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Fertile Times for May-December Couples

30 August 2007 (All day)
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John Bohannon

Fertility in their future.
This 2003 wedding was an evolutionary coup for the groom, who is 5 years older than the bride. They now have a 2-year-old, with another on the way.

Mrs. Robinson notwithstanding, men tend to prefer younger women as mates, and women aim for older men. There's an evolutionary reason for this: It gives both parties a reproductive boost.

Men's preference for younger women exists across cultures, and this finding has been replicated by decades of survey data. Martin Fieder and Susanne Huber, a married pair of anthropologists at the University of Vienna, Austria, wondered if there might be an evolutionary explanation for this inclination.

To learn more, the couple delved into a massive demographic database of Swedish baby boomers born between 1945 and 1955. They focused on 10,000 men and women who had only married once. Then they compared the age differences of those couples with the number of children they had together. If the older man-younger woman couples produced more offspring, they reasoned, then genes that make women find older men sexy--and vice versa--may have been the inevitable result of natural selection.

Among these Swedes, choosing a younger wife or an older husband paid off, at least in the evolutionary sense. Couples in which the husband was about 5 years older produced approximately 5% more children than same-age couples, Fieder and Huber report online this week in Biology Letters. That may not sound like much for a family unit, but it represents a "huge" effect on the evolutionary time scale, says Fieder. (He adds that he and Huber are both 42 and have only one child--which at their age ranks them as evolutionary losers.)

The data also reveal that when men divorced and married a new partner, they nearly always chose a younger woman and women, an older man. But the difference is that the new husbands tended to be only a few years older than the women, whereas the men's newly acquired wives tended to be of reproductive age, and hence the age gap grew ever larger the later the men remarried.

The study shows that there is a "fitness basis" to people's choice of mate age, says Robin Dunbar, an anthropologist at the University of Liverpool, U.K. It also reveals a "clash of interest between the two sexes," he says, because women have a smaller window of reproductive opportunity. Once beyond that window, they are out of the game, whereas older men can continue to mate with ever-younger women.

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