Play behavior is not only the origin of our cultural ingenuity, but is intimately linked to the shape and function of that most ingenious feature of our biology, our brain. According to the social brain hypothesis, our large human brains have evolved to deal with the increasing complexity that characterizes the social life of primates. It is not only our ability to maintain different relationships with large numbers of people that makes unprecedented cognitive demands, but the sophisticated forms of play behaviour that facilitate such bonds – ritual, dancing, singing and laughter. Neuroscientists have begun to unravel how play affects brain maturation, social competency, impulse control and stress reduction, how it engenders positive emotions by stimulating endorphins and dopamine, the role of mirror neurons in collective enactments of joy, or the effect of rough-and-tumble play in increasing dentrital arborization in the orbito-frontal cortex, which is involved with cooperation and social competency.