A Novel Method for Studying the Effect of Older Brothers on Sexual Orientation and Its Robustness to Stopping Rule Distortions

Birth Order and Family Size of UK Biobank Subjects Identified as Asexual, Bisexual, Heterosexual, or Homosexual According to Self-Reported Sexual Histories

This study used a recently developed statistical technique to investigate the relations between various elements of a subject's family background and the odds of that subject reporting a sexual history indicative of a minority sexual orientation. The subjects were 78,983 men and 92,150 women who completed relevant questionnaire items in the UK Biobank, a large-scale biomedical database of volunteers aged 40-69 years. The men were divided into three sexual minority groups-homosexual, bisexual, and asexual-and a comparison group of heterosexual men. The same was done for the women. The analytic procedure consisted of logistic regressions specifically designed to disentangle the effects of birth order and family size. The results showed that older brothers increased the odds of homosexuality in both men and women, and that older sisters increased the odds in men. In contrast, neither older brothers or older sisters affected the odds of bisexuality or asexuality in men or in women. These results suggest that birth order effects may be specific to homosexuality and not common to all minority orientations. The only family size finding was the negative association between family size and the odds of asexuality in both men and women. The outcomes of this study indicate that the maternal immune hypothesis, which was advanced to explain the relation between older brothers and homosexuality in later-born males, might have to be abandoned or else expanded to explain the findings concerning females. A few such modifications are considered.

Examining the Fraternal Birth Order Effect and Sexual Orientation: Insights from an East European Population

The development of human sexual orientation remains a complex and multifaceted subject. It is often studied but its origins continue to elude us. In this preregistered study, our primary objective was to demonstrate the fraternal birth order effect (FBOE), which assumes a higher prevalence of older brothers in gay men than in their straight counterparts and which has also been recently recorded in lesbian women. Our second aim was to explore any potential impact of the FBOE on anal-erotic role orientation (AERO), both in gay and straight men. Our study sample included 693 gay men, 843 straight men, 265 lesbian women, and 331 straight women from Czechia and Slovakia. Employing a conventionally parameterized logistic regression model, we substantiated the FBOE among both gay men (OR = 1.35 for maternal older brothers) and lesbian women (OR = 1.71). These outcomes were confirmed by a more nuanced parameterization recently proposed by Blanchard (2022). Nonmaternal older brothers did not exhibit a significant influence on their younger brothers' sexual orientation. Contrary to some earlier reports, however, our data did not establish the FBOE as exclusive to gay men with the receptive AERO. Furthermore, our observations indicated a lower offspring count for mothers of gay men compared to mothers of straight men. Emphasizing the positive FBOE outcomes, we acknowledge the need for caution regarding the various options that can be used to estimate the familial influences on sexual orientation.

Does the Fraternal Birth Order Effect Influence Handedness?

The fraternal birth order effect (FBOE) is the phenomenon whereby the probability that a man has a same-sex sexual orientation in adulthood increases with each biological older brother. Several studies have found evidence that the FBOE is limited to right-handed men, and left-handed men do not show an FBOE. Recent debates about the appropriate methods for quantifying the FBOE center on distinguishing the FBOE from other effects, such as the female fecundity effect (FFE), whereby mothers more prone to bearing gay sons are also more fecund. The FBOE and FFE are confounded in that a real FFE will result in data consistent with the FBOE under some analyses. Here, we applied some recent proposed analytic methods for the FBOE to the property of handedness. A straightforward application of Khovanova's technique to the binary trait of handedness yielded support for a fraternal birth order effect consistent with the maternal immune hypothesis, in that the ratios of handedness differed between men with one older brother only, and men with one younger brother only, while no such effect was seen in women. This effect was not seen, however, when the confounding effects of parental age were controlled for. Models including factors to simultaneously test multiple posited effects find significant female fecundity effects, as well as paternal age and birth order effects on handedness in men, but no FBOE. The effects seen in women were different, with no fecundity or parental age effects, but birth order and sex of older siblings had effects. We conclude, based on this evidence, that many of the factors thought to contribute to sexual orientation in men may also have an influence on handedness, and further note that parental age is a potential confound which may be overlooked by some analyses of the FBOE.

Male androphilia, fraternal birth order, and female fecundity in Samoa: A 10-y retrospective

Two separate but related literatures have examined familial correlates of male androphilia (i.e., sexual attraction and arousal to masculine adult males). The fraternal birth order effect (FBOE) is a widely established finding that each biological older brother a male has increased the probability of androphilia 20-35% above baseline rates. Other family demographic variables, such as reproduction by mothers, maternal aunts, and grandmothers, have been used to test evolutionary hypotheses that sexually antagonistic genes lead to androphilia among males, lowering or eliminating reproduction, which is offset by greater reproductive output among their female relatives. These proposed female fecundity effects (FFEs), and the FBOE, have historically been treated as separate yet complementary ways to understand the development and evolution of male androphilia. However, this approach ignores a vital confound within the data. The high overall reproductive output indicative of an FFE results in similar statistical patterns as the FBOE, wherein women with high reproductive output subsequently produce later-born androphilic sons. Thus, examination of the FBOE requires analytic approaches capable of controlling for the FFE, and vice-versa. Here, we present data simultaneously examining the FBOE and FFE for male androphilia in a large dataset collected in Samoa across 10 y of fieldwork, which only shows evidence of the FBOE.

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.

New Data on Birth Order in Homosexual Men and Women and a Reply to Vilsmeier et al. (2021a, 2021b)

The fraternal birth order effect (FBOE) is the repeated finding that older brothers increase the odds of homosexuality in later-born males. It has been our working assumption, based on the majority of previous studies, that a similar FBOE does not occur in females. In an elaborate quantitative review posted last year to a preprint server, Vilsmeier et al. (2021a) concluded that there is no valid evidence for an FBOE in men or women. Ablaza et al. (2022) subsequently published a study of population-level data from the Netherlands with conclusions completely opposite to those of Vilsmeier et al., namely, that there is robust evidence of an FBOE in both men and women. The present research was initially undertaken to refute the assertion of Vilsmeier et al. that there is no proof of an FBOE in men and to investigate how they obtained such a discrepant conclusion. We found evidence that the discrepancy may relate to Vilsmeier et al.'s use of the large and demonstrably unreliable sample published by Frisch and Hviid (2006). After the publication by Ablaza et al., we expanded our article to address their finding of an FBOE in women. We argue that our preferred explanation of the FBOE in men-that it reflects the progressive immunization of some mothers to Y-linked antigen by each succeeding male fetus and the concomitantly increasing effects of anti-male antibody on sexual differentiation in the brain in each succeeding male fetus-could plausibly be extended to female homosexuality.