For the first time in a western society, Swiss researchers have shown that women born to parents related to each other tend to have fewer children themselves.

The findings by a team led by evolutionary biologist Erik Postma from Zurich University revealed that, contrary to popular belief, parents who are closely related to each other do not have fewer children. Rather, the effects of inbreeding can be seen one generation later, when their children have children.

Previous studies have looked at societies with large economic and social differences. This study is the first of its kind in a western, egalitarian society, Postma said.

To conduct the study, Postma trawled through church records and family registers for each family living in selected, remote hamlets in southern Switzerland. He was able to reconstruct complete family trees for entire villages, sometimes going back as early as the 1600s.

“In most cases the couples were rarely aware of their common roots,” Postma said, whose findings with co-authors Pietro and Luigi Martini are published in the Journal of Evolutionary Biology.

For each couple, the scientists calculated how closely a man and a wife were related and then factored in the number of children they’d had together. The results showed that inbred parents had fewer grandchildren but the exact reasons are not clearly understood. Scientists simply know from tests on plants and animals that inbreeding can lead to various genetic disorders, some of which may affect fertility.

The relatively low rate of inbreeding could be one reason why the study could not show the effects that men from closely related parents have on family size. Since a woman must carry the foetus, there are more chances for things to go genetically wrong, whereas a man needs only to deliver one healthy sperm, Postma said.

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