And the age gap that maximises males’ “evolutionary fitness” is a decade and a half – much more than previous studies have suggested.
A team at Finland’s University of Turku studied three populations of people, called Sami, who lived in northern Scandinavia from the 17th to the 19th centuries.
They chose these groups because they were monogamous with extra marital sex banned, meaning marital reproduction was the only way of forwarding one’s genes.
Males with partners 15 years younger were found to be the most productive.
But researcher Dr Samuli Helle and colleagues say despite their findings, published in Biology Letters, just one in ten of the 706 Sami couples studied had this fifteen year optimal age difference, possibly due to cultural factors such as marriage practices or ecological constraints caused by a lack of available mates and resources.
They chose these people because, in contrast to the contemporary populations studied so far, they lived in isolated regions and had little advanced medical care or birth control methods, which meant they experienced what the researchers called “natural fertility and mortality”.
Recent studies have indicated there is an age gap reproductive benefit for men and women, but the results are ambiguous.
One carried out in modern day Sweden found men maximized their offspring by wedding women six years younger, whereas women had most children by marrying men four years older.
Another in Hungary found couples with an older husband produced slightly more surviving offspring than those with an older wife. Likewise, research in England said marriages in which the husband was two to three years older produced the largest family size.
But in contrast, marriages in rural Bangladesh in which the husband was several years older had the lowest total marital fertility.
Dr Helle said most men marry younger women, but the fitness consequences of such “mate preferences” are poorly known.
They have been suggested to be favoured by natural selection, as men should prefer young women due to their high fertility and women should prefer older men due to their wealth and high social status, which make them good providers for the offspring.
Dr Helle said: “We found that men maximized their fitness by marrying approximately fifteen years younger woman and vice versa.
“However, most couples failed to marry optimally. Only 10% of marriages fell within the optimal parental age difference, suggesting that cultural and ecological constraints for maximizing fitness were considerable. Those who succeeded in marrying optimally were the most preferred partners: young women and old men.
“Our findings suggest that, in Sami, parental age difference was under natural and sexual selection, as suggested by the evolutionary theory.”
He said the results fit well with our current understanding of why most men marry younger women, even though the average age gap between the Sami men and their partners was just three years.
Dr Helle said: “Young Sami women were the most fertile and had the highest reproductive value, whereas older Sami men had acquired enough skills needed for successful hunting, fishing and reindeer herding and, most importantly, wealth to be good providers for the progeny and thus desirable mates.
“A similar example of sexual selection with regard to sex-specific mate preference in humans, and realised in marriage markets in western societies, is adulthood stature, as selection seems to favour high stature in men but not in women.”
But the recommended age gap is nothing compared with some of those in showbusiness where it is customary for wealthy elderly men to settle down with young women.
Michael Douglas, 63, married 38-year-old Catherine Zeta-Jones seven years ago. They now have two children together. And Rod Stewart, 62, has a two year old son with new wife Penny Lancaster.