The causation could easily go the other way. In most societies, males are typically sent out to war because males can easily have more children than females si

The causation could easily go the other way. In most societies, males are typically sent out to war because males can easily have more children than females since they don't have go go through a period of pregnancy, so a society that sacrifices its males on high risk high reward activities can rebuild faster than if it sacrificed females instead. This is why typically you see male soldiers, raiders, and explorers. But as a result, more warlike societies tend to have less males in the population than females because the males keep getting sent out to die. Remaining females now have less potential mates to choose from, so polygamy becomes socially acceptable to ensure more people get a share of the economic rewards those surviving males bring back. So you could easily flip this around and say that it's not polygamy that causes war. Rather, war causes polygamy, and polygamy is a reward for the few male "winners" that survive to bring home the loot because now they have less genetic competition. This model of warfare reducing the proportion of males could also explain why most societies have tended to be patrachical. Scarce males means each males has more bargaining power. In return for the economic loot they bring back, they could demand more say in the household than their counterpart or counterparts. Stretch this population imbalance over a few centuries and you get a patrachical society.