Kissing cousins have more kids

A family affair
Stefansson and his colleagues studied more than 160,000 Icelandic couples going back 200 years, starting with those born in 1800, using the deCODE Genetics genealogical database. Stefansson has served as president and chief executive officer of deCODE since he co-founded the company in 1996.

The Icelandic population, they say, is relatively small and homogeneous with little variation in family size, use of contraception and marriage practices. So the results are not confounded by other variables, such as economic status, which have biased results from past studies of kinship and reproduction.

The team found that women born between 1800 and 1824 and who partnered with a third cousin had an average of about four children and nine grandchildren, while those related to their mates as eighth cousins or more distantly had three children and seven grandchildren. A similar pattern showed up for women born between 1925 and 1949. Third cousins had an average of three children and about seven grandchildren, compared with two children and five grandchildren for eighth cousins and beyond.

One caveat: More closely related couples may just start making babies earlier than others. Past research has revealed "strong evidence that couples who were first cousins married earlier and were less likely to use contraception, the wives had their first child earlier, and they continued child-bearing at later ages," Bittles told LiveScience.

Forbidden love
The newly discovered positive link between closely related couples, called consanguinity, and offspring is clouded by social norms.

"These are not the results we expected to find," Stefansson said in a telephone interview. "I have been brought up in a culture that looks down on consanguinity so I feel somewhat ill at ease with such data in many ways. This is not what I expected, but it only goes to show how incredibly complicated nature is."

The "ick" factor associated with marrying a close relative has a long history. Citing a Bible verse, Pope Gregory I said in the sixth century that sacred law forbad a man “to uncover the nakedness of his near kin” and that such unions were infertile.

But Bittles notes that first-cousin unions were quite common and highly regarded in Western Europe and the United States in the first half of the 19th century. "But as the century progressed, a suspicion that the offspring might not be healthy began to emerge," he said, "and this trend continued throughout the 20th century, resulting in ever fewer first cousin marriages."

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