Polygamy harms birthrates, study finds

Mormonism has always had the ideals of polygamy attached to it, even though it was phased out more than 120 years ago. Some have wondered if sharing wives would affect the ability to produce children.

Professor Michael Wade, an evolutionary biologist, began theoretical work guiding a study based on that very idea. 

Not only does polygamy seem wrong by today’s standards, this research could show that it is harmful to mating selection as well.

“Although it’s great in terms of number of children for successful males to have harems, the data show that for every new woman added to a male’s household, the number each wife produced goes down by one child or so,” Wade said. 

Quite simply, when a Mormon woman had more sister-wives, she was likely to produce fewer children. Polygamy, then, benefits men more so than women, as the men have a higher chance to produce offspring than the women, as each wife added to a plural marriage reduced the average number of children per wife by one. 

So is polygamy actually better for mating purposes not just in humans but in other species as well? The answer, Wade said, is a definite no. The difference between the “have” and “have-nots” in a polygamous system is larger than those in monogamous ones. In an equally divided system, for example, a male with three wives leaves two men without any.

When the Mormons drastically changed the system, there was a dramatic change in sexual selection between men. More males now had access to wives, and the scientists said ending polygamy reduced the strength of sexual selection by 58 percent, according to the press release. Sexual selection is defined by Charles Darwin as the effects of the “struggle between the individuals of one sex, generally the males, for the possession of the other sex.” A case of this is the peacock and its extravagant plumage competing with the other males of its species, according to the press release. 

The researchers didn’t choose the Mormon society just because of its choice to be a polygamous system however. They kept accurate and thorough records of every marriage, birth and death at the time when they changed mating systems. By using these records, they could discern the effects of the two types of systems.

Wade and his co-authors were not able to explain why women weren’t able to produce as many children in the polygamous society. As an evolutionary biologist battling the skeptics, Wade can only guess why this phenomenon occurs.

“Nothing in the data set itself provides an answer to the why question,” Wade said. “Society has implications for biology and vice versa.”