Testosterone, which induces sexual differentiation of the male fetus, is believed to transfer from males to their littermates in placental mammals. Among humans, individuals with a male twin have been found to exhibit greater masculinization of sexually dimorphic attributes relative to those with a female twin. We therefore regard twinning as a plausible natural experiment to test the link between prenatal exposure to testosterone and labor market earnings. For men, the results suggest positive returns to testosterone exposure. For women, however, the results indicate that prenatal testosterone does not generate higher earnings and may even be associated with modest declines.