Last updated November 10, 2017 at 4:54 pm
Other factors are undoubtedly at play in determining the sexual orientation of individuals. There’s evidence, for example, of developmental factors from the order of birth in boys to the sex hormones one is exposed to during development in the womb.
One effect dubbed the “fraternal birth order effect” or sometimes the “older brother effect” derives from studies which show that, the greater the number of older brothers a man has, the more likely he is to be gay. By some accounts, for every older brother a male child has, there is a 33 per cent greater chance of the male child being homosexual. That assumes that the progression is linear and it should be noted that other studies suggest that the first one or two brothers only has a small effect, growing rapidly with three or more brothers.
But why should this be? As it doesn’t appear to apply in the case of adopted siblings, it seems unlikely to be due to the social influence of the older brothers.
Hormonal processes in the womb have been suggested as one possible explanation.
Canadian researcher Ray Blanchard, the leader of the team which identified the effect, has hypothesised that the older brothers have an effect on the younger sibling, not directly, but through influencing the mother’s immune system. This effect, Blanchard and his colleagues suggest, sees the mother immunised against male-specific antigens in earlier pregnancies, leading to an effect on later ones.
But there are significant question marks over the research. Other studies have not been in complete agreement with the findings of the Toronto group. LeVay, in his comprehensive 2016 review of the literature, Gay Straight and the Reasons Why, points to one Northwestern University study of 1,700 gay and straight subjects that did find a strong correlation between the number of older brothers and sexual orientation. But it also found that the gay men also had more older and younger sisters – finding replicated in a later British study.
LeVay makes the further point that family sizes have changed dramatically over the years, which should have an impact on the overall percentage of the population that is same-sex oriented. But that rate has remained remarkable stable. He acknowledges the effect, which he says needs further research, suggesting the mechanism may work through a more specific effect on the brain circuitry that is responsible for sexual orientation.
There is some crossover into endocrinology in terms of sex hormones. There is strong evidence that prenatal exposure to different levels of testosterone can masculinise the foetal brain and influence male-typical behaviour. Regarding INAH 3 differences noted from the earlier mentioned neuroscience study, there is a possibility that it would be a marker of a downstream effect from sex hormones during development.
Sexual differentiation in the brain and the genitals occur during different stages of development. It is therefore possible for each to be influenced independently.
“Sex differences in cognition, gender identity (an individual’s perception of their own sexual identity), sexual orientation (heterosexuality, homosexuality or bisexuality), and the risks of developing neuropsychiatric disorders are programmed into our brain during early development,” state the authors of one review paper about sexual differentiation of the brain.
This brings up the important area of gender identity within sexuality. This further shows the complexity between sexuality, that it’s more of a spectrum of varying degrees rather than defined categories.
“Once we stop viewing sexuality through the lens of preconceived categories, it’s pretty clear that sexuality is a very complex trait,” clarifies Dr Thomas Conway.