Sexual orientation-related differences in prepulse inhibition of the human startle response

Prepulse inhibition (PPI) refers to a reduction in the startle response to a strong sensory stimulus when this stimulus is preceded by a weaker stimulus--the prepulse. PPI reflects a nonlearned sensorimotor gating mechanism and also shows a robust gender difference, with women exhibiting lower PPI than men. The present study examined the eyeblink startle responses to acoustic stimuli of 59 healthy heterosexual and homosexual men and women. Homosexual women showed significantly masculinized PPI compared with heterosexual women, whereas no difference was observed in PPI between homosexual and heterosexual men. These data provide the first evidence for within-gender differences in basic sensorimotor gating mechanisms and implicate the known neural substrates of PPI in human sexual orientation.     

Familiality of female and male homosexuality

We examined data from a large cohort of homosexual and heterosexual females and males concerning their siblings' sexual orientations. As in previous studies, both male and female homosexuality were familial. Homosexual females had an excess of homosexual brothers compared to heteroxual subjects, thus providing evidence that similar familial factors influence both male and female homosexuality. Furthermore, despite the large sample size, homosexual females and males did not differ significantly from each other in their proportions of either homosexual sisters or homosexual brothers. Thus, results were most consistent with the possibility that similar familial factors influence male and female sexual orientation. However, because results conflicted with those of some other studies, and because siblings' sexual orientations were obtained in a manner likely to yield more errors than in these other, smaller studies, further work is needed using large samples and more careful methods before the degree of cofamiliality of male and female homosexuality can be resolved definitively. We also examined whether some parental influences comprised shared environmental effects on sexual orientation. Scales attempting to measure such influences failed to distinguish subjects with homosexual siblings from subjects with only heterosexual siblings and, thus, did not appear to measure shared environmental determinants of sexual orientation.     

Large sexual-orientation-related differences in performance on mental rotation and judgment of line orientation tasks

This study examined the performance of heterosexual and homosexual men and women on 2 tests of spatial processing, mental rotation (MR) and Benton Judgment of Line Orientation (JLO). The sample comprised 60 heterosexual men, 60 heterosexual women, 60 homosexual men, and 60 homosexual women. There were significant main effects of gender (men achieving higher scores overall) and Gender x Sexual Orientation interactions. Decomposing these interactions revealed large differences between the male groups in favor of heterosexual men on JLO and MR performance. There was a modest difference between the female groups on MR total correct scores in favor of homosexual women but no differences in MR percentage correct. The evidence suggests possible variations in the parietal cortex between homosexual and heterosexual persons.     

Neural correlates of sexual arousal in homosexual and heterosexual men

Men exhibit much higher levels of genital and subjective arousal to sexual stimuli containing their preferred sex than they do to stimuli containing only the nonpreferred sex. This study used event-related functional magnetic resonance imaging to investigate how this category-specific pattern would be reflected in the brains of homosexual (n = 11) and heterosexual (n = 11) men. Comparisons of activation to preferred sexual stimuli, nonpreferred sexual stimuli, and sports stimuli revealed large networks correlated with sexual arousal, spanning multiple cortical and subcortical areas. Both homosexual and heterosexual men exhibited category-specific arousal in brain activity. Within the amygdala, greater preference-related activity was observed in homosexual men, but it is unclear whether this is a cause or a consequence of their sexuality. In a subsequent analysis of regions hypothesized to support arousal, both participant groups demonstrated widespread increases in evoked activity for preferred stimuli. Aggregate data from these regions produced significant differences between stimulus types in 16 out of 22 participants. Significant activational differences matched reported sexual orientation in 15 of these 16 participants, representing an advance in psychophysiological measures of arousal.     

Additive Effects of Older Brothers and Homosexual Brothers in the Prediction of Marriage and Cohabitation

Research has shown that male homosexuality tends to cluster in families and that homosexual males have, on average, a greater number of older brothers than do heterosexual males. This study investigated whether the former, between-families effect and the latter, within-families effect are additive. The subjects were 717 full siblings over age 40 reported by 343 heterosexual and homosexual male probands examined in Southern Ontario in 1994–1995. The sibling's history of legal marriage or cohabitation in a heterosexual relationship was taken as a proxy variable for sexual orientation. There were no significant findings for the female siblings. As expected, the never-married male siblings were more likely to come from the sibships of the homosexual probands, and they had a greater average number of older brothers. A bootstrapped logistic regression analysis showed that an additive model best explained the male siblings' data. The results suggest that the familial aggregation of male homosexuality cannot be explained by the birth order effect and that older brothers and family membership reflect separate influences on sexual orientation or sexual orientation-correlated behavior.     

Gender, sexuality, body image and eating behaviours

One hundred and twenty one participants reported sexual orientation, body mass index, body shape concerns, eating motives and eating styles. Measures of body dissatisfaction were greater in heterosexual women and homosexual men (ps <.05), while heterosexual women had smaller (ps <.001) ideal body shapes. Eating weight control motive was lower in heterosexual men compared to women (ps <.05). Restrained eating was lower in heterosexual men compared to heterosexual women or homosexual men (ps <.001). The findings support the role of socially prescribed body shapes on body shape concerns, eating motivations, and eating styles in men and women and suggest impacts are greater for heterosexual women and homosexual men.     

Sexual orientation and mental health in a birth cohort of young adults

BACKGROUND: This paper sought to examine the relationship between sexual orientation and mental health in a New Zealand birth cohort studied to age 25 years. METHOD: The analysis is based on a sample of 967 participants (469 males; 498 females) in the Christchurch Health and Development Study. As part of this study information was gathered on: (a) measures of sexual orientation, same-sex behaviour and sexual attraction obtained at ages 21 and 25 years; (b) measures of mental disorders and suicidal behaviours over the interval 21-25 years; (c) measures of childhood and family background. RESULTS: Latent class analysis was used to combine indicators of sexual orientation, same sex behaviour and attraction to form an empirically based classification of sexual orientation. The best-fitting model classified the sample into three groups: exclusively heterosexual orientation (87.6%); predominantly heterosexual but with same-sex inclinations or experience (9.6%); predominantly homosexual (2.8%). Proportionately more women than men were classified as predominantly heterosexual (14.2% v. 4.8% respectively) or predominantly homosexual (3.9% v. 1.5% respectively). Cohort members with a predominantly homosexual orientation had rates of mental disorder and suicidal behaviours that were between 1.5 and 12 times higher than for those with an exclusively heterosexual orientation. These associations persisted after adjustment for confounding. The associations between sexual orientation and mental health were more marked for males than females. CONCLUSIONS. The findings suggest a continuum of sexual preferences amongst young adults. Variations in sexual orientation were clearly associated with mental health. These associations tended to be stronger for males.     

A sex difference in features that elicit genital response

Previous research suggests that women's genital arousal is an automatic response to sexual stimuli, whereas men's genital arousal is dependent upon stimulus features specific to their sexual interests. In this study, we tested the hypothesis that a nonhuman sexual stimulus would elicit a genital response in women but not in men. Eighteen heterosexual women and 18 heterosexual men viewed seven sexual film stimuli, six human films and one nonhuman primate film, while measurements of genital and subjective sexual arousal were recorded. Women showed small increases in genital arousal to the nonhuman stimulus and large increases in genital arousal to both human male and female stimuli. Men did not show any genital arousal to the nonhuman stimulus and demonstrated a category-specific pattern of arousal to the human stimuli that corresponded to their stated sexual orientation. These results suggest that stimulus features necessary to evoke genital arousal are much less specific in women than in men.     

Circum-menopausal changes in women's preferences for sexually dimorphic shape cues in peer-aged faces

Recent studies suggest that post-menopausal women demonstrate stronger preferences for feminine characteristics in male and female faces than do pre-menopausal women, potentially reflecting stronger preferences for healthy men and greater derogation of attractive women among more fertile women. A limitation of this work was that it assessed circum-menopausal women's face preferences using images of young adults only. Here, we found that post-menopausal women demonstrated stronger preferences for feminine characteristics in male and female peer-aged faces that did pre-menopausal women. These data present novel evidence for circum-menopausal variation in face perception and confirm that the circum-menopausal variation in face preferences observed previously was not an artefact of the young faces employed as stimuli.     

Are all males equal? Anatomic and functional basis for sexual orientation in males.

In non-human primates anterior hypothalamic nuclei are closely involved in heterosexual activity in males. In humans, hypothalamic nuclei which correspond to these nuclei in non-human primates have been shown to have a neuronal density in homosexual men that is approximately half of that seen in heterosexual men. In addition, homosexual men exhibit a positive luteinizing hormone response to acutely administered estrogen that is intermediate between women (flat response) and heterosexual men (exaggerated response). Furthermore, on the basis of serum testosterone concentrations in similarly feminized transsexual males on estrogen/progestogen treatment 3 distinct groups can be identified. It is postulated that anatomic differences in the anterior hypothalamic nuclei that regulate sexual orientation in males may lead to alteration in the gonadotropin releasing hormone (GnRH) pulse/frequency leading to a more female-type pattern of gonadotropin secretion in homosexual males. Based on data in transsexual males this pattern may be more or less of the female-type in subsets of male homosexuals.     

Development and familiality of sexual orientation in females

The development and familial clustering of sexual orientation were studied in 358 heterosexual, bisexual, and homosexual women. Sexual orientation, as measured by the Kinsey scales, was diverse yet showed statistical congruity and stability over a 1- to 1.5-year time span. Developmental patterns, as measured by retrospective reports on the ages of first sexual or romantic attraction and of self-acknowledgment of sexual orientation, were very similar in the heterosexual and lesbian subjects except for the difference in object choice. The bisexual subjects displayed intermediate patterns that were more similar to the heterosexuals' on most facets yet closer to the lesbian subjects' on other dimensions. Familial clustering of nonheterosexual orientation was significant. Using two criteria, elevated rates of nonheterosexuality were found in four classes of relatives: sisters, daughters, nieces, and female cousins through a paternal uncle. The current data are not sufficient to distinguish between genetic and shared environmental sources of this familial aggregation. We discuss the possibility of using developmental criteria to differentiate between inherited and cultural sources of variation in female sexual orientation.     

Proportion of homosexual men who owe their sexual orientation to fraternal birth order: An estimate based on two national probability samples

Homosexuality in men correlates with an individual's number of older brothers, greater numbers of older brothers being associated with a greater probability of homosexuality. There are reasons to believe that this relationship is causal rather than merely statistical, that is, that older brothers produce the increase in the probability of homosexuality for later‐born males. It is possible, under this assumption, to estimate the proportion of homosexual men who can attribute their sexual orientation to their birth order among their brothers (fraternal birth order). This statistic, the population attributable fraction (PAF), was computed on the combined archival data of 2,256 heterosexual and 71 homosexual men examined in survey studies of sexual behavior in the UK and the USA. The PAF was 28.6%, with 95% confidence limits of 14.8% and 48.0%. These limits encompass the PAF of 15.1% previously estimated with a Canadian sample. The results indicate that the proportion of homosexual men whose sexual orientation is attributable to fraternal birth order constitutes a minority, but not a negligible minority, of all homosexual men. The fraternal birth order effect may reflect the progressive immunization of some mothers to Y‐linked antigens by each succeeding male fetus, and the concomitantly increasing effects of antimale antibodies on the sexual differentiation of the brain in each succeeding male fetus. Am. J. Hum. Biol. 16:151–157, 2004. © 2004 Wiley‐Liss, Inc.     

Effects of the disclosure of homosexuality on heterosexual undergraduates' behaviors with a same-sex close friend and their attitudes toward homosexuality

This study investigated heterosexual undergraduates' behavior with a same-sex close friend and their attitudes toward homosexuality after this friend disclosed his/her sexual orientation. The study also examined whether the heterosexual friend was regarded as a romantic love object or not. Participants were 77 male and 139 female undergraduates. Males decreased their behaviors with their close friend and adopted more positive attitudes toward gay men after they knew their friend's sexual orientation. Females decreased their behavior with their close friend more after learning that they were a romantic love object of their friend, compared to when tehy were not. Also females adopted more positive attitudes toward lesbians only after knowing they were not a romantic love object. These gender differences are discussed.     

Gonococcal sensitivity to fecal lipids can be mediated by an Mtr-independent mechanism.

Various factors affect the sensitivity of Neisseria gonorrhoeae to physiological levels of hydrophobic molecules. A total of 98 N. gonorrhoeae strains from rectal, cervical, and urethral cultures of homosexual men and heterosexual men and women were examined for their sensitivities to fecal lipids. Isolates were characterized according to cell envelope phenotype, auxotype, and protein I serogroup. Although cell envelope phenotype was an important factor in the resistance of this organism to fecal lipids (Mtr phenotype greater than wild type greater than Env phenotype), other factors were also of importance. AHU- strains (strains having a requirement for arginine, hypoxanthine, and uracil) uniformly exhibited a wild-type envelope phenotype but were as sensitive to fecal lipids as were Env strains. The protein I serogroup was not a factor in determining the sensitivity of wild-type envelope phenotype non-AHU- strains to fecal lipids. However, sexual preference and site of isolation were important factors. Wild-type envelope phenotype (non-AHU-) strains from homosexual men and heterosexual women were more resistant to fecal lipids than were similar isolates from heterosexual men. When these strains were compared by isolation site, it was observed that rectal isolates from homosexual men and heterosexual women were more resistant than were cervical isolates from heterosexual women or urethral isolates from heterosexual men. Urethral isolates from homosexual men were also more resistant to fecal lipids than were urethral isolates from heterosexual men. These data suggest that the host environment can select for increased resistance to hydrophobic molecules by an Mtr-independent mechanism. The basis for this Mtr-independent resistance is presently unknown, but it is likely that it involves an alteration of the target site(s) for fecal lipid inhibition.     

Two HIV-1 epidemics in Argentina: different genetic subtypes associated with different risk groups

This study determined the risk behaviors and viral subtypes of HIV-1 found in 134 heterosexual HIV-seroprevalent maternity patients, 41 of their sexual partners (men who have sex with women [MSW]), and 95 homosexual men (men who have sex with men [MSM]) from Buenos Aires, Argentina. Peripheral blood mononuclear cells (PBMCs) were purified from blood and used for DNA extraction, amplification, and genotyping by the envelope heteroduplex mobility assay (env HMA).Most of the women had been infected by having sex with an already infected partner (84%), whereas most of the male partners had been infected via drug use (76%). Both the patients and their sexual partners were poorly educated, only 30% having completed secondary school. The MSM study subjects, however, were significantly better educated and had a lower prevalence of injecting drug use.Env HMA subtype F was found in 77% (103 of 134) of the maternity patients, with similar rates in their partners (73%). Most of the remaining samples were env subtype B. All but one of the couples was concordant in subtype. In the MSM risk group, however, only 10% were env HMA subtype F. Ninety percent of the MSM samples were subtype B.There are at least two independent epidemics of HIV-1 infection in Buenos Aires, Argentina. One, in heterosexual men and women, is dominated by env subtype F whereas the other, in homosexual men, is dominated by env subtype B, as determined by env HMA.     

Prevalence and correlates of HIV infection among young injection drug users in San Francisco

OBJECTIVE: We assessed the prevalence of HIV infection and associated risk behaviors among street-recruited young injection drug users (IDUs) in San Francisco. METHODS: In a cross-sectional study, 304 young (age <30 years) IDUs with a history of injecting in the previous 30 days were interviewed and tested for antibodies to HIV. Analyses assessing independent associations with HIV infection were limited to males only, due to the low number of infections in women. RESULTS: The prevalence of HIV infection was 5.3% overall but was highly stratified by gender and sexual preference (15.6% among homosexual/bisexual men vs. heterosexual men) and recruitment neighborhood (18% in the Polk Street area). Of 16 HIV infections, 14 (88%) were in males. Factors independently associated with HIV infection in males included sexual preference (homosexual/bisexual vs. heterosexual: adjusted odds ratio [AOR], 7.5; 95% confidence interval [CI], 1.5-36.6), recruitment neighborhood (Polk Street neighborhood vs. other neighborhoods: AOR, 4.8; 95% CI, 1.4-16.7), and duration of residence in San Francisco (>or=1 year vs. <1 year: AOR, 11.8; 95% CI, 1.4-95.8). CONCLUSIONS: The prevalence of HIV infection was highest among male IDUs who have sex with men. The strong associations between HIV infection and sexual orientation and HIV infection and recruitment locale suggest that risk may be attributable largely to sexual risk. In addition to successful prevention efforts aimed at reducing needle-associated risk, current intervention models aimed at young IDUs should target high-risk neighborhoods and emphasize sexual risk reduction measures, in particular among men who have sex with men.     

Men and their bodies: a comparison of homosexual and heterosexual men

The study examined influences on body satisfaction, disordered eating, and exercise behavior of a male subculture that places a heightened emphasis on appearance: the homosexual male subculture. Subjects were 71 homosexual and 71 heterosexual men. Relative to heterosexual men, homosexual men showed more body dissatisfaction and considered appearance more central to their sense of self. Also, their exercise was more motivated by a desire to improve attractiveness. Among the homosexual but not the heterosexual group, men who desired to be thinner showed more attitudes and behaviors associated with disordered eating than men who were thinner than their desired size. In contrast, heterosexual but not homosexual men who wished to be heavier had lower self-esteem scores than men who were heavier than or equal to their desired size. The findings support a view that a male subculture that emphasizes appearance may heighten the vulnerability of its members to body dissatisfaction and disordered eating.     

A specific sexual orientation-related difference in navigation strategy

During spatial navigation, women typically navigate an environment using a landmark strategy, whereas men typically use an orientation strategy. To examine the as yet unknown effects of sexual orientation on these normative sex differences, this study required 80 healthy heterosexual and homosexual adult men and women to provide directions from experimental maps for 4 routes. The frequency and type of strategy used by each participant were computed. Expected sex differences were demonstrated, and a robust cross-sex shift was shown by homosexual men in using landmarks. This remained after controlling for differences in mental rotation, directional sense, and general intelligence. The findings may limit the number of putative neurodevelopmental pathways responsible for sex differences in navigation strategy utility.     

Sexual orientation and height, weight, and age of puberty: new tests from a British national probability sample

Research using non-representative samples indicates that certain sex-dimorphic physical, developmental and behavioral variables predict sexual orientation. In this study, three sex-dimorphic physical development variables-height, weight, and age of puberty/sexual maturation-and sexual orientation were examined in a British national probability sample (N=18,876). Women with same-sex inclinations (particularly bisexuals) had an earlier first sexual experience (approx. 1.4 years) and were taller (approx. 1 cm) than heterosexual women, but the height/sexual orientation relationship was complex. Lesbian/bisexual women did not differ from heterosexual women in age at menarche or in weight. Men with same-sex inclinations (particularly bisexuals) had an earlier first sexual experience relative to heterosexual men (approx. 3 months). Homosexual men did not significantly differ from heterosexual men in height or in weight, although there was some evidence that bisexual men were taller than heterosexual men (approx. 1 cm). The results challenge some prior research on sexual orientation and physical development using non-representative samples.     

Gender differences in experiences of sexual harassment: data from a male-dominated environment

The goal of this investigation was to examine gender differences in experiences of sexual harassment during military service and the negative mental health symptoms associated with these experiences. Female (n = 2,319) and male (n = 1,627) former reservists were surveyed about sexual harassment during their military service and current mental health symptoms. As expected, women reported a higher frequency of sexual harassment. Further, women had increased odds of experiencing all subtypes of sexual harassment. Being female conferred the greatest risk for experiencing the most serious forms of harassment. For both men and women, sexual harassment was associated with more negative current mental health. However, at higher levels of harassment, associations with some negative mental health symptoms were stronger for men than women. Although preliminary, the results of this investigation suggest that although women are harassed more frequently than men, clinicians must increase their awareness of the potential for sexual harassment among men in order to provide the best possible care to all victims of harassment.