Fluctuating asymmetry and sexual orientation in men and women

Finger-Length Ratios and Sexual Orientation in Southern Spain: A Large Sample Study on Sex-Linked Personality and Cognitive Traits

Some similarities between sex differences and sexual orientation differences, with gay men shifted in female-typical directions and lesbians shifted in male-typical directions, have been hypothesized to reflect the effects of prenatal androgen exposure. This report assessed sex-linked personality and cognitive traits in a Spanish sample of 1117 straight (i.e., heterosexual) and non-straight (i.e., non-heterosexual) men and women, using finger-length ratio (2D:4D) as an index of prenatal androgen exposure. Observed sex differences were in the expected direction: Measures of right-hand 2D:4D, left-hand 2D:4D, and anxiety were higher in straight women than men, whereas impulsivity, dominance, cognitive reflection, and mathematical proficiency were higher in straight men than women. Gay men proved to be shifted in a female-typical direction compared to straight men for right-hand 2D:4D, left-hand 2D:4D, anxiety, and cognitive reflection; they were intermediate for dominance and mathematics, and were shifted in a male-typical direction for impulsivity. Lesbians were shifted in a male-typical direction compared to straight women for left-hand 2D:4D, anxiety, cognitive reflection, and mathematics; and they were shifted in a highly female-typical direction for impulsivity and dominance (scoring lower than straight women). Observed sex-linked shifts in gay men and lesbians were larger for psychological traits that displayed larger sex differences, and smallest for impulsivity (which displayed the smallest sex difference). Evidence also suggested that sex-related measures were linked to 2D:4D in theoretically meaningful ways. In conclusion, this report suggests a plausible role of androgens as prenatal determinants of sex-linked personality and cognitive traits in straight and non-straight men and women.

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.

In support of 2D:4D: More data exploring its conflicting results on handedness, sexual orientation and sex differences

In the last few years, several studies have questioned the value of the second-to-fourth digit ratio (2D:4D) as a measure of exposure to sex hormones before birth. Controversy has also extended to the 2D:4D association with individual features previously related to this exposure such as handedness and sexual orientation. Given that it has been argued that sex differences in 2D:4D could be a consequence of body-size differences, we have tested in a large sample the allometric relationship between finger lengths and body size. Our results show that the association is either allometric or isometric, depending on the analyses performed. In any case, the deviation from isometry is not large enough to explain the typically observed sex difference in this trait. We have also tested the association between sexual orientation and 2D:4D, finding a relationship between 2D:4D and sexual orientation in men but not in women. We attribute this discordance with previously published meta-analysis to differences in genetic background, a variable that has gained relevance in recent years in studies involving 2D:4D. Finally, we did not find any relationship between 2D:4D and handedness, evaluated through self-reported preference and hand performance. Our main conclusion is that 2D:4D shows differences between sexes beyond their disparity in body size. In our opinion, 2D:4D can be used cautiously as an indicator of intrauterine exposure to sex hormones taking into account some considerations, such as analysing a very large sample and taking careful measurements of the ethnicity of the sample.

Handedness and Birth Order Among Heterosexual Men, Gay Men, and Sao Praphet Song in Northern Thailand

Previous research has examined handedness and birth order to inform sexual orientation and gender identity/role expression development; however, sexual orientation and gender identity/role expression have rarely been disentangled to provide a more nuanced perspective. In Thailand, we investigated sexual orientation and gender identity simultaneously via comparison of 282 heterosexual men, 201 gay men, and 178 sao praphet song-i.e., androphilic, markedly feminine males recognized as a "third" gender. Handedness was examined as: extremely left-handed, moderately left-handed, ambidextrous, moderately right-handed, or extremely right-handed. Birth order was examined as numbers of older and younger brothers and sisters, by using Berglin's, fraternal, and sororal indices, and by examining the older brother odds ratio and sibling sex ratio. Compared with heterosexual men, gay men and sao praphet song were more likely to be extremely right-handed. Sao praphet song were also more likely to be extremely left-handed than heterosexual and gay men. Heterosexual men and sao praphet song had later sororal birth order compared with the expected Thai population value, suggesting stopping rules influenced when probands' mothers ceased having children. These findings provide new insights and replicate previous findings in a non-Western sample. Regarding handedness, in males, mechanisms related to extreme right-handedness likely influence the development of androphilia, whereas mechanisms related to both extreme right- and extreme left-handedness likely explain the combination of androphilia and feminine gender identity/role expression. Regarding birth order, similar to the conclusions of some prior research, stopping rules pose a challenge for testing the fraternal birth order effect.

The molecular mechanisms of sexual orientation and gender identity

Differences between males and females are widely represented in nature. There are gender differences in phenotypes, personality traits, behaviors and interests, cognitive performance, and proneness to specific diseases. The most marked difference in humans is represented by sexual orientation and core gender identity, the origins of which are still controversial and far from being understood. Debates continue on whether sexual behavior and gender identity are a result of biological (nature) or cultural (nurture) factors, with biology possibly playing a major role. The main goal of this review is to summarize the studies available to date on the biological factors involved in the development of both sexual orientation and gender identity. A systematic search of published evidence was performed using Medline (from January 1948 to June 2017). Review of the relevant literature was based on authors' expertise. Indeed, different studies have documented the possible role and interaction of neuroanatomic, hormonal and genetic factors. The sexual dimorphic brain is considered the anatomical substrate of psychosexual development, on which genes and gonadal hormones may have a shaping effect. In particular, growing evidence shows that prenatal and pubertal sex hormones permanently affect human behavior. In addition, heritability studies have demonstrated a role of genetic components. However, a convincing candidate gene has not been identified. Future studies (e.i. genome wide studies) are needed to better clarify the complex interaction between genes, anatomy and hormonal influences on psychosexual development.

Does Sexual Orientation Precede Childhood Sexual Abuse? Childhood Gender Nonconformity as a Risk Factor and Instrumental Variable Analysis

Research suggests that there is a relation between childhood sexual abuse (CSA) and adulthood nonheterosexual orientation. To explore whether nonheterosexual orientation increases the risk of CSA, we recruited a large sample, added the variable of childhood gender nonconformity (CGNC), and applied the instrumental variable method. We found that heterosexual and nonheterosexual men who were more gender nonconforming in childhood were significantly more likely to report having a history of CSA than their gender-conforming counterparts. There was no relation between CSA and CGNC for heterosexual and nonheterosexual women. The instrumental variable analysis revealed that the increased prevalence of CSA experienced by nonheterosexuals compared with heterosexuals may be due to the influence of sexual orientation on CSA. In sum, the results suggest that nonheterosexuality may increase the risk of childhood sexual abuse.

Handedness is a biomarker of variation in anal sex role behavior and Recalled Childhood Gender Nonconformity among gay men

Developmental theories of the biological basis of sexual orientation suggest that sexually differentiated psychological and behavioural traits should be linked with sexual orientation. Subgroups of gay men delineated by anal sex roles differ according to at least one such trait: gender expression. The present study assessed the hypothesis that handedness, a biologically determined sexually differentiated trait, corresponds to differences in subgroups of gay men based on anal sex role. Furthermore, it assessed whether handedness mediates the association between gender nonconformity and male sexual orientation. Straight and gay men (N = 333) completed the Edinburgh Inventory of Handedness and the Recalled Childhood Gender Nonconformity Scale. Gay men also completed measures of anal sex role preference. As in previous studies, gay men showed greater non-right-handedness and gender nonconformity than straight men. Also, among gay men, bottoms/versatiles (i.e., gay men who take a receptive anal sex role, or who take on both a receptive and insertive anal sex role) were more gender-nonconforming than tops (i.e., gay men who take an insertive anal sex role). In support of the hypothesis, bottoms/versatiles were more non-right-handed than tops and handedness mediated the male sexual orientation and anal sex role differences in Recalled Childhood Gender Nonconformity. Together, these findings suggest that developmental processes linked to handedness underpin variation among men in sexual orientation and gender nonconformity as well as variation among subgroups of gay men that are delineated by anal sex roles.

Pubertal Stress and Nutrition and their Association with Sexual Orientation and Height in the Add Health Data

A number of studies have indicated that gay men tend to be shorter, on average, than heterosexual men. Less evidence exists that lesbian women are taller, on average, than heterosexual women. The most popular explanation of the association between sexual orientation and height involves prenatal factors, such that, for example, gay men may have been exposed to lower than typical androgens during fetal development, which impacts their height and sexual orientation as adults. An alternative explanation involves stress, given that stress has been associated with sexual minority identification and with lower height. Another alternative explanation involves nutrition, although its relationship is less clear with sexual minority identification. Using the Add Health data, which is a large, nationally representative and longitudinal sample of American adolescents (n = 14,786), we tested a mediation model, such that sexual orientation → pubertal stress/nutrition → height. Within men, we found that gay men (n = 126) were shorter, on average, than heterosexual men (n = 6412). None of the 24 pubertal stress-related and 15 pubertal nutrition-related variables assessed in the Add Health data mediated the relationship between sexual orientation and height in men. Within women, lesbians (n = 75) did not differ significantly in stature compared to heterosexual women (n = 6267). Thus, prenatal mechanisms (e.g., hormones, maternal immune response) are likely better candidates for explaining the height difference between gay men and heterosexual men.

Fraternal Birth Order, Handedness, and Sexual Orientation in a Chinese Population

We examined the relationship between handedness, fraternal birth order, and sexual orientation in a Chinese population, and analyzed the influence of the components assessing sexual orientation and criteria classifying individuals as homosexual on this relationship. A large sample of heterosexual, bisexual, and homosexual men and women participated in a web-based survey. Our results showed that homosexual women are more likely to be non-right-handed than heterosexual women, regardless of how sexual orientation was defined, whereas bisexual women are more likely to be non-right-handed than heterosexual women when sexual orientation was assessed via sexual attraction and sexual identity. Bisexual men are more likely to be non-right-handed than heterosexual men when sexual orientation was assessed via sexual attraction. We found neither a fraternal birth-order effect nor an interaction between sibling sex ratio, handedness, and sexual orientation. The small number of siblings may be the reason why we could not replicate the fraternal birth-order effect in this Chinese population, which highlights the importance of cultural differences in the understanding of handedness, fraternal birth order, and sexual orientation.

The Relationship Between Digit Ratio (2D:4D) and Sexual Orientation in Men from China

We examined the relationship between 2D:4D digit ratio and sexual orientation in men from China and analyzed the influences of the components used to assess sexual orientation and the criteria used to classify individuals as homosexual on this relationship. A total of 309 male and 110 female participants took part in a web-based survey. Our results showed that heterosexual men had a significantly lower 2D:4D than heterosexual women and exclusively homosexual men had a significantly higher left 2D:4D than heterosexual men whereas only exclusively homosexual men had a significantly higher right 2D:4D than heterosexual men when sexual orientation was assessed via sexual attraction. The left 2D:4D showed a significant positive correlation with sexual identity, sexual attraction, and sexual behavior, and the right 2D:4D showed a significant positive correlation with sexual attraction. The effect sizes for differences in 2D:4D between homosexual and heterosexual men varied according to criteria used to classify individuals as homosexual and sexual orientation components; the more stringent the criteria (scores closer to the homosexual category), the larger the effect sizes; further, sexual attraction yielded the largest effect size. There were no significant effects of age and latitude on Chinese 2D:4D. This study contributes to the current understanding of the relationship between 2D:4D and male sexual orientation.

Fraternal Birth Order and Extreme Right-Handedness as Predictors of Sexual Orientation and Gender Nonconformity in Men

The present study explored whether there were relationships between number of older brothers, handedness, recalled childhood gender nonconformity (CGN), and sexual orientation in men. We used data from previous British studies conducted in our laboratory (N = 1,011 heterosexual men and 921 gay men). These men had completed measures of demographic variables, number and sex of siblings, CGN, and the Edinburgh Handedness Inventory. The results did not replicate the fraternal birth order effect. However, gay men had fewer "other siblings" than heterosexual men (even after controlling for the stopping-rule and family size). In a sub-sample (425 gay men and 478 heterosexual men) with data available on both sibling sex composition and handedness scores, gay men were found to show a significantly greater likelihood of extreme right-handedness and non-right-handedness compared to heterosexual men. There were no significant effects of sibling sex composition in this sub-sample. In a further sub-sample (N = 487) with data available on sibling sex composition, handedness, and CGN, we found that men with feminine scores on CGN were more extremely right-handed and had fewer other-siblings compared to masculine scoring men. Mediation analysis revealed that handedness was associated with sexual orientation directly and also indirectly through the mediating factor of CGN. We were unable to replicate the fraternal birth order effect in our archived dataset but there was evidence for a relationship among handedness, sexual orientation, and CGN. These data help narrow down the number of possible neurodevelopmental pathways leading to variations in male sexual orientation.

Pitch and pitch variation in lesbian women

OBJECTIVES

The purpose of this study was to investigate to what extent lesbian women demonstrate pitch and pitch variation that is different from that of heterosexual women.

STUDY DESIGN

Static group comparison.

METHODS

The average pitch and pitch variation of a group of 34 self-identified lesbian women and an age-matched group of 68 heterosexual women were compared. The speech sample consisted of read speech. Acoustic analysis was performed by means of PRAAT.

RESULTS

Mean fundamental frequency in the group of lesbian women was significantly lower than that of the group of heterosexual women. The lesbian woman also showed significantly less pitch variation.

CONCLUSIONS

Lesbian women tend to demonstrate a lower average pitch and less pitch variation than heterosexual women, but this does not mean a confirmation of the popular stereotype that lesbian women are masculine women. In their assessment of clients with voice disorders, clinicians should reckon with the sociophonetic variation that is associated with sexual orientation.

Prenatal Influences on Sexual Orientation: Digit Ratio (2D:4D) and Number of Older Siblings

Prenatal androgen levels are suggested to influence sexual orientation in both sexes. The 2D:4D digit ratio has been found to associate with sexual orientation, but published findings have often been contradictory, which may partly be due to the large ethnic diversity between and within studied populations. In men, number of older brothers has been found to correlate positively with homosexuality. This phenomenon has been explained with a maternal immune reaction, which is provoked only by male fetuses and which gets stronger after each pregnancy. Here we assessed the relationship of sexual orientation to 2D:4D ratios and number of older siblings in Finland, where the population is found to be genetically relatively homogeneous. As in many previous studies, heterosexual men had lower 2D:4D than non-heterosexual men, which supports the notion that non-heterosexual men experience higher androgen levels in utero than population norms. Contrary to previous reports, non-heterosexual women had higher 2D:4D than heterosexual women. Non-heterosexual men had more older brothers and older sisters than heterosexual men. The greater number of older sisters in non-heterosexual men indicates that there are other factors that contribute to the higher birth order of homosexual men than the maternal immunization.