Khytam Dawood

Sexual Orientation and the Second to Fourth Finger Length Ratio: A Meta-Analysis in Men and Women

The ratio of the lengths of the second and fourth fingers (2D:4D) may serve as a marker for prenatal androgen signaling. Because people are typically unaware of their 2D:4D, its use allows possible effects of early sex hormone regimes and socialization to be disentangled. We conducted a meta-analysis on relationships between 2D:4D and sexual orientation in men and women in 18 independent samples of men and 16 independent samples of women. Collectively, these samples comprised 1,618 heterosexual men, 1,693 heterosexual women, 1,503 gay men, and 1,014 lesbians. In addition to identifying the normative heterosexual sex difference in 2D:4D for both hands, we found that heterosexual women had higher (more feminine) left- and right-hand 2D:4D than did lesbians, but we found no difference between heterosexual and gay men. Moderator analyses suggested that ethnicity explained some between-studies variation in men. These results add to a literature suggesting that early sex hormone signaling affects sexual orientation in women, and highlight the need for further research exploring the relationships among 2D:4D, sexual orientation, and ethnicity in men.

Genetic and environmental influences on the frequency of orgasm in women.

This study reports on genetic and environmental influences on the frequency of orgasm in women during sexual intercourse, during other sexual contact with a partner, and during masturbation. Participants were drawn from the Australian Twin Registry, and recruited from a large, partly longitudinal twin-family study. Three thousand and eighty women responded to the anonymous self-report questionnaire, including 667 complete monozygotic (MZ) pairs and 377 complete dizygotic (DZ) same-sex pairs, 366 women from complete DZ opposite-sex pairs, and 626 women whose co-twins did not participate. Significant twin correlations were found for both MZ and DZ twin pairs for all three items of interest. Age effects were statistically significant for some items. Models incorporating additive genetic, shared and nonshared environmental influences provided the best fit for Items 1 and 3, while a model with additive and nonadditive genetic influences along with nonshared environment fitted the data from Item 2. While an independent pathway model fits the data most par-simoniously, a common pathway model incorporating additive genetic (A), shared environment (C), and unique environment (E) effects cannot be ruled out. Overall, genetic influences account for approximately 31% of the variance of frequency of orgasm during sexual intercourse, 37% of the variance of frequency of orgasm during sexual contact other than during intercourse, and 51% of the variance of frequency of orgasm during masturbation. Following Baker (1996), we speculate that this additive genetic variance might arise from frequency-dependent selection for a variety of female sexual strategies.

A family history study of male sexual orientation using three independent samples

Available evidence suggests that male homosexuality is both familial and somewhat heritable and that some cases may be caused by an X-linked gene. However, most studies have recruited subjects in a relatively unsystematic manner, typically via advertisements, and hence suffer from the potential methodological flaw of ascertainment bias due to volunteer self-selection. In the present study we assessed the familiality of male homosexuality using two carefully ascertained samples and attempted to replicate findings consistent with X-linkage in three samples. The percentage of siblings of the probands rated as either homosexual or bisexual, with a high degree of certainty, ranged from 7 to 10% for brothers and 3 to 4% for sisters. These estimates are higher than recent comparable population-based estimates of homosexuality, supporting the importance of familial factors for male homosexuality. Estimates of λs for male homosexuality ranged from 3.0 to 4.0. None of the samples showed a significantly greater proportion of maternal than paternal homosexual uncles or homosexual male maternal first cousins. Although our results differed significantly with those of some prior studies, they do not exclude the possibility of moderate X-linkage for male sexual orientation.

Women's attractiveness changes with estradiol and progesterone across the ovulatory cycle

In many species, females are more sexually attractive to males near ovulation. Some evidence suggests a similar pattern in humans, but methodological limitations prohibit firm conclusions at present, and information on physiological mechanisms underlying any such pattern is lacking. In 202 normally-cycling women, we explored whether womens attractiveness changed over the cycle as a function of two likely candidates for mediating these changes: estradiol and progesterone. We scheduled women to attend one session during the late follicular phase and another during the mid-luteal phase. At each session, facial photographs, voice recordings and saliva samples were collected. All photographs and voice recordings were subsequently rated by men for attractiveness and by women for flirtatiousness and attractiveness to men. Saliva samples were assayed for estradiol and progesterone. We found that progesterone and its interaction with estradiol negatively predicted vocal attractiveness and overall (facial plus vocal) attractiveness to men. Progesterone also negatively predicted womens facial attractiveness to men and female-rated facial attractiveness, facial flirtatiousness and vocal attractiveness, but not female-rated vocal flirtatiousness. These results strongly suggest a pattern of increased attractiveness during peak fertility in the menstrual cycle and implicate estradiol and progesterone in driving these changes.

Why Women Have Orgasms: An Evolutionary Analysis

Whether women’s orgasm is an adaptation is arguably the most contentious question in the study of the evolution of human sexuality. Indeed, this question is a veritable litmus test for adaptationism, separating those profoundly impressed with the pervasive and myriad correspondences between organisms’ phenotypes and their conditions of life from those who apply the “onerous concept” of adaptation with more caution, skepticism or suspicion. Yet, the adaptedness of female orgasm is a question whose answer will elucidate mating dynamics in humans and nonhuman primates. There are two broad competing explanations for the evolution of orgasm in women: (1) the mate-choice hypothesis, which states that female orgasm has evolved to function in mate selection and (2) the byproduct hypothesis, which states that female orgasm has no evolutionary function, existing only because women share some early ontogeny with men, in whom orgasm is an adaptation. We review evidence for these hypotheses and identify areas where relevant evidence is lacking. Although additional research is needed before firm conclusions can be drawn, we find that the mate-choice hypothesis receives more support. Specifically, female orgasm appears to have evolved to increase the probability of fertilization from males whose genes would improve offspring fitness.

Sexual selection on male vocal fundamental frequency in humans and other anthropoids

In many primates, including humans, the vocalizations of males and females differ dramatically, with male vocalizations and vocal anatomy often seeming to exaggerate apparent body size. These traits may be favoured by sexual selection because low-frequency male vocalizations intimidate rivals and/or attract females, but this hypothesis has not been systematically tested across primates, nor is it clear why competitors and potential mates should attend to vocalization frequencies. Here we show across anthropoids that sexual dimorphism in fundamental frequency (F0) increased during evolutionary transitions towards polygyny, and decreased during transitions towards monogamy. Surprisingly, humans exhibit greater F0 sexual dimorphism than any other ape. We also show that low-F0 vocalizations predict perceptions of mens dominance and attractiveness, and predict hormone profiles (low cortisol and high testosterone) related to immune function. These results suggest that low male F0 signals condition to competitors and mates, and evolved in male anthropoids in response to the intensity of mating competition.

Genetic and Environmental Influences on Sexual Orientation

The primary focus of this chapter is to provide an overview of the evidence to date on the quantitative genetics of sexual orientation, including family and twin studies. The bulk of the available evidence suggests moderate heritability for male sexual orientation. Female sexual orientation has been studied much less extensively, but current studies are consistent with a genetic contribution for women as well (Kirk, Bailey, Dunne, & Martin, 2000; Pattatucci & Hamer, 1995). Familial aggregation has been reported in several family studies of both male and female homosexuality (Dawood & Bailey, 2000), although the genetic and environmental influences on this familial clustering have not been clearly defined by the largest twin studies published thus far, which have produced contradictory results. Recent molecular genetic studies will also be reviewed, including the two main strategies that have been used to date – linkage and association analysis. We will also discuss the implications of recent advances in molecular genetic studies.

The face of female dominance: Women with dominant faces have lower cortisol

The human face displays a wealth of information, including information about dominance and fecundity. Dominance and fecundity are also associated with lower concentrations of the stress hormone cortisol, suggesting that cortisol may negatively predict facial dominance and attractiveness. We digitally photographed 61 womens faces, had these images rated by men and women for dominance, attractiveness, and femininity, and explored relationships between these perceptions and womens salivary cortisol concentrations. In a first study, we found that women with more dominant-appearing, but not more attractive, faces had lower cortisol levels. These associations were not due to age, ethnicity, time since waking, testosterone, or its interaction with cortisol. In a second study, composite images of women with low cortisol were perceived as more dominant than those of women with high cortisol significantly more often than chance by two samples of viewers, with a similar but non-significant trend in a third sample. However, data on perceptions of attractiveness were mixed; low-cortisol images were viewed as more attractive by two samples of US viewers and as less attractive by a sample of Mexican viewers. Our results suggest that having a more dominant-appearing face may be associated with lower stress and hence lower cortisol in women, and provide further evidence regarding the information content of the human face.

Genome-Wide Association Study of Male Sexual Orientation

Family and twin studies suggest that genes play a role in male sexual orientation. We conducted a genome-wide association study (GWAS) of male sexual orientation on a primarily European ancestry sample of 1,077 homosexual men and 1,231 heterosexual men using Affymetrix single nucleotide polymorphism (SNP) arrays. We identified several SNPs with p < 10−5, including regions of multiple supporting SNPs on chromosomes 13 (minimum p = 7.5 × 10−7) and 14 (p = 4.7 × 10−7). The genes nearest to these peaks have functions plausibly relevant to the development of sexual orientation. On chromosome 13, SLITRK6 is a neurodevelopmental gene mostly expressed in the diencephalon, which contains a region previously reported as differing in size in men by sexual orientation. On chromosome 14, TSHR genetic variants in intron 1 could conceivably help explain past findings relating familial atypical thyroid function and male homosexuality. Furthermore, skewed X chromosome inactivation has been found in the thyroid condition, Graves’ disease, as well as in mothers of homosexual men. On pericentromeric chromosome 8 within our previously reported linkage peak, we found support (p = 4.1 × 10−3) for a SNP association previously reported (rs77013977, p = 7.1 × 10−8), with the combined analysis yielding p = 6.7 × 10−9, i.e., a genome-wide significant association.

The Genetics of Human Sexual Orientation

The primary focus of this chapter is on the evidence to date regarding the population genetics of sexual orientation. The bulk of the available evidence suggests moderate heritability for male sexual orientation. Female sexual orientation has been studied much less extensively, but recent studies are consistent with a genetic contribution for women as well (Pattatucci & Hamer, 1995). Familial aggregation has been observed in nuclear family studies of both male and female homosexuality, although the genetic and environmental contributions to this familial clustering have not been resolved by the main twin studies published thus far, which have yielded somewhat contradictory results. Recent molecular genetics studies will also be reviewed, including the two main strategies that have been employed so far - linkage and association analysis.