Scientists say they have carried out the first study that confirms that evolutionary pressure - the drive to have more children, in short - is what causes the typical age gap among couples.
Researchers explored the theory that men go for younger, sexually attractive women in order to boost their chances of reproductive success, while women prefer older, successful men to provide the resources and security that increase their offspring's chance of survival.
The investigators trawled through a Swedish population database, covering 11 500 men and women born between 1945 and 1955, to see at what age these individuals became parents.
4 to 6 years difference best for reproduction
Among couples who stayed together, the most children were born in households where there was an age difference of four to six years.
When couples split up and mated again, they each opted for partners who were younger than the first.
That was especially so for older men, who went for women who were much younger. Women looking for a new mate generally chose a male who was slightly older than herself.
"The age preference for the partner increases the individual reproductive fitness of both men and women," say the authors, who speculate that this trait has been acquired through millennia of evolution.
The study, which appears in the British journal Biology Letters, is written by Martin Fieder, an anthropologist at the University of Vienna, and Susanne Huber, a wildlife ecologist at the University of Veterinary Medicine Vienna. - (Sapa)
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