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From some basic googling, I found that nobody has ever proven that people are born gay and that environment plays a great part in homosexuality.

I wish to know if there is a genetic component to homosexual behaviour. If so, how significant is that component (in some measure such as percentage contribution to the behaviour). What are other factors that play a role in homosexuality i.e. what constitutes environmental factors?

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This is not an area I know well, but I'm familiar with a couple of studies that have tried to estimate the heritable (genetic) component of homosexuality in humans. A review paper by Rice et al (2012) points out that:

Pedigree and twin studies indicate that homosexuality has substantial heritability in both sexes, yet concordance between identical twins is low and molecular studies have failed to find associated DNA makers.

Here is a selection of papers that should be of interest to you:

Kirk et al (2000) find a rather strong heritable component (30-60%), which also differs between males and females (stronger in females).

Abstract
Multivariate structural equation modeling techniques have been applied to examine the causes of individual differences in responses to several items concerning sexual orientation. To minimize potential ascertainment and response biases, the study sample involved a large (N = 4901) community-based cohort of Australian twins aged 18-52 who answered an anonymous questionnaire on sexual behavior and attitudes. The statistical power of the analysis was increased by the availability of multiple measures of sexual orientation (behaviors, attitudes and feelings), providing stronger evidence for the existence of additive genetic influences on this phenotype than in a previous analysis (Bailey et al., 2000). Estimates of the heritability of homosexuality in this sample ranged between 50 and 60% in females but were significantly lower (heritability of approximately 30%) in males.

Note however that is it often difficult to separate genetic and environmental components in these types of epidemiological data, especially when twins have grown up in the same environment. I'm also not in a position to judge or comment on the quality or methods of these studies.

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    $\begingroup$ Be careful about the term 'environment' in these studies. In the breeders equation, which is the model used in most twin studies, "Non-shared environment" is just a miscellaneous bin used to account for all the variance not explained by the other terms. It therefore includes measurement error, developmental noise, etc etc. So even if environment as we usually think of it plays zero role, and it's all chance events during embryonic development, 'non-shared environment' will suck up that variance. $\endgroup$ – Dermot Harnett Apr 27 '16 at 16:03
  • $\begingroup$ There is a 'genetic component' to homosexual behavior in that genes do predict it to some extent. They don't determine it though. As usual it's an interaction between many factors, most of which we don't understand. $\endgroup$ – Dermot Harnett Apr 27 '16 at 16:03
  • $\begingroup$ For those you are not familiar with the definition of heritability ($h=\frac{V_A}{V_P}$), you might want to have a look at this post. $\endgroup$ – Remi.b Apr 27 '16 at 16:35
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I doubt that there is a gene or group thereof directly responsible for homosexuality.

Rather, if there is genetic correlation, it would be something like, a gene which makes it more likely for the mother to not produce enough sex hormones during pregnancy, or for the fetus's brain to react differently or not at all to them. Other causes of an unusual chemical composition of the womb are malnutrition in the mother or environmental causes.

And thus the tendency of the brain of a male to be aroused by female forms, or vice versa, is limited, and this makes it more likely, but not pre-determined, for homosexuality to develop.

(Educated guess)

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  • $\begingroup$ Please add some references to your answer. $\endgroup$ – another 'Homo sapien' Mar 24 '17 at 14:10

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