My guesses are that there ate multiple factors at play:
- less educated people make more kids
- "nice guys finish last" - assertive and more socially active people are more likely to find a partner and make children
- forcing others to follow perceived social dynamics means internal pressure to follow them oneself; having children is a social norm and expectation that is expected to be followed
I'd try and look for papers to see if my guesses hold any water, but time eludes me.