In some cultures, having a number of wives is considered a symbol prosperity and power. However, a new study points out that for every new addition to the harem, a man reduces his spouses' power to procreate.

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For the study, researchers analyzed the detailed genealogies of Mormons during the period when polygamous marriages were being phased out abd discovered that the more wives a man had, the less fecund the females:
"In the study of Mormon families, published in the US journal Evolution and Human Behaviour, the researchers surveyed birth, marriage and death records from the Utah population database, which covers nearly 186,000 adults and 630,000 children who lived or died between 1830 and 1894.

It was during this period that polygamy was slowly being phased out under pressure from state legislators. The results were clear: the more women partnered with a man, the fewer children each of those women had. Exactly why is not clear. Like the Soay rams, men may simply not have had the stamina. Wade says: "It could be owing to competition between women within a plural marriage for shared resources, or it could be owing to other unknown factors.

Although it is great in terms of numbers of children for successful males to have harems, the data show that, for every new woman added to a male's household, the number of children that each wife produced goes down by one," said biologist Dr Michael Wade, of Indiana University.
This  decreased ability to breed (a.k.a. the Bateman gradient) has also been observed in fruit flies and the Soay sheep. The researchers hope to continue exploring why this phenomenon would have such an affect on women in a polygamous situation.

via Guardian