Biology and sexual orientation

The relationship between biology and sexual orientation is a subject of ongoing research. While scientists do not know the exact cause of sexual orientation, they theorize that it is caused by a complex interplay of genetic, hormonal, and environmental influences. However, evidence is weak for hypotheses that the postnatal social environment impacts sexual orientation, especially for males.

Biological theories for explaining the causes of sexual orientation are favored by scientists. These factors, which may be related to the development of a sexual orientation, include genes, the early uterine environment (such as prenatal hormones), and brain structure. While the evolutionary explanation for heterosexuality in organisms that reproduce sexually is straightforwardly understood to be a psychological adaptation resulting from greater reproductive success, evolutionary explanations for homosexuality rely upon other mechanisms of evolution such as kin selection and inclusive fitness, or antagonistic pleiotropy that favors heterozygotes causing homosexuality among homozygotes as a by-product.

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