It's (apparently) true. I'm not sure this can be directly extrapolated to playing videogames, mind, but this is a wonderful read: The Science of Fun.
But the best argument for fun and play happens when you take it away.
Deny play to young mammals and the consequences are colossal. This has been demonstrated powerfully in rats, with some of the most significant new science coming out of Canada.
"What's been shown repeatedly," said University of Lethbridge neuroscientist Sergio Pellis, author of The Playful Brain, "if you prevent juvenile rats from engaging in rough and tumble play, you get animals who have cognitive problems, emotional problems and they're socially incompetent. "
In short, stop rats from horsing around and you get socially-awkward, troubled, dumbo rats. Worse still, male rats who never play become bad lovers.
Read on. It gets better.
Fun is the brain's workshop.
"If you look at countries with the lowest amount of recess time, and scholastic achievement, there's almost an exact correlation," says Pellis. "The U.S., which has the lowest recess rates, often does the worst" in scholastic achievement.
It might even be the brains of children grow more at recess than in the classroom.
I'm not in the least bit surprised by that last bit!
Now I am actually really curious as to whether the brain can extrapolate out to fun like (heh) co-op L4D, or teamplay in MMOs. Is it the physical interaction that counts more, or the social interaction - or both equally? Does physical interaction extend to "mirror neuron" stuff, so that what happens to your avatar is effectively also happening to you? So many questions. I'd better buy that book.