Because of the role of play in the epigenetic construction of social brain functions, the young of all mammalian species need sufficient play. For the same reason, the nature of that play becomes an important social policy issue for early childhood development and education. Animal research on this topic indicates that play can facilitate the maturation of behavioral inhibition in growing animals, while psychostimulants reduce playfulness. Our failure to provide adequate opportunities for natural play in modern societies, the author argues, may have contributed to the steady growth in the diagnosis of Attention Deficit Hyperactivity Disorders (ADHD) in children.