Nature, Nurture, and Orientation
My take from Selfish Reasons to Have More Kids:
Parents have a small effect on sexual orientation. Psychologists used to label homosexuality a “mental illness” caused by overprotective mothers and distant fathers.[i] Now we tend to see sexual orientation as a preference inherent in our genes. When you look at the actual evidence from twin and adoption studies, however, neither story quite works. Genes definitely play a strong role – every major twin study finds that identical twins are more alike in their sexual orientation than fraternal twins.[ii] Yet genes are far from the whole story – if you’re gay, your identical twin is usually still straight. Furthermore, family environment clearly affects sexual orientation. The smoking gun: Adoptive brothers of gay men and adoptive sisters of gay women are about six times more likely to be gay that you would expect from chance.[iii] On balance, the evidence that parents make kids gay is a little stronger than evidence that parents make kids well-educated, rich, pious, or chaste. While heredity is the star of the story, upbringing plays a modest supporting role.
[ii] “Genetic and Environmental Influences on Sexual Orientation,” in Handbook of Behavior Genetics, p.271.