Sex-Specificity in the Reward Value of Facial Attractiveness

Studies of the sex-specificity of sexual arousal in adults (i.e., the tendency to respond more strongly to preferred-sex individuals than non-preferred sex individuals) have suggested that heterosexual men, homosexual men, and homosexual women show stronger sex-specific responses than do heterosexual women. Evidence for a similar pattern of results in studies investigating the reward value of faces is equivocal. Consequently, we investigated the effects of (1) sexual orientation (homosexual vs. heterosexual), (2) sex (male vs. female), (3) image sex (preferred-sex vs. non-preferred-sex), and (4) the physical attractiveness of the individual shown in the image on the reward value of faces. Participants were 130 heterosexual men, 130 homosexual men, 130 heterosexual women, and 130 homosexual women. The reward value of faces was assessed using a standard key-press task. Multilevel modeling of responses indicated that images of preferred-sex individuals were more rewarding than images of non-preferred-sex individuals and that this preferred-sex bias was particularly pronounced when more physically attractive faces were presented. These effects were not qualified by interactions involving either the sexual orientation or the sex of our participants, however, suggesting that the preferred-sex bias in the reward value of faces is similar in heterosexual men, homosexual men, heterosexual women, and homosexual women.     

Detection of Sexual Orientation (“Gaydar”) by Homosexual and Heterosexual Women

Although there has been considerable research investigating the ability to identify sexual orientation from static images, or “gaydar,” few studies have considered the role of female sexual orientation or sexual interest (for example, sociosexual orientation) in judgment accuracy. In two studies, we investigated the sexuality detection ability, and masculinity and femininity as cues used in judgment. In Study 1, we recruited heterosexual (N = 55) and homosexual (N = 71) women to rate the sexual orientation of homosexual and heterosexual male and female targets (N = 80: 20 heterosexual men, 20 homosexual men, 20 heterosexual women, and 20 homosexual women). We found that detection accuracy was better than chance levels for both male and female targets and that male targets were more likely to be falsely labeled as homosexual than female targets were. Overall, female faces were more accurately identified as heterosexual or homosexual than male faces and homosexual female raters were biased towards labeling targets as homosexual. Sociosexuality did not influence the accuracy with which targets were identified as heterosexual or homosexual. In Study 2, 100 heterosexual and 20 homosexual women rated the stimulus for masculinity and femininity. Heterosexual women were rated as more feminine and less masculine than homosexual women and homosexual men were rated as more feminine and less masculine than heterosexual men. Sexual orientation of the judges did not affect the ratings. The results were discussed with a reference to evolutionary and cultural influences affecting sexual orientation judgment accuracy.     

Patterns of Sexual Arousal in Homosexual,Bisexual, and Heterosexual Men

The purpose of this study was to determine if self-identified bisexual, heterosexual, and homosexual men show differential genital and subjective arousal patterns to video presentations of bisexual, heterosexual, male homosexual, and lesbian sexual interactions. It was predicted that, relative to heterosexual and homosexual stimuli, bisexual men would show the highest levels of sexual arousal to bisexual erotic material, while this stimulus would induce relatively low levels of response in heterosexual and homosexual men. A sample of 59 men (19 homosexual, 13 bisexual, and 27 heterosexual) were presented with a series of 4-min sexual videos while their genital and subjective sexual responses were measured continuously. Bisexual men did not differ significantly in their responses to male homosexual stimuli (depicting men engaging in sex) from homosexual men, and they did not differ significantly in their responses to heterosexual (depicting two women, without same-sex contact, engaged in sex with a man) and lesbian (depicting women engaging in sex) stimuli from heterosexual men. However, bisexual men displayed significantly higher levels of both genital and subjective sexual arousal to a bisexual stimulus (depicting a man engaged in sex with both a man and a woman) than either homosexual or heterosexual men. The findings of this study indicate that bisexuality in men is associated with a unique and specific pattern of sexual arousal.     

Shape Differences Between the Faces of Homosexual and Heterosexual Men

Previous studies have shown that homosexual men differ from heterosexual men in several somatic traits and lay people accurately attribute sexual orientation based on facial images. Thus, we may predict that morphological differences between faces of homosexual and heterosexual individuals can cue to sexual orientation. The main aim of this study was to test for possible differences in facial shape between heterosexual and homosexual men. Further, we tested whether self-reported sexual orientation correlated with sexual orientation and masculinity–femininity attributed from facial images by independent raters. In Study 1, we used geometric morphometrics to test for differences in facial shape between homosexual and heterosexual men. The analysis revealed significant shape differences in faces of heterosexual and homosexual men. Homosexual men showed relatively wider and shorter faces, smaller and shorter noses, and rather massive and more rounded jaws, resulting in a mosaic of both feminine and masculine features. In Study 2, we tested the accuracy of sexual orientation judgment from standardized facial photos which were assessed by 80 independent raters. Binary logistic regression showed no effect of attributed sexual orientation on self-reported sexual orientation. However, homosexual men were rated as more masculine than heterosexual men, which may explain the misjudgment of sexual orientation. Thus, our results showed that differences in facial morphology of homosexual and heterosexual men do not simply mirror variation in femininity, and the stereotypic association of feminine looking men as homosexual may confound judgments of sexual orientation.     

Sexual orientation and performance on sexually dimorphic motor tasks

Performance of homosexual and heterosexual men and women on two motor tasks which reliably demonstrate sex differences in opposite directions was examined. A Throw-to-Target Task and the Purdue Pegboard were administered to undergraduate students matched for age and program of study. A two-way ANOVA (Sex × Sexual Orientation) of the Throw data showed a significant interaction,F(1, 90) = 16.22,p 0.001, and a trend for an effect of sex,F(1, 90) = 3.72,p < 0.06. Heterosexual men outperformed heterosexual women, whereas gay men threw less accurately and lesbians tended to throw more accurately than their heterosexual counterparts. Differences in sports history or hand strength did not account for these effects. Analysis of the Pegboard scores found no interaction or main effect of sexual orientation, but the effect of sex was significant,F(1,63) = 7.01,p 0.02. Regardless of sexual orientation, women outperformed men and this difference remained significant even when a measure of finger size was partialed out. This study provides new evidence suggesting an association between sexual orientation and motor-performance profiles. As with cognitive tasks, the motor-performance profiles of homosexuals are composites of some male-typical and some female-typical abilities. To the extent that sociological factors have been controlled, the study suggests that both sexual orientation and motor/cognitive predispositions may have early biological contributions.This research was supported by a grant from the Medical Research Council, Ottawa, to D. Kimura.     

Implicit Sexual Associations in Heterosexual and Homosexual Women and Men

Patterns of genital arousal to sexual stimuli are somewhat different between men and women. Heterosexual males and homosexual males show clear category specific arousal that is consistent with their self-reported sexual preference. However, heterosexual women do not show this category specificity. In the present study, we attempted to measure a person’s automatic appraisals of stimuli with respect to the concept of sex via the use of implicit measures (the Implicit Association Test and the priming task). In three experiments, we showed that heterosexual females did not show a sex-related category specific response in favor of male versus female stimuli. However, this lack of specificity was not due to a lack of sex-related appraisals, but by equal appraisals of both male and female stimuli. On the other hand, heterosexual men, homosexual men, and homosexual women all showed automatic sex-related appraisals of stimuli that were category specific and in line with their self-reported sexual preference. The study shows difference in the pattern of sexual interest between genders at the earliest stages of the evaluation of a stimulus.     

Corpus Callosum Anatomy in Right-Handed Homosexual and Heterosexual Men

The results of several studies have shown that homosexual men have an increased prevalence of non-right-handedness and atypical patterns of hemispheric functional asymmetry. Non-right-handedness in men has been associated with increased size of the corpus callosum (CC), particularly of the isthmus, which is the posterior region of the callosal body connecting parietotemporal cortical regions. We hypothesized that isthmal area would be greater in homosexual men, even among right handers. Twelve homosexual and ten heterosexual healthy young men, all consistently right-handed, underwent a research-designed magnetic resonance imaging scan. We found that the isthmal area was larger in the homosexual group, adding to the body of findings of structural brain differences between homosexual and heterosexual men. This result suggests that right-handed homosexual men have less marked functional asymmetry compared to right-handed heterosexual men. The results also indicate that callosal anatomy and laterality for motoric functions are dissociated in homosexual men. A logistic regression analysis to predict sexual orientation category correctly classified 21 of the 22 men (96% correct classification) based on area of the callosal isthmus, a left-hand performance measure, water level test score, and a measure of abstraction ability. Our findings indicate that neuroanatomical structure and cognition are associated with sexual orientation in men and support the hypothesis of a neurobiological basis in the origin of sexual orientation.     

A Comparative Analysis of Functional Cerebral Asymmetry in Lesbian Women, Heterosexual Women, and Heterosexual Men

This study tested dichotic listening performance, a measure of functional cerebral asymmetry previously shown to be sexually dimorphic, in homosexual women (n=23), heterosexual women (n=27), and heterosexual men (n=24). All participants completed a verbal consonant-vowel dichotic listening task. On three laterality indices, heterosexual men displayed a significantly greater right-ear advantage compared to heterosexual women only. Homosexual women did not differ significantly from heterosexual men or from heterosexual women. Post-hoc comparisons (with a statistical correction) for each ear separately showed that heterosexual men displayed significantly greater right-ear scores compared to heterosexual women only. These data suggest that lesbian women are somewhat masculinized in their functional cerebral asymmetry. The results were interpreted in the context of sexual variation in models of linguistic hemispheric processing.     

The Effects of Sex,Sexual Orientation,and Digit Ratio (2D:4D) on Mental Rotation Performance

In spite of the reduced level of experimental control, this large scale study brought some clarity into the relation between mental rotation task (MRT) performance and a number of variables where contradictory associations had previously been reported in the literature. Clear sex differences in MRT were observed for a sample of 134,317 men and 120,783 women, with men outperforming women. There were also MRT differences as a function of sexual orientation: heterosexual men performed better than homosexual men and homosexual women performed better than heterosexual women. Although bisexual men performed better than homosexual men but less well than heterosexual men, no significant differences were observed between bisexual and homosexual women. MRT performance in both men and women peaked in the 20-30 year range, and declined significantly and markedly thereafter. Both men and women showed a significant negative correlation between left and right digit finger ratio and MRT scores, such that individuals with smaller digit ratios (relatively longer ring finger than index finger) performed better than individuals with larger digit ratios.     

Hand Asymmetry in Heterosexual and Homosexual Men and Women: Relationship to 2D:4D Digit Ratios and Other Sexually Dimorphic Anatomical Traits

Sexual differentiation leads to the development of distinctive anatomical structures (e.g., gonads and genitalia); it also produces less obvious anatomical shifts in brain, bones, muscles, etc. This study is a retrospective analysis of growth patterns in the hands in relation to sex and sexual orientation. Using data from three published studies, we analyzed four hand traits in adults: hand width, hand length, second digit length, and fourth digit length. Using these measurements, we derived estimates of trait laterality (directional asymmetry or DA) and developmental instability (fluctuating asymmetry or FA). High FA is a putative indicator of interference with the cellular and molecular mechanisms regulating development. We focused on how these derived variables were related to sex, sexual orientation, and putative markers of early sex steroid exposure (e.g., the second to fourth digit ratio or 2D:4D). Our data point to three principal conclusions. First, individual differences in DA appeared to be a major source of variation in the 2D:4D ratio. The 2D:4D ratios of heterosexual men differed depending on whether they had leftward or rightward DA in their digits. Homosexual women showed the same pattern. Individuals with leftward DA in both digits had lower 2D:4D ratios than those with rightward DA. This effect was absent in heterosexual women and homosexual men. This led to sex differences in 2D:4D and sexual orientation differences in 2D:4D in the leftward DA group, but not in the rightward DA group. The second conclusion was that DA in digit length and hand width varied with sex; women were more likely to have rightward asymmetry than men. Homosexual men and women were generally sex typical in DA. The third conclusion was that homosexuality is unlikely to be a result of increased developmental instability. Although limited in scope, the present evidence actually suggests that homosexuals have lower FA than heterosexuals, raising the question of whether the positive fitness components associated with low FA may contribute to selection that maintains homosexuality in a population.     

Dermatoglyphics, Handedness, Sex, and Sexual Orientation

Both handedness and dermatoglyphic asymmetry reflect early, prenatal influences and both have been reported to be associated with male sexual orientation; handedness has been related to female sexual orientation as well. Neurohormonal and developmental perturbation are two competing hypothesis that attempt to explain these connections. We attempted to replicate these associations and to extend dermatoglyphic asymmetry findings to women. Dermatoglyphic directional asymmetry and fluctuating asymmetry were unrelated to sexual orientation. Homosexual women, but not homosexual men, had highly significant increases in non–right-handedness compared with same-sex heterosexual controls. Although this pattern of results does not allow resolution of the two competing models, it does lend additional support to a biological basis of sexual orientation.     

Sexual orientation and cognitive abilities

We asked whether homosexual and heterosexual men and women differ on standard measures of cognitive ability. Subjects were adults recruited for a family study of sexual orientation. They completed the Fy Scale of the California Psychological Inventory, a self-administered measure of psychological femininity, and were given subtests of the Wechsler Adult Intelligence Scale and the Primary Mental Abilities test which had been previously shown to discriminate between the sexes. We predicted that homosexual subjects would score significantly in the gender-atypical direction on the Fy scale. We also predicted that they would achieve scores on the tests of cognitive abilities significantly in the direction of the other gender. Comparisons were made between homosexual and heterosexual individuals within gender. Also, comparisons were made between heterosexual men and women to see if our results replicate differences typically found between the genders. As predicted, both homosexual men and women were strongly gender-atypical on the Fy scale relative to their heterosexual counterparts. However, neither differed from heterosexuals on any of the measures of cognitive abilities. Heterosexual men did better than women on the Primary Mental Abilities spatial relations test. We conclude that sexual orientation is not related to specific cognitive abilities.Supported in part by NIMH grant 32,170 to Dr. Pillard.     

Mate Retention Behavior of Men and Women in Heterosexual and Homosexual Relationships

Comparing the behavior of heterosexual and homosexual persons can provide insight into the origins of heterosexual sex differences in psychology. Evidence indicates that, aside from sexual partner preference, the mating psychology of homosexual men is sex-typical whereas that of homosexual women tends to be more sex-atypical. The current study examined one aspect of mating psychology, mate retention behavior, and tested whether homosexual men and women were sex-typical or sex-atypical for those mate retention tactics where heterosexual men and women differed. Men and women in heterosexual and homosexual relationships were asked to provide information regarding their partners' mate retention behavior by using the Mate Retention Inventory Questionnaire. Heterosexual men and women differed significantly for six of the 19 mate retention tactics considered. With respect to the six mate retention tactics where heterosexual sex differences existed, homosexual men behaved in a sex-typical manner for five of the tactics, whereas homosexual women behaved in a sex-atypical manner for all six tactics. We discuss the significance of these findings for explaining the origins of the mate retention behavior of heterosexual men and women. In addition, we consider what the pattern of sex-typical and sex-atypical mating psychology among homosexual men and women, respectively, suggests in regard to sex differences in the development of mating psychology and the development of homosexual persons.     

Sexual Partner Age Preferences of Homosexual and Heterosexual Men and Women

The sexual age preferences of 192 adults (equal groups of heterosexual men, heterosexual women, homosexual men, and homosexual women) were examined. Participants rated the sexual attractiveness of pictures of 15 male and 15 female faces arranged into five apparent average age categories ranging from 18 to 60 years. It was predicted that homosexual and heterosexual men would prefer younger partners of their preferred sex than would homosexual and heterosexual women and that age preference would not vary with participant age. Both predictions were supported, although homosexual women preferred older partners than expected. Results suggest that age and sex preferences develop independently.     

The Cognitive, Behavioral, and Personality Profiles of a Male Monozygotic Triplet Set Discordant for Sexual Orientation

The neurohormonal theory of sexual orientation proposes that homosexual men and homosexual women are exposed prenatally to a hormonal environment that is similar to that of the other sex. Prenatal exposure to an opposite-sex hormonal environment may lead the nervous system to develop in a manner consistent with the opposite sex. If this cross-sex exposure occurs, one prediction would be that the cognitive ability profile of homosexual men would be similar to that of heterosexual women. This study examined a set of male monozygotic triplets, aged 21 years, discordant for sexual orientation: 2 of the triplets were heterosexual, 1 was homosexual. The triplets were administered measures of 23 domains of cognitive ability, as well as measures of sexual orientation and masculinity/femininity. On the measures of cognitive ability, the triplets performed similarly, yet consistent differences were found between the 2 heterosexual triplets and the 1 homosexual cotriplet. Differences having the same pattern were found for the number of Schafer homosexuality signs on the Rorschach, and on a homosexuality scale derived from items on the Minnesota Multiphasic Personality Inventory--2 (MMPI-2). Responses from the homosexual triplet were in a more feminine direction than responses from his 2 heterosexual cotriplets on measures of masculinity-femininity, which included measures derived from Rorschach responses, the MMPI-2 Masculinity-Femininity scale, the Bem Sex Role Inventory, and the Boyhood Gender Conformity Scale. Responses to the 16 Personality Factor Questionnaire also distinguished the 1 homosexual triplet from the 2 heterosexual cotriplets. These findings support the view that the prenatal hormonal environment may have enduring effects on selected behavioral traits.     

Visuospatial Performance on an Internet Line Judgment Task and Potential Hormonal Markers: Sex,Sexual Orientation,and 2D:4D

We investigated whether performance on a visuospatial line judgment task, the Judgment of Line Angle and Position-15 test (JLAP-15), showed evidence of sensitivity to early sex steroid exposure by examining how it related to sex, as well as to sexual orientation and 2D:4D digit ratios. Participants were drawn from a large Internet study with over 250,000 participants. In the main sample (ages 12–58 years), males outperformed females on the JLAP-15, showing a moderate effect size for sex. In agreement with a prenatal sex hormone hypothesis, line judgment accuracy in adults related to 2D:4D and sexual orientation, both of which are postulated to be influenced by early steroids. In both sexes, better visuospatial performance was associated with lower (more male-typical) digit ratios. For men, heterosexual participants outperformed homosexual/bisexual participants on the JLAP-15 and, for women, homosexual/bisexual participants outperformed heterosexual participants. In children aged 8–10 years, presumed to be a largely prepubertal group, boys also outperformed girls. These findings are consistent with the hypothesis that visuospatial ability is influenced by early sex steroids, although they do not rule out alternative explanations or additional influences. More broadly, such results support a prenatal sex hormone hypothesis that degree of androgen exposure may influence the neural circuitry underlying cognition (visuospatial ability) and sexual orientation as well as aspects of somatic (digit ratio) development.     

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.     

Gender-Related Traits of Heterosexual and Homosexual Men and Women

Two studies investigated the relation between sexual orientation and gender-related traits. Analyzing data from an Internet survey, Study 1 found that gay men and lesbians differed from same-sex heterosexuals most strongly on gender diagnosticity (GD) measures, which assess male- versus female-typicality of occupational preferences (effect sizes were 1.14 for men and 0.53 for women) and least strongly on instrumentality (I) and expressiveness (E). Study 2 found that GD measures showed large differences between 289 gay and 200 heterosexual men (d = 0.95) and between 296 lesbian and 435 heterosexual women (d = 1.32), whereas I and E showed much smaller differences. In Study 2 homosexual–heterosexual diagnosticity measures, computed from men's and women's occupational preferences, correlated very strongly with GD measures (r = 0.88 for men and 0.89 for women), indicating that occupational preference items that distinguished men from women also tended to distinguish heterosexual from homosexual individuals. LISREL 8 analyses showed that self-ascribed masculinity–femininity did not mediate the strong relation between sexual orientation and GD for men or for women.