A newly published study suggests sexual orientation in males is influenced by prenatal development.
Blanchard, R. (2004). Quantitative and theoretical analyses of the relation between older brother and homosexuality in men. Journal of Theoretical Biology, 230, 173-187.
ABSTRACT
Meta-analysis of aggregate data from 14 samples representing 10,143 male subjects shows that homosexuality in human males is predicted by higher numbers of older brothers, but not by higher numbers of older sisters, younger brothers, or younger sisters. The relation between number of older brothers and sexual orientation holds only for males. This phenomenon has therefore been called the fraternal birth order effect. Research on birth order, birth weight, and sexual orientation suggests that the developmental pathway to homosexuality initiated by older brothers operates during prenatal life. Calculations assuming a causal relation between older brothers and sexual orientation have estimated the proportion of homosexual men who owe their sexual orientation to fraternal birth order at 15 percent in one study and 29 percent in another.
The maternal immune hypothesis proposes that the fraternal birth order effect reflects the progressive immunization of some mothers to male-specific antigens by each succeeding male fetus and the increasing effects of such immunization on sexual differentiation of the brain in each succeeding male fetus. There are at least three possible mechanisms by which the mother’s immune response could influence the fetus:
the transfer of anti-male antibodies across the placenta from the maternal into the fetal compartment, the transfer of maternal cytokines across the placenta, and maternal immune reactions affecting the placenta itself. This hypothesis is consistent with recent studies showing that the quantity of fetal cells that enter the maternal circulation is greater than previously thought, and that the number of male-specific proteins encoded by Y-chromosome genes is greater than previously thought.
I haven’t read the study, so I don’t know how the sampling of 10,000 men was defined or derived.
And as we’ve seen many times before, no one study is conclusive.
The abstract makes the study sound much more impressive than it really is. The author was actually very careful to say that the 10,000-odd men were not actually what was sampled. Here’s why, from the article:
The core of the above-mentioned research program consists of 14 samples of homosexual and heterosexual males from 12 published studies (Table 1). These samples comprise 10,143 subjects (3181 homosexuals and 6962 heterosexuals) examined in Canada, the Netherlands, the UK, and the USA. The sources of information were archival data from several previous studies as well as data collected by the writer and his colleagues in Toronto. The combination of new and old data meant that some subjects were examined in recent years and others were examined decades ago; for similar reasons, their years of birth ranged from 1861 to 1989. The samples include psychiatric patients and non-patient volunteers, subjects examined in adulthood and subjects examined in childhood, men who wished to become women and men contented with their male role and anatomy, and men sexually attracted to adults as well as men attracted to children. Thus, with the exception of race, which was Caucasian for the great majority of subjects, they represent an extremely diverse collection of males.
…
The 14 samples were divided into 28 groups (14 heterosexual and 14 homosexual). The sample sizes of these groups are shown in Table 1. As explained below, however, sample sizes were not taken into account in the statistical analyses.
The data points were four measurements taken from each group: the number of older brothers per subject, older sisters per subject, younger brothers per subject, and younger sisters per subject. An example of a single data point would be the number of older brothers per subject for the homosexual males interviewed by Alfred Kinsey and his colleagues from 1938 to 1963 (see Table 1). Thus, the aggregate data consisted of 112 data points (28 groups×4 measurements) rather than 40,572 data points (10,143 subjects×4 measurements).
The numbers weren’t weighted by sample size of the study, and the author doesn’t account for why he doesn’t do this. It gets worse. Fully half of the sampled studies drew their population from pedophiles, hebephiles, sex offenders, and/or gender-dysphoric/transsexual men. One other study was of psychiatric patients. These may not be the only shortcomings, but just a quick glance reveals these major flaws.
Again, not having read the study, it sounds to me like a classic meta-analysis – an attempt to get over the statistical problems in a whole lot of smaller studies by combining them. Thus weighting results may not have been the best method – the authors are trying to treat each male in each sub-study as a distinct individual – as if they were participating in a new study. The whole reason to do a meta-analysis is to get large data samples, and they certainly did – even with the sub-studies based on clearly non-representative samples (pedophiles, psychiatric patients – although remember when a lot of these kinds of studies were being done, Kinsey for one example, being gay was still considered a mental illness and you could be institutionalized for it against your will) the sheer volume of overall data makes the finding very “robust” as they say in the literature.
More importantly, this finding is one that has been identified several times – and it is replication and repetition that makes a scientific theory.
This kind of thing isn’t exactly new, is it? I recall reading something along these lines a few years ago. Is the study just being published? (In other words, did the authors finally get some publisher to publish it for them?)
Another explanation could be that some boys growing up with older brothers gets more insight in the heteronormative society earlier and life and as a result forms another sexual identity.
I’m not necessarily saying that I believe this, I’m just trying to say that one can find explanations from statistical material such as this that fits one’s world view.