Well, they are making the assumption that one woman "mated" with one man only. If you drop that assumption, and take "sperm competition" (https://en.wikipedia.

Well, they are making the assumption that one woman "mated" with one man only. If you drop that assumption, and take "sperm competition" (https://en.wikipedia.org/wiki/Sperm_competition https://en.wikipedia.org/wiki/Sperm_competition) into account, other scenario's than "wealth-and-power" are possible. Such as: groups of hunter gatherers had frequent orgies, but due to "sperm competition", a few of the man fathered all children.