Sexual differentiation in humans development can only occur when the fetal testis secretes key hormones at a critical period in early gestation. The testes begin to secrete three hormones that influence the male internal and external genitalia: they secrete anti-müllerian hormone (AMH), testosterone, and dihydrotestosterone (DHT). Anti-müllerian hormone causes the paramesonephric ducts to regress. Testosterone converts the mesonephric ducts into male accessory structures, including the epididymis, vas deferens, and seminal vesicle. Testosterone will also control the descending of the testes from the abdomen into the scrotum. Many other genes found on other autosomes, including"WT-1", "SOX9", "SF-1" also play a role in gonadal development. Females:
Fraternal birth order and male sexual orientation sexual orientation via the fraternal birth order effect (for example, homosexual men with no older brothers) owe their homosexuality to other causes such as certain polymorphic genes (colloquially called "gay genes") or prenatal hormone levels during fetal development. Both direct and indirect evidence have demonstrated that the mechanism by which the fraternal birth order effect operates is prenatal in timing and biological in nature, rather than postnatal in timing or psychosocial in nature. Indirect evidence was found first when it was discovered that the fraternal birth order effect interacts with birth weight. Then, Bogaert (2006) produced direct evidence that the