Source: The link between polygamy and war
Plural marriage, bred of inequality, begets violence
Wherever it is widely practised, polygamy (specifically polygyny, the taking of multiple wives) destabilises society, largely because it is a form of inequality which creates an urgent distress in the hearts, and loins, of young men. If a rich man has a Lamborghini, that does not mean that a poor man has to walk, for the supply of cars is not fixed. By contrast, every time a rich man takes an extra wife, another poor man must remain single. If the richest and most powerful 10% of men have, say, four wives each, the bottom 30% of men cannot marry. Young men will take desperate measures to avoid this state.