Gay identical twins

In a study where scientists looked at the sexual arousal patterns of identical twins with different sexualities—specifically, where one was gay and the other was straight—they found that gay twins demonstrated more genital arousal in response to same-sex images, whereas straight twins demonstrated more arousal in response to opposite-sex.

Identical twins resulting from splitting occurring before day five are free to develop their own sexual orientation. If one twin has a same-sex orientation, there is a 30% chance that the second twin will have this same identity.