Fluctuating asymmetry, second to fourth finger length ratios and human sexual orientation

Sexual orientation in humans may be influenced by levels of prenatal sex steroids which canalise neurodevelopment along sex-typical (heterosexual) or sex-atypical (homosexual) lines. Some evidence for sexual-orientation-related differences in putative somatic markers of prenatal sex hormones supports this view. A competing theory asserts that human homosexuality is due to developmental instability (DI) because it represents a shift from the species-typical pattern of heterosexual orientation. Evidence for elevated rates of non-right handedness among homosexuals provides limited support for this account. The current study tested both theories by examining nine bilateral somatic traits in 120 healthy heterosexual and homosexual men and women in order to compute second to fourth finger length ratios (2D:4D), a measure ascribed to levels of prenatal sex steroids, and fluctuating asymmetry (FA), a measure of DI. Homosexual men and women had significantly lower right hand 2D:4D ratios (even after controlling for handedness, height and weight differences) in comparison to heterosexuals, but sexual orientation did not relate to composite FA scores. The findings constrain the number of possible neurodevelopmental pathways responsible for sexual orientation in humans.     

A cognitive profile of homosexual men compared to heterosexual men and women

Matched groups of homosexual men, heterosexual men, and heterosexual women (n=38 per group) were tested on three measures of spatial ability and two measures of fluency that typically reveal sex differences. For the three spatial tests and one of the fluency tests, the mean performance of homosexual men fell between those of the heterosexual men and women. The pattern of cognitive skills of homosexual men was different from that of heterosexual men: homosexual men had lower spatial ability relative to fluency. The cognitive pattern of homosexual men was not significantly different from that of heterosexual women. In addition, the results suggest that homosexual men classified on the basis of hand preference may form two subgroups that differ in cognitive pattern. These findings are compatible with the hypothesis that there is a neurobiological factor related to sexual differentiation in the etiology of homosexuality.     

Performance Differences Between Adult Heterosexual and Homosexual Men on the Digit-Symbol Substitution Subtest of the WAIS-R

Previous evidence suggests that sexual orientation influences performance on a number of cognitive functions known to be sexually dimorphic. This investigation examined the performance of 240 right-handed subjects (60 heterosexual men, 60 homosexual men, 60 heterosexual women and 60 homosexual women) on one of the most commonly used neuropsychological tests to show normative sex differences, the Digit-Symbol Substitution test of the WAIS-R. Analysis of scaled Digit-Symbol scores revealed that heterosexual women and homosexual men outperformed heterosexual men. The magnitude of these differences were modest by standard criteria. No differences were found between heterosexual and homosexual women. The findings implicate within-sex variation in one test that relies on intact executive function.     

Performance differences between adult heterosexual and homosexual men on the Digit-Symbol Substitution subtest of the WAIS-R

Previous evidence suggests that sexual orientation influences performance on a number of cognitive functions known to be sexually dimorphic. This investigation examined the performance of 240 right-handed subjects (60 heterosexual men, 60 homosexual men, 60 heterosexual women and 60 homosexual women) on one of the most commonly used neuropsychological tests to show normative sex differences, the Digit-Symbol Substitution test of the WAIS-R. Analysis of scaled Digit-Symbol scores revealed that heterosexual women and homosexual men outperformed heterosexual men. The magnitude of these differences were modest by standard criteria. No differences were found between heterosexual and homosexual women. The findings implicate within-sex variation in one test that relies on intact executive function.     

The impact of relational factors on sexual satisfaction among heterosexual and homosexual men

This study analyzed the correlates of sexual satisfaction among exclusively heterosexual and homosexual men. Analyses were carried out using data from a 2011 web survey about male sexual interest in two European countries. A total of 2968 men with exclusive heterosexual behavior and 285 men with exclusive homosexual behavior in the previous five years from Portugal and Croatia, participated in this study. Hierarchical multiple regressions were performed in order to explore which variables predicted sexual satisfaction. Among heterosexuals, in step one, both age and sexual difficulties emerged as highly significant predictors, and in step two, the frequency of intercourse and the number of sexual partners also emerged as significant predictors. Finally, in the third step, the intimacy and the length of the relationship emerged as highly significant predictors of sexual satisfaction. Among homosexuals, in step one, sexual difficulties and country were significant predictors, in step two, only the frequency of sexual intercourse was a significant predictor, and in step three, both relational variables (relationship length and relationship intimacy) were significant predictors. The findings suggest that relational factors, particularly couple intimacy were the strongest predictors of sexual satisfaction for both heterosexual and homosexual men.     

Brain response to visual sexual stimuli in heterosexual and homosexual males

Although heterosexual and homosexual individuals clearly show differences in subjective response to heterosexual and homosexual sexual stimuli, the neurobiological processes underlying sexual orientation are largely unknown. We addressed the question whether the expected differences in subjective response to visual heterosexual and homosexual stimuli may be reflected in differences in brain activation pattern. Twenty-four healthy male volunteers, 12 heterosexuals and 12 homosexuals, were included in the study. BOLD signal was measured while subjects were viewing erotic videos of heterosexual and homosexual content. SPM02 was used for data analysis. Individual sexual arousal was assessed by subjective rating. As compared to viewing sexually neutral videos, viewing erotic videos led to a brain activation pattern characteristic for sexual arousal in both groups only when subjects were viewing videos of their respective sexual orientation. Particularly, activation in the hypothalamus, a key brain area in sexual function, was correlated with sexual arousal. Conversely, when viewing videos opposite to their sexual orientation both groups showed absent hypothalamic activation. Moreover, the activation pattern found in both groups suggests that stimuli of opposite sexual orientation triggered intense autonomic response and may be perceived, at least to some extent, as aversive.     

Finger length ratio (2D:4D) and dimensions of sexual orientation

BACKGROUND: It has been suggested that both adult sexual orientation and the 2nd to 4th finger length ratio (2D:4D) are influenced by prenatal testosterone levels. Bearing this in mind, 2D:4D has been tested as a proxy measure of the putative prenatal testosterone impact on adult human homosexuality, but the results are inconsistent. To date, most studies in this field of research comprise categorical group comparison of heterosexuals and homosexuals with respect to their 2D:4D. AIMS: The purpose of our study was to evaluate 2D:4D and the dimensional perspective of sexual orientation in addition to categorical results. METHODS: We examined a community-based sample of 409 subjects and calculated correlations of 2D:4D and sexual orientation as a continuum ranging from homosexual to heterosexual. RESULTS: We found a significant negative correlation of 2D:4D with homosexual orientation (fantasy, attraction, activity and general score) in women, but not in men. CONCLUSIONS: Our results indicate that with higher prenatal testosterone levels in women, the likelihood of homosexual orientation might increase. We hypothesize a continuous neurohormonal sexual differentiation of the brain, most notably for women, that overrides categories and results in varying dimensions of sexual orientation. This hypothesis contrasts with the predominant suggestion of fixed organizational effects of androgens in the brain and a categorical sexual orientation.     

Biosocial factors, sexual orientation and neurocognitive functioning

It has been proposed that sexual orientation related differences in cognitive performance are either due to the actions of prenatal factors early in development or the influence of gender role learning. This study examined the performance of 240 healthy, right-handed heterosexual and homosexual males and females (N = 60 per group) on a battery of cognitive tasks comprising mental rotation, judgement of line orientation (JLO), verbal fluency, perceptual speed and object location memory. Measures were also taken of the psychological gender, birth order, sibling sex composition and the 2nd to 4th finger length ratios of the right and left hands. A series of stepwise regression analyses revealed that sex and sexual orientation were the strongest predictors of cognitive performance, with IQ also contributing considerable variance. Psychological gender (M/F scores) added a small, but significant, amount of variance to mental rotation and perceptual speed scores in addition to these main factors, but prenatal hormone related indices, such as 2nd to 4th finger ratios, birth order and sibling sex composition added no independent predictive power. These findings are discussed in relation to biosocial influences on cognitive differences between heterosexuals and homosexuals.     

Neuropsychological development of cognitive abilities: a new research strategy and some preliminary evidence for a sexual orientation model

The organizing action that prenatal sex hormones exert on the brain has been implicated in the aetiology of sex differences in cognitive abilities and cerebral asymmetries. Prenatal sex hormones are known to determine neuroendocrine responses and subsequent adult sexual behaviour in nonhuman animals and these hormones may also influence human sexual orientation. To unify these observations, we elaborate a sexual orientation model of neuropsychological development which predicts that sex differences in neuroendocrine responses, cerebral asymmetries, and cognitive abilities are related to sexual orientation rather than to sex per se. A common problem in the study of sex differences is that biological and sociocultural explanations are confounded. The sexual orientation model suggests a new research strategy, the comparison of homosexual and heterosexual groups, for which explanations in terms of sociocultural factors would be more difficult to sustain. The available evidence supports the model: In terms of a neuroendocrine response, cerebral asymmetry, and cognitive abilities, homosexual males resemble heterosexual females rather than heterosexual males. Our conclusions lead to questions that further research in this area should address, and to a consideration of problems which such research may have to face.     

Sibship size, sibling sex ratio, birth order, and parental age in homosexual and nonhomosexual gender dysphorics.

This study investigated whether demographic variables previously reported to differentiate ordinary homosexuals from heterosexuals also differentiate gender-dysphoric homosexuals from gender-dysphoric persons with other sexual orientations. Subjects were outpatients aged 16 and older who were referred by physicians to a specialty clinic for the assessment of gender identity disorders. The subjects were divided into three groups: 204 homosexual women, 193 homosexual men, and 273 nonhomosexual men; the last category included men sexually attracted to females, to both sexes, and to neither sex. Demographic data on patients' families of origin were extracted from their clinical charts and from structured interview protocols. A multiple-range test at the .05 level showed that the homosexual men had significantly more siblings than the homosexual women, who, in turn, had significantly more siblings than the nonhomosexual men. The sibling sex ratio of the homosexual men, 131 brothers per 100 sisters, was significantly higher than the sex ratio of live births for the population as a whole (106 men per 100 women; p = .01); the sibling sex ratios of the other groups did not differ from the expected value. The homosexual men had a significantly later birth order than the nonhomosexual men (p = .004); the homosexual women, who fell in between, did not differ significantly from either male group. There were no between-groups differences in paternal or maternal age at the time of the subject's birth. The results concerning sibling sex ratio and birth order are consistent with previous findings for homosexual men.     

A crossover interaction between sex, sexual orientation, and handedness

The distribution of handedness scores was analysed in 1414 heterosexual, bisexual, and homosexual men and women. Hand preference was assessed by a questionnaire that distinguishes consistently right-handed from partially or completely left-handed individuals, and sexual orientation was measured by the Kinsey scales. As expected from population-based studies, heterosexual men were, on average, more left-handed than heterosexual women. By contrast, gay men were more right-handed than lesbians or heterosexual men, and lesbians were more left-handed than gay men or heterosexual women. This crossover interaction suggests that a common variable influences sex, sexual orientation, and hand preference.     

Sex differences in face processing are mediated by handedness and sexual orientation

Previous research has demonstrated sex differences in face processing at both neural and behavioural levels. The present study examined the role of handedness and sexual orientation as mediators of this effect. We compared the performance of LH (left-handed) and RH (right-handed) heterosexual and homosexual male and female participants on a face recognition memory task. Our main findings were that homosexual males have better face recognition memory than both heterosexual males and homosexual women. We also demonstrate better face processing in women than in men. Finally, LH heterosexual participants had better face recognition than LH homosexual participants and also tended to be better than RH heterosexual participants. These findings are consistent with differences in the organisation and laterality of face-processing mechanisms as a function of sex, handedness, and sexual orientation.     

Male and female homosexuality: natural history

The aim of our study was to describe the natural history of the homosexual phenomenon through describing its development and characteristics from early childhood and into adult life. Our findings indicate that the homosexual orientation develops very early in life and that the prehomosexual child displays behavioral characteristics that are more typical of the opposite sex than of his own biologic sex. The homosexual male begins his sexual involvement in early adolescene, while the homosexual female starts her homosexual involvement several years later. There was no significant difference in the prevalence of neurotic disorders between homosexuals and heterosexuals. However, homosexual males and females had more depression, suicide attempts, and usually abused alcohol more often. Still, the majority of the homosexuals functioned adequately and were productive, with no significant disabilities.     

Relation between language lateralisation and spatial ability in gay and straight women and men

Delineating the relationship between language lateralisation and spatial ability remains an elusive goal. To explore the association of sexual orientation to the language lateralisation/spatial ability relationship, heterosexual (HT) women, HT men, lesbians, and gay men (20 per group) completed a divided-visual-field lexical-decision task to assess relative degrees of language lateralisation, as well as a Water Level Task (WLT) and two Mental Rotation (MR) Tasks designed to assess spatial ability. A significant cross-sex shift was revealed in the language lateralisation of gay men. Further, language lateralisation positively correlated with MR ability in HT men and tended to positively correlate with spatial perception in HT women. No significant associations were revealed in homosexual subjects. Different patterns of functional hemispheric asymmetry in homosexual and heterosexual individuals are discussed in relation to early neurobiological factors influencing the development of sexual orientation.     

Psychological adjustment of nonpatient homosexuals: critical review of the research literature.

This paper critically reviews research studies comparing adjustment levels of nonpatient homosexuals and heterosexuals, and concludes that findings to date have not demonstrated that the homosexual individual is any less psychologically adjusted than his heterosexual counterpart.     

Sexual orientation related differences in spatial memory.

The purpose of this study was to investigate and extend previously reported sex differences in object location memory by comparing the performance of heterosexual and homosexual males and females. Subjects were 240 healthy, right-handed heterosexual and homosexual males and females. They were instructed to study 16 common, gender-neutral objects arranged randomly in an array and subsequently tested for object recall, object recognition and spatial location memory. Females recalled significantly more objects than males, although there were no group differences in object recognition. Decomposition of significant interactions between sex and sexual orientation on spatial location memory (controlling for differences in object recall, age and IQ) revealed that heterosexual females and homosexual males scored better than heterosexual males, and no different from each other. There were no differences between homosexual and heterosexual females. The findings suggest that homosexual males and heterosexual females encode, store and retrieve positional and relational information about spatial layouts similarly, pointing to within-sex variations in the neural architecture underlying spatial memory.     

Sexual orientation-related differences in allocentric spatial memory tasks

Spatial memory in mammals, including humans, appears highly sexually dimorphic. The present investigation sought to examine if spatial learning and spatial memory in humans is also linked to sexual orientation. This was achieved by using virtual reality versions of two classic paradigms developed in animal models of hippocampal functioning, the Morris Water Maze (MWM) and Radial Arm Maze (RAM). Here, we show that in contrast to heterosexual men, and in congruence with heterosexual women, homosexual men displayed significantly greater search latencies (spatial learning) during a virtual Morris Water Maze. During a virtual 8-arm Radial Arm Maze, heterosexual males had significantly shorter search latency than heterosexual females, and did not differ from homosexual males. Statistical modeling revealed that variations in neurodevelopmental markers previously associated with human sexual orientation (2nd to 4th finger length ratios and older fraternal siblings) differentially predicted MWM probe trial performance and RAM search latencies only. These data may limit the number of possible neurodevelopmental pathways responsible for sexual variation in components of spatial learning and memory.     

Proportion of unmarried siblings of homosexual and non homosexual gender-dysphoric patients.

Among the siblings of homosexuals, a lack of sexual and romantic interest in women may be independent of erotic feelings for men. This study investigated the sexual histories of siblings of gender-dysphoric outpatients. The patients were classified into three groups: non homosexual males, homosexual males and homosexual females. Their siblings consisted of 301 brothers and 284 sisters over the age of 25. Logistic regression showed that the brothers of the homosexual male patients were significantly less likely to have been married, either legally or common-law, than the brothers of the other gender-dysphoric groups, even with age and relative birth order taken into account. This finding suggests that the low propensity for long term heterosexual relationships seen in homosexual male gender-dysphoric patients also tends to appear in their brothers.     

Sexual Orientation and Gender Differences in Markers of Inflammation and Immune Functioning

Background

Sexual minorities have documented elevated risk factors that can lead to inflammation and poor immune functioning.

Purpose

This study aims to investigate disparities in C-reactive protein (CRP) and Epstein–Barr virus (EBV) by gender and sexual orientation.

Methods

We used the National Longitudinal Study of Adolescent Health to examine disparities in CRP (N?=?11,462) and EBV (N?=?11,812).

Results

Among heterosexuals, women had higher levels of CRP and EBV than men. However, sexual minority men had higher levels of CRP and EBV than heterosexual men and sexual minority women. Lesbians had lower levels of CRP than heterosexual women.

Conclusions

Gender differences in CRP and EBV found between men and women who identify as 100 % heterosexual were reversed among sexual minorities and not explained by known risk factors (e.g., victimization, alcohol and tobacco use, and body mass index). More nuanced approaches to addressing gender differences in sexual orientation health disparities that include measures of gender nonconformity and minority stress are needed.     

Evidence of familial nature of male homosexuality

We recruited 51 predominantly homosexual and 50 predominantly heterosexual men as index subjects for a family study of sexual orientation. The sexual orientation of siblings (115 sisters and 123 brothers) was ascertained in two ways: via interview with the index subjects and via interview of and/or mailed questionnaire to the siblings themselves. Index subjects' knowledge of the sexual orientation of their siblings was, for the most part, quite accurate. Moreover, heterosexual index men had about as many homosexual brothers as would be predicted given national prevalence figures for homosexuality, but homosexual index men had about four times as many homosexual brothers, although in both cases most brothers were heterosexual. There was no significant difference between the index groups in number of homosexual sisters. We conclude that there is a significant familial component to male homosexuality.