Why people cry: A Darwinist explanation

One theory is that crying may have evolved as a kind of signal — a signal that was valuable because it could only be picked up by those closest to us who could actually see our tears. Tears let our intimates in — people within a couple of feet of us — who would be more likely to help.

“You can imagine there’d be a selection pressure to develop a signaling system that wouldn’t let predators in on the fact that you’re vulnerable.” says Randy Cornelius, a psychologist at Vassar College.