What does science say about sexual orientation?

Sexual orientation: biology or choice?

The scientific research regarding sexual orientation has ongoing political, religious and social implications. This post highlights the specifically scientific conclusions on the causes of sexual orientation. I've structured it in a Question and Answer format with quotes from the relevant scientific literature.

In particular, a comprehensive meta-analysis study conducted by J. Michael Bailey and colleagues in the journal Psychological Science in the Public Interest in 2016 is referenced which provides a systematic review of the scientific research.

What percentage of people have a same-sex attraction?

From the 2016 meta-analysis study:

"Those with predominantly same-sex attractions comprise fewer than 5% of respondents in most Western surveys. Data from non-Western cultures are consistent with this conclusion. There is no persuasive evidence that the rate of same-sex attraction has varied much across time or place." [1]

What causes one's sexual orientation?

From the 2016 meta-analysis study:

"No causal theory of sexual orientation has yet gained widespread support. The most scientifically plausible causal hypotheses are difficult to test. However, there is considerably more evidence supporting nonsocial causes of sexual orientation than social causes. This evidence includes the cross-culturally robust finding that adult homosexuality is strongly related to childhood gender nonconformity; moderate genetic influences demonstrated in well-sampled twin studies; the cross-culturally robust fraternal-birth-order effect on male sexual orientation; and the finding that when infant boys are surgically and socially “changed” into girls, their eventual sexual orientation is unchanged (i.e., they remain sexually attracted to females). In contrast, evidence for the most commonly hypothesized social causes of homosexuality—sexual recruitment by homosexual adults, patterns of disordered parenting, or the influence of homosexual parents—is generally weak in magnitude and distorted by numerous confounding factors." [1]

To elaborate on the above nonsocial causes:

1. Childhood gender nonconformity: "Behaving like the other sex—is a strong correlate of adult sexual orientation that has been consistently and repeatedly replicated." [1]
2. Moderate genetic influences demonstrated in well-sampled twin studies: "Twin siblings of homosexual males are more likely to be homosexual than non-twin siblings." [4]
3. The cross-culturally robust fraternal-birth-order effect on male sexual orientation: "The number of biological older brothers, including those not reared with the participant (but not the number of nonbiological older brothers), increases the probability of homosexuality in men. These results provide evidence that a prenatal mechanism(s), and not social and/or rearing factors, affects men’s sexual orientation development." [4]
4. When infant boys are surgically and socially “changed” into girls, their eventual sexual orientation is unchanged (i.e., they remain sexually attracted to females): "These results comprise the most valuable currently available data concerning the broad nature-versus-nurture question for sexual orientation. They show how difficult it is to derail the development of male sexual orientation by psychosocial means." [1]

From Wikipedia:

"Biological theories for explaining the causes of sexual orientation are favored by scientists and involve a complex interplay of genetic factors, the early uterine environment and brain structure. These factors, which may be related to the development of a heterosexual, homosexual, bisexual, or asexual orientation, include genes, prenatal hormones, and brain structure." [2][3]

What are the relevant peer-reviewed scientific studies?

• The 2016 meta-analysis has a comprehensive survey of the scientific research [1].
• A summary of 19 peer-reviewed studies relevant to the question Is sexual orientation determined at birth? Of the 19 studies, 15 conclude in the affirmative, one in the negative and three are non-conclusive. [4]

Is the presence of same sex orientation in the population a Darwinian paradox?

From Australia’s Science Channel:

"Jenny Graves, professor of genetics at LaTrobe University in Melbourne, has no problem with the concept of gay genes.

'The idea that a person’s genetic makeup affects their mating preference is unsurprising,' she writes in The Conversation. 'We see it in the animal world all the time. There are probably many genes that affect human sexual orientation.'

But rather than thinking of them as 'gay genes', perhaps, she says, we should consider them “male-loving genes”.

'They may be common because these variant genes, in a female, predispose her to mate earlier and more often, and to have more children.'

Graves cites an Italian study [5] that shows female relatives of gay men having 1.3 times as many children as the female relatives of straight men. One possible explanation is that “male-loving” alleles – our gene variants – in a female they predispose her to mate earlier and have more children, so making up for the fewer children of gay males." [6]

References:

[1] "Sexual Orientation, Controversy, and Science", Psychological Science in the Public Interest, 2016.

[2] https://en.wikipedia.org/wiki/Biology_and_sexual_orientation

[3] https://en.wikipedia.org/wiki/Sexual_orientation

[4] Peer-Reviewed Studies on the Origin of Sexual Orientation Since 1990

[5] "Evidence for maternally inherited factors favouring male homosexuality and promoting female fecundity", Proceedings, Biological Sciences, 2004

[6] https://australiascience.tv/science-of-sexuality/