The origin of human sexual orientation is often debated as a choice, but neuroscientist Marc Breedlove reveals that the die is cast in the womb. From the length of your fingers to the number of older brothers you have, your biology carries traces of prenatal hormones and maternal immune responses that bias who you will love decades later. The discovery of specific hypothalamic structures that differ between gay and straight individuals suggests that attraction is not a social construct, but a hardwired neural reality.