to this idea. A histogram of scores from 0 (exclusive opposite-sex inclinations) to 18 (exclusive same-sex inclinations) forms a near-normal distribution. Although this distribution was likely caused to some extent by sampling bias, it may also reflect the unusual honesty people show when taking online tests anonymously, as an increasing body of evidence demonstrates. We present a formal mathematical expression of a social pressure theory of sexual orientation, along with empirical evidence and computational explorations that support the theory. We also present an analysis of the new data set. Among other findings: sexual orientation labels corresponded to broad, skewed, overlapping distributions of scores. Self-labeled gays/lesbians and, to a greater extent, self-labeled straights, reported that the larger the mismatch between their sexual orientation label and their actual sexual inclinations, the more distress they felt regarding their sexual orientation, a finding that is predictable from cognitive dissonance theory. Educating the public about the true nature of sexual orientation might quell the often rancorous public debates on this topic, as well as give comfort to a large number of mislabeled people.

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.

Dimensions over categories: a meta-analysis of taxometric research

Taxometric procedures have been used extensively to investigate whether individual differences in personality and psychopathology are latently dimensional or categorical ('taxonic'). We report the first meta-analysis of taxometric research, examining 317 findings drawn from 183 articles that employed an index of the comparative fit of observed data to dimensional and taxonic data simulations. Findings supporting dimensional models outnumbered those supporting taxonic models five to one. There were systematic differences among 17 construct domains in support for the two models, but psychopathology was no more likely to generate taxonic findings than normal variation (i.e. individual differences in personality, response styles, gender, and sexuality). No content domain showed aggregate support for the taxonic model. Six variables - alcohol use disorder, intermittent explosive disorder, problem gambling, autism, suicide risk, and pedophilia - emerged as the most plausible taxon candidates based on a preponderance of independently replicated findings. We also compared the 317 meta-analyzed findings to 185 additional taxometric findings from 96 articles that did not employ the comparative fit index. Studies that used the index were 4.88 times more likely to generate dimensional findings than those that did not after controlling for construct domain, implying that many taxonic findings obtained before the popularization of simulation-based techniques are spurious. The meta-analytic findings support the conclusion that the great majority of psychological differences between people are latently continuous, and that psychopathology is no exception.

Large-scale GWAS reveals insights into the genetic architecture of same-sex sexual behavior

Twin and family studies have shown that same-sex sexual behavior is partly genetically influenced, but previous searches for specific genes involved have been underpowered. We performed a genome-wide association study (GWAS) on 477,522 individuals, revealing five loci significantly associated with same-sex sexual behavior. In aggregate, all tested genetic variants accounted for 8 to 25% of variation in same-sex sexual behavior, only partially overlapped between males and females, and do not allow meaningful prediction of an individual's sexual behavior. Comparing these GWAS results with those for the proportion of same-sex to total number of sexual partners among nonheterosexuals suggests that there is no single continuum from opposite-sex to same-sex sexual behavior. Overall, our findings provide insights into the genetics underlying same-sex sexual behavior and underscore the complexity of sexuality.

Identifying Basic Classes of Sexual Orientation with Latent Profile Analysis: Developing the Multivariate Sexual Orientation Classification System

Despite considerable progress, research on sexual minorities has been hindered by a lack of clarity and consistency in defining sexual minority groups. Further, despite recent recommendations to assess the three main dimensions of sexual orientation-identity, behavior, and attraction-it remains unclear how best to integrate such multivariate information to define discrete sexual orientation groups, particularly when identity and behavior fail to match. The current study used a data-driven approach to identify a parsimonious set of sexual orientation classes. Latent profile analysis (LPA) was run within a large (N = 3182) and sexually diverse sample, using dimensions of sexual identity, behavior, and attraction as predictors. LPAs supported four fundamental sexual orientation classes not only in the overall sample, but also when conducted separately in men (n = 980) and women (n = 2175): heterosexual, homosexual, bisexual, and heteroflexible (a class representing individuals who self-identify as heterosexual or mostly heterosexual but report moderate same-sex sexual behavior and attraction). Heterosexuals reported the highest levels of psychological functioning and lowest risk behaviors. Homosexuals showed similarly high levels of psychological functioning to heterosexuals, but higher levels of risk behaviors. Bisexuals and heteroflexibles showed similarly low levels of psychological functioning and high risk taking. To facilitate applications of this classification approach, the study developed the Multivariate Sexual Orientation Classification System, reproducing the four LPA groups with 97% accuracy (kappa = .95) using just two items. Implications of this classification approach are discussed.

Sexual Orientation: Categories or Continuum? Commentary on Bailey et al. (2016)

Bailey et al. (2016) have provided an excellent, state-of-the-art overview that is a major contribution to our understanding of sexual orientation. However, whereas Bailey and his coauthors have examined the physiological, behavioral, and self-report data of sexual orientation and see categories, I see a sexual and romantic continuum. After noting several objections concerning the limitations of the review and methodological shortcomings characteristic of sexual-orientation research in general, I present evidence from research investigating in-between sexualities to support an alternative, continuum-based perspective regarding the nature of sexual orientation for both women and men. A continuum conceptualization has potential implications for investigating the prevalence of nonheterosexuals, sexual-orientation differences in gender nonconformity, causes of sexual orientation, and political issues.

Gay, Mostly Gay, or Bisexual Leaning Gay? An Exploratory Study Distinguishing Gay Sexual Orientations Among Young Men

This exploratory study assessed physiological, behavioral, and self-report measures of sexual and romantic indicators of sexual orientation identities among young men (mean age = 21.9 years) with predominant same-sex sexual and romantic interests: those who described themselves as bisexual leaning gay (n = 11), mostly gay (n = 17), and gay (n = 47). Although they were not significantly distinguishable based on physiological (pupil dilation) responses to nude stimuli, on behavioral and self-report measures a descending linear trend toward the less preferred sex (female) was significant regarding sexual attraction, fantasy, genital contact, infatuation, romantic relationship, sex appeal, and gazing time to the porn stimuli. Results supported a continuum of sexuality with distinct subgroups only for the self-report measure of sexual attraction. The other behavioral and self-report measures followed the same trend but did not significantly differ between the bisexual leaning gay and mostly gay groups, likely the result of small sample size. Results suggest that romantic indicators are as good as sexual measures in assessing sexual orientation and that a succession of logically following groups from bisexual leaning gay, mostly gay, to gay. Whether these three groups are discrete or overlapping needs further research.