Evolutionary psychologist Dr Peter Gray on why children need the freedom to play

This is an enjoyable TEDx Talk - and it's always good to hear an expert endorse your prejudices.

The blurb for it on YouTube runs:

In this talk, Dr. Peter Gray compellingly brings attention to the reality that over the past 60 years in the United States there has been a gradual but, overall dramatic decline in children's freedom to play with other children, without adult direction.

Over this same period, there has been a gradual but overall dramatic increase in anxiety, depression, feelings of helplessness, suicide, and narcissism in children and adolescents. 

Based on his own and others' research, Dr. Gray documents why free play is essential for children's healthy social and emotional development and outlines steps through which we can bring free play back to children's lives.

I find evolutionary psychology compelling. It can tell you why children don't like spinach - it's because it would be dangerous if children liked bitter green things. They would poison themselves.

And I have heard Peter Gray explain why children never want to go to bed. It's because for most of our time on this planet there really have been monsters underneath it. Children want to stay with the adults because that's where safety is. After all, it's what we tell them the rest of the time.

But as a good Popperian I have to ask how you test these theories. Aren't they just plausible stories about our ancestors and hunter-gatherer society?