Psychologists say early humans used their capacities of free play to overcome the innate tendencies toward aggression and dominance, which developed of a highly cooperative way of life.
Peter Gray, Boston College developmental psychologist, said that play and humor played a major role in human social development.
"Play and humor were not just means of adding fun to their lives. They were means of maintaining the band's existence, means of promoting actively the egalitarian attitude, intense sharing, and relative peacefulness for which hunter-gatherers are justly famous and upon which they depended for survival," said Gray.
In his opinion, the theory has implications for human development in the present-day world, claiming that social play counteracts tendencies toward greed and arrogance, and promotes concern for the feelings and well being of others.
"It may not be too much of a stretch to suggest that the selfish actions that led to the recent economic collapse are, in part, symptoms of a society that has forgotten how to play," said Gray.
Psychologists, educators, and the general public are taking an increased interest in play, according to Gray.
"People are beginning to realize that we have gone too far in the direction of teaching children to compete. We have been depriving children of the normal, non-competitive forms of social play that are essential for developing a sense of equality, connectedness, and concern for others," he said.
Gray stressed that the kind of "play" that helped hunter-gatherer children develop into cooperative adults is quite similar to the sort of play that was once a highlight of American children's summers and after-school hours in contemporary culture.
He said that this play is freely chosen, age-mixed, and, because it is not adult-organized, non-competitive. This "free play" is distinct from leisure pursuits such as video games, watching TV, or structured extracurricular activities and sports.
"Even when children are playing nominally competitive games, such as pickup baseball or card games, there is usually relatively little concern for winning," said Gray.
"Striving to do well, as individuals or teams, and helping others do well, is all part of the fun. It is the presence of adult supervisors and observers that pushes play in a competitive direction, and if it gets pushed too far in that direction it is no longer truly play," Gray added.
However, in self-organized play, children learn to get along with diverse others, to compromise, and to anticipate and meet others' needs.
Gray said: "To play well, and to keep others interested in continuing to play with you, you must be able to see the world from the other players' points of view. Children and teenagers in hunter-gatherer cultures played in this way more or less constantly, and they developed into extraordinarily cooperative, egalitarian adults. My observations indicate that age-mixed free play in our culture, in those places where it can still be found, has all of these qualities."
Apart from addressing to children's play, the study also highlights as a fundamental component of adult human nature, which allowed humans to develop as intensely social and cooperative beings.
Gray said that in the course of his research, it became increasingly apparent that play and humor lay at the core of hunter-gatherer social structures and mores.
The study has been published in the current edition of the interdisciplinary American Journal of Play.