In Utah, women used to marry young. In particular they married Brigham Young, leader of the Mormon Church. The religious leader had 55 wives by whom he had 56 children before he died, aged 76, in 1877. His followers had similar polygamous marriages.

But scientists have now uncovered an odd fact about 19th-century Mormons: the more women in a household, the lower the average birthrate. In other words, the more sister-wives a Mormon woman had, the fewer children she was likely to produce.

"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.

The result is intriguing, because this is the first time scientists have observed humans being affected by what is known as the Bateman gradient, a phenomenon that gets its name from the geneticist who first observed it in fruit flies. The more sexual partners the male fruit fly had, the lower was the fecundity of each of those partners, the 20th-century geneticist Angus Bateman noted....