Over the past 50-60 years, play time in kids' lives has been drastically cut. School days and years are longer and parents often schedule enrichment activities for their children instead of giving them space to direct their own play. Children are rarely given the freedom to direct their own activities, leading to a persistent rise in children feeling that they have no control over their lives. And, while correlation doesn't prove causation, Dr. Peter Gray, who has been studying play for years, says there's strong evidence that in this case, the decline in play is leading to a rise in depression and acute anxiety among young people.

Check out his TEDx talk for all the details on this fascinating area of research. Read more about play here.