A possible second type of maternal-fetal immune interaction involved in both male and female homosexuality

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.

Reassessing the Effect of Older Sisters on Sexual Orientation in Men

This research reanalyzed questionnaire data from 8279 homosexual and 79,519 heterosexual men who participated in 2005 in an internet-based research project sponsored by the British Broadcasting Corporation. It focused on parameters of sibship composition (older brothers, older sisters, younger siblings) previously shown or hypothesized to influence sexual orientation in males. The results included the usual finding that older brothers increase the odds of homosexuality in later-born males. As predicted, older sisters also increase those odds, although by a lesser amount than older brothers. Other results confirmed that the odds of homosexuality are increased in only-children, the amount of increase being equal to that produced by one older brother and greater than that produced by one older sister. Finally, the results indicated that younger siblings have no effect on the odds of homosexuality in males. These results might be explained by the hypothesis that two different types of immune responses in pregnant women can affect the future sexual orientation of their male fetuses. One type of response affects fetuses in first pregnancies and reduces subsequent fertility. The other type affects fetuses in later pregnancies and has little or no effect on fertility. Finally, we conducted an estimate of combined sibship effects. Men who were exposed to any of the influences that we identified (being an only-child or having an older sibling) had 27% greater odds of homosexuality than did subjects who were exposed to none of these influences (i.e., the first-born of two or more children).

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.

Evidence for distinct biodevelopmental influences on male sexual orientation

Several biological mechanisms have been proposed to influence male sexual orientation, but the extent to which these mechanisms cooccur is unclear. Putative markers of biological processes are often used to evaluate the biological basis of male sexual orientation, including fraternal birth order, handedness, and familiality of same-sex sexual orientation; these biomarkers are proxies for immunological, endocrine, and genetic mechanisms. Here, we used latent profile analysis (LPA) to assess whether these biomarkers cluster within the same individuals or are present in different subgroups of nonheterosexual men. LPA defined four profiles of men based on these biomarkers: 1) A subgroup who did not have these biomarkers, 2) fraternal birth order, 3) handedness, and 4) familiality. While the majority of both heterosexual and nonheterosexual men were grouped in the profile that did not have any biomarker, the three profiles associated with a biomarker were composed primarily of nonheterosexual men. We then evaluated whether these subgroups differed on measures of gender nonconformity and personality that reliably show male sexual orientation differences. The subgroup without biomarkers was the most gender-conforming whereas the fraternal birth order subgroup was the most female-typical and agreeable, compared with the other profiles. Together, these findings suggest there are multiple distinct biodevelopmental pathways influencing same-sex sexual orientation in men.

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.

Gay Male Only-Children: Evidence for Low Birth Weight and High Maternal Miscarriage Rates

Recent findings suggest that there may be a maternal immune response underpinning the etiology of sexual orientation of gay male only-children. This maternal immune response appears to be distinct from that which is purported to explain the classic fraternal birth order effect found in studies of male sexual orientation. We tested two predictions related to the hypothesized maternal immune response in mothers of gay male only-children: (1) elevated fetal loss among mothers who have had gay male only-children and (2) lower birth weight in gay male only-children. Mothers of at least one gay son (n = 54) and mothers of heterosexual son(s) (n = 72) self-reported their pregnancy histories, including the birth weights of newborns and number of fetal losses (e.g., miscarriages). Mothers of gay male only-children (n = 8) reported significantly greater fetal loss compared with mothers of males with four other sibship compositions (gay with no older brothers, gay with older brothers, heterosexual only-children, heterosexual with siblings) (n = 118). Also, firstborn gay male only-children (n = 4) had a significantly lower birth weight than firstborn children in the four other sibship compositions (n = 59). Duration of pregnancy was not significantly different among the groups of firstborn children in the birth weight analyses. Thus, this study found further support for a distinct pattern of maternal immune response implicated in the etiology of male sexual orientation. Mechanisms that may underlie this potential second type of maternal immune response are discussed.

Birth weight and two possible types of maternal effects on male sexual orientation: a clinical study of children and adolescents referred to a Gender Identity Service

This study tested predictions regarding two hypothesized maternal immune responses influencing sexual orientation: one affecting homosexual males with high fraternal birth order and another affecting firstborn homosexual individuals whose mothers experience repeated miscarriage after the birth of the first child. Low birth weight was treated as a marker of possible exposure to a maternal immune response during gestation. Birth weight was examined relative to sibship characteristics in a clinical sample of youth (N = 1,722) classified as heterosexual or homosexual based on self-reported or probable sexual orientation. No female sexual orientation differences in birth weight were found. Homosexual, compared to heterosexual, males showed lower birth weight if they had one or more older brothers--and especially two or more older brothers--or if they were an only-child. These findings support the existence of two maternal immune responses influencing male sexual orientation and possibly also cross-gender behavior and identity.

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.