Studies of male twins have suggested that about 50 per cent of the variability of sexual orientation is due to genes. This would leave about 50 per cent due to various environmental factors. There is a growing sense that genes play a role in male sexual orientation. The evidence for a genetic contribution to female homosexuality is less well developed, but the case is hardly closed. (Juan, Stephen, PhD, U of Sydney. Whatever Happened to the Gay Gene?)

Study: Homosexuality tends to run in families. 3x greater chance of a male being gay if he has brothers, uncles, cousins, or parents (more on the mother’s side and fewer on the father’s side) who are also gay. The real percentage of gay twins who were separated at birth and who have identical genetic makeup is 60-70%, or 2 out of 3. (Pease, Barbara and Allan. Why Men Don’t Listen and Women Can’t Read Maps. p 179-186. NY: Broadway Books, 1998.)