Jordan Peterson sometimes talks about how play in rats stimulates prefrontal cortex growth. He thinks this is an important aspect of child development too. Kids need physical play at an early age or else they'll become anti-social and likely criminals.
I connect that up with birth order effects. If you're familiar with birth order, you might know that most personality effects are on very shaky ground. It doesn't seem like birth order predicts much of anything. Except research has consistently found gains in IQ for younger siblings.
Now I wonder, is that because of the extra play younger children get from having older siblings to play with?
Just speculation.
