Disorders affecting penis development are among the most common birth defects seen in humans, and rates are on the rise.
There are many cultural and social factors involved in making a baby into a man or a woman. But biologically speaking, sex starts when you're just a tiny group of cells in your mother's uterus.
We have the penis of a monogamous primate yet our body sizes suggest our ancestors slept around a lot.
Men with Klinefelter’s syndrome are infertile, as the extra X chromosome affects the ability to produce sperm.