There are reasons, then and now, that people might gather in plural marriages. Some people are inclined to have more than one partner in life. Others believe that they should have multiple spouses for religious reasons. Still others are practical: Many spouses can mean much less work when it comes to childcare and household chores. It appears, though, that lighter domestic work has an evolutionary cost.
The early 19th century Mormons are an ideal group to study when it comes to polygamy. They were open practitioners of polygamy and they were avid geneological record keepers. They dispensed with the former practice, while keeping the latter, which provides a detailed account of a society transitioning from one mode of marriage to another. Those records have recently been perused by evolutionary biologists, and they found an interesting phenomenon at work.
The researchers discovered that "the Bateman gradient" was lowering the birth rates of women in polygamous relationships. The Bateman gradient was first observed in fruit flies. The more sexual partners the male fruit fly had, the lower the fecundity of each of those partners. A similar dynamic operated in the Mormon families with multiple wives. The more women partnered with a man, the fewer children each of those women had. Every extra wife lowered the overall number of children each wife had by one. In terms of spreading their genes around, polygamy was a raw deal for women.
It may also be a very raw deal for men. For every man who has two wives there is a man who has none. Men may consider competition for women fierce now, but what would happen if marriage didn't take a man off the market? Why would men even participate in a system that could leave them without options?
The scientists compared such a system to a 'predator cap'. In most wild species, males are far more elaborately decorated than females. The shining feathers of a hummingbird or the long tail feathers of a pheasant look great, until they try to hide from predators. Then those displays get the males killed. And yet, the sexual pressure is so intense, and the genes of the successful males are spread so thoroughly, that the fact that the system chokes off a good deal of male opportunity doesn't matter. And so polygamy has its downside, for men and for women.
Via Official Journal of the Human Behavior and Human Society and Science Daily.