Play is innate, and our instincts to participate in it can be traced back through our animal ancestry. The fact that other mammals, such as chimpanzees and great apes, have a natural urge to play suggests that play occurred well before our species evolved.

Prehistoric 2-year-olds created finger art on cave walls. Ancient Greek children played games like knucklebones, an ancient version of jacks, for fun. In the Middle Ages, boys and girls made their own toys from wood or stones. Even children in Nazi concentration camps played. Play has always been present in the history of humankind, regardless of culture or living condition.

If you're that parent who tells your kids that back in your day, you played outside much more than they do now, you're telling the truth: In another survey, 70 percent of American mothers reported playing outside daily when they were kids, while only 31 percent of their children do so [source: Clements]. And when youngsters do get to play, it's often adult-supervised rather than free play (spontaneous, self-directed play for play's sake).