Any man dreams of being Hugh Hefner, with at least one of his odalisques. It is a fact that men prefer younger women for sex, and women want by their side an older man (well, for sex, we don't really know...). This male preference is universal across cultures worldwide.
A Finnish team tried to see if there is a connection between culture, genetics or environment influencing the age differences and which would be the optimal one within the couple. Their study published in the Biology Letters investigated the case of the Sami (Laps) people of preindustrial Finland, finding that men had to marry a woman about 15 years younger to increase at maximum their chances of having the highest number of surviving children.
"We studied how parental age difference at marriage affected [families'] reproductive success among Sami people who married only once in their lifetime[s]. 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." said ecologist Samuli Helle of the University of Turku in Finland.
The team gathered data from church records of 700 couples from the Utsjoki, Inari and Enontekiö settlements going from 17th to 19th century, with the purpose of taking out of the equation the child survival boosted by modern medicine.
Just 10% of these families were made of spouses having the ideal age difference. The age difference varied from wives as much as 20 years older to those as much as 25 years younger; the average age difference between spouses was men being three years older.
"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", said Helle. The woman's age was the main factor influencing the biological success (translated in the number of surviving children): the younger the woman, the more healthy children she could deliver. A too old wife or husband decreased significantly the couple's fertility.
Another research carried in Sweden found the ideal reproductive success was when the man was 6 years older than the woman. Anyhow, the reasons why people marry now are very different. "Wealth was the most important factor in a [Sami] marriage. Love played almost no role in it." said Helle. And I cannot imagine anyone today in the western world regarding himself/herself biologically unsuccessful with no more than 1-2 kids. On the contrary...