Over the last half century, the US and other industrialized nations have seen a dramatic decrease in the free time that children have to play. This decline been attributed to a several factors, including a decrease in school recess time and an increase in parental fear of the outside world – especially of injury or germs.

This decrease in play has been paralleled by an increase in mental illness in children and young adults. Some reports have argued for a causal link between the two. That hasn't been definitively established, but there is new evidence to suggest that playing is indeed crucial to healthy development and functioning.

Last year, Gordon Burghardt, a psychology and ecology/evolutionary biology professor at the University of Tennessee, wrote a paper with psychology colleague Matthew Cooper that explored what happens when barring juvenile golden hamsters from playing with one another. The study was led by Cody Burleson, then a research technician in Cooper's laboratory. These researchers were interested in how this play deprivation would alter the hamsters' brains, specifically in the prefrontal cortex, an area of the frontal lobe that is primarily responsible for planning, personality, and coping. Furthermore, they wanted to know how social play affects responses to stress in adulthood.

Imagine for a moment that dendrites, the pathways that send and receive signals from neurons, are like the branches on a tree. Branches are typically good for trees, but too many branches leaves the tree cramped, inefficient, and in need of a good pruning. Scientists think that our neurons work in a similar fashion; during adolescence the brain goes through a “pruning” process to eliminate excess, inefficient connections.

Burleson and colleagues' study (echoing the results of a prior, similar study in rats) suggest that juvenile social play facilitates this dendritic pruning. More specifically, they found that play deprivation increases the complexity of dendrites in the prefrontal cortex. This increased complexity may be why these researchers also found that when animals weren’t allowed to play as juveniles they were more susceptible to the effects of an acute social stressor when they were tested as adults. Playing not only rewires brains for the better; it also mediates how adults respond to stress.

Subsequent analyses of that data set, led by a PhD student at the University of Tennessee, indicated that animals who played more as juveniles spent less time engaging in aggressive behavior as adults, suggesting that play may facilitate the development of competent adult social behavior.

In addition to the rise in mental illness in humans that has coincided with this decrease in play, young adults also report a decline in their sense of control. This isn’t surprising, as the prefrontal cortex plays a critical role the perception of controllability.

Of course, Burleson and colleagues’ study was done in hamsters, not humans. But there’s plenty to learn about the brain and behavior by studying rodents. If play deprivation changes the dendrites of neurons in the rodent prefrontal cortex, studying this issue in human brains is a reasonable next step.