Essentially, there's a sort-of phase diagram for the optimal human reproduction strategy where the axes are things like "income inequality" and "environmental stress". You can then color in patches of the diagram with what strategy members of a society will settle on.
For example, high-stress, high-inequality societies practice polygamy. The game-theoretic logic is that women need a partner that can provide consistent excess calories. If you can't feed your kids in just one year out of twenty, they die. It doesn't matter what the average income is, just how steady the income is. In societies like that, only the richest 10% of men can provide this, so they all have 10 wives. The other 90% of men remain bachelors and often participate in tribal warfare, exacerbating the problem. Saudi Arabia used to be like this, and parts of Africa still is.
Similarly, in "one corner" of this graph is super rich men in stable societies, where their optimal reproduction strategy is serial monogamy to young wives. Look at Hollywood A-listers or billionaries to see this play out in real life.
Japan got stuck in a part of the phase diagram where the optimal reproduction strategy is "don't."