Other species play, but none play for as much of their lives as humans do, or as imaginatively, or with as much protection from the family circle. Human children are unique in using play to explore hypothetical situations rather than to rehearse actual challenges they’ll face later. Kittens may pretend to be cats fighting, but they will not pretend to be children; children, by contrast, will readily pretend to be cats or kittens — and then to be Hannah Montana, followed by Spider-Man saving the day.

And in doing so, they develop some of humanity’s most consequential faculties. They learn the art, pleasure and power of hypothesis — of imagining new possibilities. And serious students of play believe that this helps make the species great.

The idea that play contributes to human success goes back at least a century. But in the last 25 years or so, researchers like Elizabeth S. Spelke, Brian Sutton-Smith, Jaak Panksepp and Alison Gopnik have developed this notion more richly and tied it more closely to both neuroscience and human evolution. They see play as essential not just to individual development, but to humanity’s unusual ability to inhabit, exploit and change the environment.