Yawn! Pass it on.

Ever spy your neighbor yawning in math class and then find yourself following suit? Turns out, yawning chimpanzees can get fellow chimps hooked on yawning too.

Scientists at Japan's Primate Research Institute wanted to find out whether yawning is as contagious for nonhuman primates (animal group that includes humans and monkeys) as it is for adult humans. They showed six adult chimpanzees videos of yawning and nonyawning chimps. During and immediately after the yawning videos, the chimps yawned more than twice as often as they yawned after watching nonyawning chimps.

Why is a gaping mouth so infectious? The most widely accepted belief: "Humans have built-in social instincts to do things as a group," says Robert Provine, a psychologist at the University of Maryland. "Contagious yawning helps synchronize a group to the same behavior [like going to sleep] at the same time." This latest study suggests that chimps, too, are wired to recognize group activities--even naptime.