Men marry younger women and women prefer to marry older men, in general. But is it culture, genetics or the environment that drives such a choice—and is there an optimal age difference? New research shows that, at least for the Sami people of preindustrial Finland, men should marry a woman almost 15 years their junior to maximize their chances of having the most offspring that survive.

"We studied how parental age difference at marriage affected [families'] reproductive success among Sami people who married only once in their lifetime[s]," says ecologist Samuli Helle of the University of Turku in Finland. "We found that marrying women 14.6 years younger maximized men's lifetime reproductive success—in other words, the number of offspring surviving to age 18."

The researchers did this by examining church records of 700 marriages from the Utsjoki, Inari and Enontekiö populations from the 17th through 19th centuries (in order to eliminate the effects of modern medicine on child survival).

Yet, only 10 percent of these marriages were between men and women with that optimal age difference. The span ranged from men marrying women as much as 20 years older to women marrying men as much as 25 years older; the average age difference between husband and wife was three years. Marriage customs or the availability of reindeer to support a new family (the Sami people are reindeer herders) might be the reason that more Sami marriages did not display the optimum age difference, Helle says.

Ultimately, it is the age at which the woman begins bearing children that is the biggest factor in survival, the paper in Biology Letters suggests: Younger women, in general, bear more healthy children. Marrying an older woman or much older man proved the most detrimental to reproductive success.

Other research in modern day Sweden has shown that the ideal reproductive match is for a man to marry a woman six years his junior. But the cultural constraints on marriage may have changed. "Wealth was the most important factor in a [Sami] marriage," Helle notes. "Love played almost no role in it."