One contributing factor is human beings self-selecting for passivity. We lock violent people away, and that dramatically reduces their chances of procreation. Over time, the population is likely to become more passive.
Also, comfort. Life in North America, even for the poor, is more comfortable than life elsewhere on the planet.
Finally, the average human in modern societies is far more cerebral now, because society requires complex interactions to get even simple things done. Thinking about the number of steps required to obtain temporary shelter now (book a hotel or AirBnB) vs 10000 years in the past.
Just speculation in all cases, no hard data to back any of it up.
The study is focused on the last couple of decades, doesn't it seem unlikely any evolutionary selective pressures are a cause in such a short and recent time span?
Also, comfort. Life in North America, even for the poor, is more comfortable than life elsewhere on the planet.
Finally, the average human in modern societies is far more cerebral now, because society requires complex interactions to get even simple things done. Thinking about the number of steps required to obtain temporary shelter now (book a hotel or AirBnB) vs 10000 years in the past.
Just speculation in all cases, no hard data to back any of it up.