Number of older brothers and sexual orientation: new tests and the attraction/behavior distinction in two national probability samples

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.

Are Autism Spectrum Disorder and Asexuality Connected?

Asexuality is a lack of sexual attraction to any gender. There is some evidence to suggest that many self-identified asexuals have a formal diagnosis of autism spectrum disorder which is characterized by deficits in social interaction and communication, as well as by restricted and repetitive interests and behaviors. Additionally, the literature shows that asexuality and lack of sexual attraction or low sexual interest is overrepresented in people with autism spectrum disorder compared with neurotypical samples. Nevertheless, no studies have been conducted to investigate the relationship between autism and asexuality in depth. We conducted a systematic review of the literature to examine whether asexuality and autism spectrum disorder are connected. We conclude that asexuality and autism share various aspects, such as a possible role of prenatal factors, reference to romantic dimensions of sexual attraction and sexual orientation, and non-partner-oriented sexual desire, but future research should explore and clarify this link.

Infertility and same-sex attraction in women

A polygyny hypothesis of female sexual fluidity proposes that women may have been evolutionarily selected to be sexually fluid, in order to have occasional sex with their cowives in polygynous marriage to reduce tension and conflict inherent in such marriage, while at the same time reproducing children with their husbands. Among others, the hypothesis predicts that women who are biologically (but not surgically) infertile would experience greater same-sex attraction. Biological infertility stems from natural, evolutionarily familiar causes such as menopause, whereas surgical infertility stems from artificial, evolutionarily novel causes such as tubal ligation or hysterectomy. Consistent with the prediction, the analyses of the National Survey of Family Growth data showed that biological infertility, but not surgical infertility, was significantly associated with same-sex self-identified labels, behavior and sexual attraction in women. Biological infertility nearly doubled the odds of women having engaged in same-sex behavior and the number of same-sex partners in the last 12 months and nearly tripled the number of same-sex partners in life. In sharp contrast, biological infertility was not associated (and surgical infertility was significantly negatively associated) with same-sex attraction in men.

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.

A short review of biological research on the development of sexual orientation

We review research supporting biological mechanisms in the development of sexual orientation. This research includes studies on neural correlates, prenatal hormones and related physical/behavioral correlates, genetics, and the fraternal birth order effect (FBOE). These studies, taken together, have provided substantial support for biological influences underlying the development of sexual orientation, but questions remain unanswered, including how biological mechanisms may differ in contributing to men's and women's sexual orientation development.

Fraternal Birth Order, Only-Child Status, and Sibling Sex Ratio Related to Sexual Orientation in the Add Health Data: A Re-analysis and Extended Findings

The fraternal birth order (FBO) effect related to men's sexual orientation refers to the finding that the number of older brothers that a man has increases his chance of being androphilic. The FBO effect has generally been well replicated in diverse samples; one instance of non-replication was by Francis (2008) using Waves I and III of the Add Health data. We attempted to replicate the FBO effect in the Add Health data taking into account family size and other limitations of Francis' (2008) analyses. Also, we examined other sibling characteristics related to the FBO effect: sibling sex ratio and only-child status. We used two subsamples from Waves I (n = 20,745) and IV (n = 15,701) of the Add Health data, consisting of adolescents who were followed longitudinally from 1994 to 1995 until 2008. Wave I data were used to compute numbers of younger and older brothers and sisters from household roster information. Wave IV information about sexual orientation identity was used. Analyses were conducted within men and within women. We found modest support for the FBO effect in men, but not in women, using the older brother odds ratio, logistic regression analyses, and sibling sex ratio, which provided the strongest support for FBO. We found that gynephilic/biphilic women, but not androphilic/biphilic men, were more likely to be only-children compared to androphilic women and gynephilic men, respectively. We discuss limitations of the Add Health data and purported mechanisms for the FBO effect in men and the only-child effect in women.

Father absence, sociosexual orientation, and same-sex sexuality in women and men

A recent evolutionary theory of female sexual fluidity suggests that women may not have sexual orientations in the same sense that men do, and that women's apparent sexual orientation may instead be a byproduct of their sociosexual orientation. One developmental factor that has consistently been shown to influence sociosexual orientation is father absence in childhood. Consistent with the prediction of the theory, the analyses of the National Longitudinal Study of Adolescent to Adult Health (Add Health) data show that father absence significantly increases women's, but not men's, same-sex sexuality in adulthood, whether it is measured by self identity, sexual behaviour, or romantic attraction. Further consistent with the theory, the association between father absence and same-sex sexuality in women is entirely mediated by their sociosexual orientation.

Personality and Sexual Orientation: Extension to Asexuality and the HEXACO Model

Previous research has examined self-identified sexual orientation in relation to self-reports on the Big Five personality factors. Here we extend this research by including asexuality as a fourth sexual orientation and by assessing the HEXACO personality factors, using self-report data from a large anonymous online sample (N ≈ 100,000). A large overlap was observed among all sexual orientation groups on all six HEXACO dimensions, but notable group differences also emerged. All nonheterosexual groups, particularly bisexual people, averaged higher in openness to experience than did heterosexuals. Heterosexual men averaged much lower in emotionality than heterosexual women, but only a small difference was observed between gay men and lesbians on this dimension. Asexual men and women averaged lower in extraversion and in some facets of emotionality (specifically, dependence and sentimentality) than did the other sexual orientation groups of the same sex. Results are discussed in relation to the different social realities that each sexual orientation group often experiences, along with the (gender-related) developmental influences hypothesized to underlie in part the origins of sexual orientation.

Gender Nonconformity and Birth Order in Relation to Anal Sex Role Among Gay Men

Androphilia is associated with an elevated number of older brothers among natal males. This association, termed the fraternal birth order effect, has been observed among gay men who exhibit marked gender nonconformity. Gender nonconformity has been linked to gay men's preferred anal sex role. The present study investigated whether these two lines of research intersect by addressing whether the fraternal birth order effect was associated with both gender nonconformity and a receptive anal sex role (243 gay men, 91 heterosexual men). Consistent with previous research, we identified the fraternal birth order effect in our sample of gay men. Also, gay men were significantly more gender-nonconforming on adulthood and recalled childhood measures compared to heterosexual men. When gay men were compared based on anal sex role (i.e., top, versatile, bottom), all groups showed significantly greater recalled childhood and adult male gender nonconformity than heterosexual men, but bottoms were most nonconforming. Only gay men with a bottom anal sex role showed evidence of a fraternal birth order effect. A sororal birth order effect was found in our sample of gay men, driven by versatiles. No significant associations were found between fraternal birth order and gender nonconformity measures. These results suggest that the fraternal birth order effect may apply to a subset of gay men who have a bottom anal sex role preference and that this subgroup is more gender-nonconforming. However, there were no significant associations between fraternal birth order and gender nonconformity at the individual level. As such, based on the present study, whether processes underpinning the fraternal birth order effect influence gender nonconformity is equivocal.

Asexuality: Sexual Orientation, Paraphilia, Sexual Dysfunction, or None of the Above?

Although lack of sexual attraction was first quantified by Kinsey, large-scale and systematic research on the prevalence and correlates of asexuality has only emerged over the past decade. Several theories have been posited to account for the nature of asexuality. The goal of this review was to consider the evidence for whether asexuality is best classified as a psychiatric syndrome (or a symptom of one), a sexual dysfunction, or a paraphilia. Based on the available science, we believe there is not sufficient evidence to support the categorization of asexuality as a psychiatric condition (or symptom of one) or as a disorder of sexual desire. There is some evidence that a subset of self-identified asexuals have a paraphilia. We also considered evidence supporting the classification of asexuality as a unique sexual orientation. We conclude that asexuality is a heterogeneous entity that likely meets conditions for a sexual orientation, and that researchers should further explore evidence for such a categorization.

Possible evolutionary origins of human female sexual fluidity

I propose an evolutionary theory of human female sexual fluidity and argue that women may have been evolutionarily designed to be sexually fluid in order to allow them to have sex with their cowives in polygynous marriage and thus reduce conflict and tension inherent in such marriage. In addition to providing an extensive definition and operationalization of the concept of sexual fluidity and specifying its ultimate function for women, the proposed theory can potentially solve several theoretical and empirical puzzles in evolutionary psychology and sex research. Analyses of the National Longitudinal Study of Adolescent Health (Add Health) confirm the theory's predictions that: (i) women (but not men) who experience increased levels of sexual fluidity have a larger number of children (suggesting that female sexual fluidity, if heritable, may be evolutionarily selected); (ii) women (but not men) who experience marriage or parenthood early in adult life subsequently experience increased levels of sexual fluidity; and (iii) sexual fluidity is significantly positively correlated with known markers of unrestricted sexual orientation among women whereas it is significantly negatively correlated with such markers among men.

Asexuality: what it is and why it matters

In this review article, human asexuality, a relatively understudied phenomenon, is discussed. Specifically, definitions and conceptualizations of asexuality (e.g., is it a unique category of sexual orientation?), biological and historical contexts, identity issues, discrimination against asexual people relative to other minorities, origins, and variations, including gender differences, are reviewed. Whether asexuality should be construed as a disorder is also discussed. The study of asexuality allows for a better understanding of an underrecognized sexual minority but also affords a unique opportunity to examine and better understand human sexuality.

Asexuality: A Multidimensional Approach

While lack of sexual attraction, lack of sexual behavior, and self-identification as asexual have been used as criteria to define asexuality, it is not known how much they overlap in describing the same group of people. This study aimed to assess how many individuals could be identified as asexual based on each of these criteria and on combinations of these criteria. Participants were recruited through the Asexuality Visibility and Education Network, social media, and posts on several health- and lifestyle-related websites. In total, 566 participants between 18 and 72 years old (M = 27.86, SD = 10.53) completed an online survey (24% male, 68.9% female, 7.1% "other"). Based on self-identification or lack of sexual attraction, 71.3% and 69.2%, respectively, of participants were categorized as asexual, while based on lack of sexual behavior only 48.5% were categorized as asexual. Gender differences were found only for those participants who indicated that they did not experience sexual attraction, with more women (72.8%) than men (58.8%) indicating a lack of sexual attraction. Given that self-identification as asexual implies familiarity with the term asexual, we argue for the use of lack of sexual attraction as the primary criterion to define asexuality.

Birth order and sibling sex ratio of children and adolescents referred to a gender identity service

In adult male samples, homosexuality is associated with a preponderance of older brothers (i.e., the fraternal birth order effect). In several studies comparing gender dysphoric youth, who are likely to be homosexual in adulthood, to clinical or non-clinical control groups, the findings have been consistent with the fraternal birth order effect in males; however, less is known about unique sibship characteristics of gender dysphoric females. The current study investigated birth order and sibling sex ratio in a large sample of children and adolescents referred to the same Gender Identity Service (N = 768). Probands were classified as heterosexual males, homosexual males, or homosexual females based on clinical diagnostic information. Groups differed significantly in age and sibship size, and homosexual females were significantly more likely to be only children. Subsequent analyses controlled for age and for sibship size. Compared to heterosexual males, homosexual males had a significant preponderance of older brothers and homosexual females had a significant preponderance of older sisters. Similarly, the older sibling sex ratio of homosexual males showed a significant excess of brothers whereas that of homosexual females showed a significant excess of sisters. Like previous studies of gender dysphoric youth and adults, these findings were consistent with the fraternal birth order effect. In addition, the greater frequency of only children and elevated numbers of older sisters among the homosexual female group adds to a small literature on sibship characteristics of potential relevance to the development of gender identity and sexual orientation in females.

Physical size and sexual orientation: Analysis of the Chinese health and family life survey

In the present study, the relationship between physical size and sexual orientation was examined. Participants were men and women comprising the Chinese Health and Family Life Survey (N > 3,500), which employed a national probability sample from China. This survey is important because no research has examined these issues in a representative, non-Western sample. Participants completed self-report measures of height, weight, and sexual orientation. Some evidence was found that men with same-sex inclinations were significantly shorter than heterosexual men. The evidence that women with same-sex inclinations were significantly taller and heavier than heterosexual women was equivocal. The results add modest support to some prior research suggesting that men with same-sex inclinations have a different pattern of growth and development relative to heterosexual comparisons.

Asexuality: few facts, many questions

Although there has been increasing interest in asexuality during the past decade, still little is known on this topic. To define asexuality, three different approaches have been proposed: a definition that is based on sexual behavior, one on sexual desire/sexual attraction, one on self-identification, and one on a combination of these. Depending on the definition used, reported prevalence rates range from 0.6% to 5.5%. In this article, the authors present characteristics of asexuality and review biological, psychological, and sociodemographic factors associated with asexuality. Given the suggestion of existing overlap with hypoactive sexual desire disorder, special attention is paid to similarities and differences between this condition and asexuality. It is further noted that theoretical models to understand (the etiology) of asexuality are underdeveloped.

Interfamily Conflict, Reproductive Success, and the Evolution of Male Homosexuality

In a preindustrial context, the mate choices of women are controlled by their parents, who require evidence of wealth and of ability to generate wealth from prospective sons-in-law in order to grant access to their daughters’ reproductive capacity. As a consequence, male siblings have to compete among themselves for controlling family resources required for successful reproduction. This competition increases interfamily conflict, decreasing each of the siblings’ inclusive fitness. Moreover, the sharing of resources among brothers reduces the chances that each one will be able to gain access to a high-quality female and/or to mate polygynously. It is proposed that male homosexuality has evolved to increase the reproductive success of male siblings and to decrease interfamily conflict. That is, by having a homosexual orientation, a younger male sibling increases his inclusive fitness by reducing competition over resources, whereas his older brothers enjoy higher reproductive success by having more resources available for reproduction. Finally, the reproductive costs of this sexual orientation are kept low by the elevated probability that a homosexual man will find himself in an arranged marriage where he is likely to have children.

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.

Male sexual orientation in independent samoa: evidence for fraternal birth order and maternal fecundity effects

In Western cultures, male androphiles tend to have greater numbers of older brothers than male gynephiles (i.e., the fraternal birth order effect). In the non-Western nation of Independent Samoa, androphilic males (known locally as fa'afafine) have been shown to have greater numbers of older brothers, older sisters, and younger brothers (Vasey & VanderLaan, 2007). It is unclear, however, whether the observed older brother effect, in the context of the additional sibling category effects, represented a genuine fraternal birth order effect or was simply associated with elevated maternal fecundity. To differentiate between these two possibilities, this study employed a larger, independent replication sample of fa'afafine and gynephilic males from Independent Samoa. Fa'afafine had greater numbers of older brothers and sisters. The replication sample and the sample from Vasey and VanderLaan were then combined, facilitating a comparison that showed the older brother effect was significantly greater in magnitude than the older sister effect. These results suggest that fraternal birth order and maternal fecundity effects both exist in Samoa. The existence of these effects cross-culturally is discussed in the context of biological theories for the development of male androphilia.

Sexual orientation, fraternal birth order, and the maternal immune hypothesis: a review

In 1996, psychologists Ray Blanchard and Anthony Bogaert found evidence that gay men have a greater number of older brothers than do heterosexual men. This "fraternal birth order" (FBO) effect has been replicated numerous times, including in non-Western samples. More recently, strong evidence has been found that the FBO effect is of prenatal origin. Although there is no direct support for the exact prenatal mechanism, the most plausible explanation may be immunological in origin, i.e., a mother develops an immune reaction against a substance important in male fetal development during pregnancy, and that this immune effect becomes increasingly likely with each male gestation. This immune effect is hypothesized to cause an alteration in (some) later born males' prenatal brain development. The target of the immune response may be molecules (i.e., Y-linked proteins) on the surface of male fetal brain cells, including in sites of the anterior hypothalamus, which has been linked to sexual orientation in other research. Antibodies might bind to these molecules and thus alter their role in typical sexual differentiation, leading some later born males to be attracted to men as opposed to women. Here we review evidence in favor of this hypothesis, including recent research showing that mothers of boys develop an immune response to one Y-linked protein (i.e., H-Y antigen; SMCY) important in male fetal development, and that this immune effect becomes increasingly likely with each additional boy to which a mother gives birth. We also discuss other Y-linked proteins that may be relevant if this hypothesis is correct. Finally, we discuss issues in testing the maternal immune hypothesis of FBO.

Physical development and sexual orientation in men and women: an analysis of NATSAL-2000

In the present study, three physical development characteristics-weight, height, and age of menarche-were examined for their relation to sexual orientation. Participants were men and women comprising the National Survey of Sexual Attitudes and Lifestyles-2000 (N > 11,000). Participants completed self-report measures of sexual orientation, height, weight, and, for women, age of menarche. Results indicated that gay/bisexual men were significantly shorter and lighter than heterosexual men. There were no significant differences between lesbians and heterosexual women in height, weight, and age of puberty. The results add to literature suggesting that, relative to heterosexual men, gay/bisexual men may have different patterns of growth and development because of early biological influences (e.g., exposure to atypical levels of androgens prenatally). However, the present results do not support a number of studies suggesting that lesbian/bisexual women are taller and heavier than heterosexual women.

Correlates of same-sex sexuality in heterosexually identified young adults

Currently, little is known about heterosexually identified individuals who nonetheless acknowledge same-sex interests. To address this shortcoming, the prevalence of same-sex attractions, fantasies, and experiences among heterosexually identified college students was examined, as well as differences between those who are exclusively heterosexual in their interests and those who are nonexclusive because they report some same-sex attractions or fantasies. Students (N = 243) at a large, Northeastern university completed an online survey providing information about their sexual orientation identity; same- and other-sex attractions, fantasies, and behaviors; and demographic, sexual history, and sexual attitudes variables. Compared to exclusive heterosexual women, nonexclusive women were more liberal in their political and sexual attitudes and had greater sexual experience. Nonexclusive men were virtually indistinguishable from exclusive heterosexual peers on assessed variables. Results are discussed in terms of implications for the nature and meaning of non-heterosexuality in contemporary Western society.

Toward a Conceptual Understanding of Asexuality

Asexuality has been the subject of recent academic ( A. F. Bogaert, 2004 ) and public (e.g., New Scientist; CNN) discourse. This has raised questions about the conceptualization and definition of asexuality. Here the author reviews some of these issues, discusses asexuality from a sexual orientation point of view (i.e., as a lack of sexual attraction), and reviews the similarities and differences between this definition and related phenomena (e.g., hypoactive sexual desire disorder). Finally, the author concludes that the term asexuality should not necessarily be used to describe a pathological or health-compromised state.

Biological versus nonbiological older brothers and men's sexual orientation

The most consistent biodemographic correlate of sexual orientation in men is the number of older brothers (fraternal birth order). The mechanism underlying this effect remains unknown. In this article, I provide a direct test pitting prenatal against postnatal (e.g., social/rearing) mechanisms. Four samples of homosexual and heterosexual men (total n = 944), including one sample of men raised in nonbiological and blended families (e.g., raised with half- or step-siblings or as adoptees) were studied. Only biological older brothers, and not any other sibling characteristic, including nonbiological older brothers, predicted men's sexual orientation, regardless of the amount of time reared with these siblings. These results strongly suggest a prenatal origin to the fraternal birth-order effect.

Interaction of fraternal birth order and handedness in the development of male homosexuality

The present study investigated evidence for an interaction between two of the best established etiologic factors, or markers of etiologic factors, in the literature on male homosexuality: fraternal birth order and hand preference. By combining five samples, the authors produced study groups of 1774 right-handed heterosexuals, 287 non-right-handed heterosexuals, 928 right-handed homosexuals, and 157 non-right-handed homosexuals. The results showed a significant (P = 0.004) handedness by older brothers interaction, such that (a) the typical positive correlation between homosexuality and greater numbers of older brothers holds only for right-handed males, (b) among men with no older brothers, homosexuals are more likely to be non-right-handed than heterosexuals; among men with one or more older brothers, homosexuals are less likely to be non-right-handed than heterosexuals, and (c) the odds of homosexuality are higher for men who have a non-right hand preference or who have older brothers, relative to men with neither of these features, but the odds for men with both features are similar to the odds for men with neither. These findings have at least two possible explanations: (a) the etiologic factors associated with non-right-handedness and older brothers-hypothesized to be hyperandrogenization and anti-male antibodies, respectively-counteract each other, yielding the functional equivalent of typical masculinization, and (b) the number of non-right-handed homosexuals with older brothers is smaller than expected because the combination of the older brothers factor with the non-right-handedness factor is toxic enough to lower the probability that the affected fetus will survive.

Sibling sex ratio and sexual orientation in men and women: new tests in two national probability samples

One line of research on the etiology of sexual orientation has examined sibling sex ratio, the ratio of brothers to sisters collectively reported by a group of individuals, but this research has only used clinical and/or convenience samples. In the present study, homosexual men and women's sibling sex ratio was examined in two national probability samples. Results indicated that homosexual men had a sex ratio of 129.54 male live births to 100 female live births. This ratio was within the range of elevated sex ratios found in some previous studies of homosexual men, although it was only marginally significant (p = .09) relative to the known human sex ratio with regard to live births. Additional analyses indicated that this effect was likely the result of a high fraternal birth order (i.e., an elevated number of older brothers) in homosexual men. The sibling sex ratio for lesbians was 122.58 male live births to 100 female live births, which did not significantly differ from the known human sex ratio with regard to live births. The results for lesbians, however, should be interpreted with caution because the sample size (and resulting power) was low. The results in men add to research suggesting that homosexual men, unselected for gender identity or gender role behavior, do not have elevated sibling sex ratios. These results also suggest that research should concentrate on finding the cause(s) of the fraternal birth order effect, the consistent finding that homosexual men have an elevated number of older brothers.

The prevalence of male homosexuality: the effect of fraternal birth order and variations in family size

The prevalence of male homosexuality probably varies over time and across societies. One reason for this variation may be the joint effect of two factors: (1) variations in fertility rate or family size; and (2) the fraternal birth order effect, the finding that the odds of male homosexuality increases with each additional older brother. Because of these effects, the rate of male homosexuality may be relatively high (at least in terms of sexual attraction if not behavior) in societies that have a high fertility rate, but this rate has probably declined somewhat in some, particularly western, societies. Thus, even if accurately measured in one country at one time, the rate of male homosexuality is subject to change and is not generalizable over time or across societies.

Asexuality: prevalence and associated factors in a national probability sample

I used data from a national probability sample (N > 18,000) of British residents to investigate asexuality, defined as having no sexual attraction to a partner of either sex. Approximately 1% (n = 195) of the sample indicated they were asexual. A number of factors were related to asexuality, including gender (i.e., more women than men), short stature, low education, low socioeconomic status, and poor health. Asexual women also had a later onset of menarche relative to sexual women. The results suggest that a number of pathways, both biological and psychosocial, contribute to the development of asexuality.

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.