“Overall, genetics accounted for around 35 per cent of the differences between men in homosexual behavior and other individual-specific environmental factors (that is, not societal attitudes, family or parenting which are shared by twins) accounted for around 64 per cent. In other words, men become gay or straight because of different developmental pathways, not just one pathway.”

For women, genetics explained roughly 18 per cent of the variation in same-sex behavior, non-shared environment roughly 64 per cent and shared factors, or the family environment, explained 16 per cent.

The study shows that genetic influences are important but modest, and that non-shared environmental factors, which may include factors operating during fetal development, dominate.