D.F. Swaab

Alfred Kinsey didn’t attract any notice when he published his doctoral thesis on gall wasps. But in 1948, when he produced the report Sexual Behavior in the Human Male and then, five years later, Sexual Behavior in the Human Female, he became a celebrity. He devised the “Kinsey scale,” which went from 0 to 6, 0 signifying exclusively heterosexual and 6 exclusively homosexual. Being bisexual, he himself would have been classified as a “Kinsey 3.”
A person’s position on the scale is determined in the womb by his or her genetic background and the effects of hormones and other substances on the developing brain. Studies of twins and families show that sexual orientation is 50 percent genetically determined, but the genes in question haven’t yet been identified.

It is curious that a genetic predisposition for homosexuality should persist in populations over the course of evolution, given that this group reproduces so much less. One explanation for why homosexuality persists is that the involved genes don’t just increase the likelihood of homosexuality but also promote fertility in the rest of the family. Heterosexual individuals with the same genes produce a larger than average number of offspring, causing the genes to remain in circulation.

ll the research indicates that our sexual orientation is programmed in the brain before birth, determining it for the rest of our lives