Humans differ in paternal investment—the degree to which fathers help mothers care for their offspring. They differ in this way between individuals, between populations, and between stages of cultural evolution. During the earliest stage, when all humans were hunter-gatherers, men invested more in their offspring with increasing distance from the equator. Longer, colder winters made it harder for women to gather food for themselves and their children. They had to rely on meat from their hunting spouses. Conversely, paternal investment was lower in the tropics, where women could gather food year-round and provide for themselves and their children with little