More than just an opportunity to break a sweat and maybe have some fun, competitive physical activity like sparring allows people to explore disagreement with respect, according to Janet O'Shea, professor of dance at UCLA, who recently wrote a book about the value of play. "If I'm trying trying to punch you and you're trying to kick me in the head, we disagree on a pretty basic level. But we agree as to the terms of our interaction. At the end, we shake hands and most of the time we actually mean it."

As teenagers become adults and are no longer required to take physical education classes in school, chances to engage in physical competition tend to drop dramatically. What we lose is far more than simply exercise, O'Shea says.