It probably relates because the marriages themselves don&#x27;t matter, it is the family that forms and the impacts that has on individuals that do.  Thus if you had some society which allowed only monogamy but also allowed concubines, you would likely see the same issues as caused by polygamy.  Or if you had a society that didn&#x27;t do marriage at all, but the relationships that formed tended to be greater towards polygamy than monogamy.  (You can also have the reverse, polyandry, but this study doesn&#x27;t seem to look into those societies to see how they differ from monogamous and polygamous ones.)