Take a look at Fundamentalist Mormons, they have to expel young men in order to have extra wives for the greybeards, which doesn't make sense on a natural level since technically the young-bloods (from a biological point of view) should kill the older and weaker competition and claim their mates. Humans exist more peacefully in monogamy because there is no need for taking additional mates, and since we no longer need alpha-male genes to ensure our survival as a species, polygamy is primitive and useless.