This is fascinating, and makes me wonder if human intelligence itself is such a cliff edge trait. For most of human history our advanced intelligence has obviously been a benefit, but now we see, as people and societies become wealthier and better educated (both correlated with intelligence), their reproduction rates drop precipitously. Perhaps we've overshot the intelligence cliff and evolution is now gradually pulling us back. (Evidence of this would be less intelligent people having more children on average than more intelligent ones.)
replies(2):