It’s common knowledge: new parents don’t get much sleep because of crying babies. It’s just a fact of life. But why do the babies cry in the first place? David Haig, an evolutionary biologist at Harvard University, claims that it came about as a way for babies to stop their mothers from having another baby! It’s just a theory, but no one seems to be willing to step up and claim otherwise.

Not only would a crying baby or extreme fatigue get in the way of sex for the mother, but frequent night nursing can also delay the fertility of the nursing woman. Additionally, babies seem to cry more often six months after birth, the time when a baby would need its own form of birth control the most, according to Haig.

If true, this would have developed through natural selection a long time ago. In the days before any real medical care, a baby would need the full attention of his or her mother to have the best chance to survive. Another baby would take away from that.

This would also mean that babies and their mothers have completely opposite evolutionary goals. Mothers would want to have the most possible babies in order to increase the likelihood of some surviving, while babies don’t want their mothers to have any more kids.

Not everyone is on board with the theory, however. Although no one has outright denounced the theory, some have commented with other possible causes. Anthropologist James McKenna says there are plenty of other reasons. Babies can get too hot, or hungry, or they could just want a cuddle from mom. Frequently waking prevents the baby from slipping into too deep a sleep, which can be dangerous, as well.

We may never know for sure if Haig’s theory is true, but it is certainly an interesting thought. Evolution is capable of some pretty amazing things.