I think that they misinterpret this. Today, poor childhood diet leads to bad behavior. Back in the hunting and gathering days, a poor diet lead to a more aggressive behavior, which was needed in times of poor food. You either needed to defend you area against others who wanted your poor food, or you wanted to move into areas where food was available and take that area away from others.
So we now have a reason why peoples in harder climates can be more aggressive: poor diet. When they migrate to areas which are rich in food, the aggressiveness declines in a generation due to good nutrition.