How much of it is self-enforcing simply by peer groups though? I certainly wouldn't say there isn't a psychological/neurological component at work here - I'd gu

How much of it is self-enforcing simply by peer groups though? I certainly wouldn't say there isn't a psychological/neurological component at work here - I'd guess that there is a proclivity for boys to choose a ball over a doll. However, I can't think it would be as much as what we see today - the environment plays the largest role I would guess, especially since after the "roles" are established experimentation with "non-role-appropriate" toys are probably highly discouraged.