No. 1 at making babies? Third cousins (Submitted by Arash)

No. 1 at making babies? Third cousins
At least, that's what's happening with Icelanders

By ELIZABETH LOPATTO
Bloomberg News

Couples with the same great-great-grandparents have more children than those who aren't as closely related, according to an Icelandic study.

Married third cousins on average had one more child and two more grandchildren than distantly related couples, according to an analysis of Icelandic families in the last two centuries published in the Feb. 8 issue of the journal Science.

Inbreeding produces children with more birth defects, so the authors said they couldn't explain the biological reason the related couples cousins have more babies. Previous research didn't examine fertility in relatives more distant than third cousins, according to the study by Iceland-based DeCode Genetics, which is developing drugs based on genetics.

"Our assumption was that the more distantly related, the more children a couple would have," said lead study author Kari Stefansson, DeCode's chief executive officer, in a telephone interview. "But nature has its ways of surprising us."

Parents who were second cousins or closer had fewer children than third-cousin couples, the study found.

Stefansson and his co-authors focused on Iceland because the population's education, wealth and status varies little and because most people descended from a few founders. Similar studies in other parts of the world have been difficult to interpret because of the population variables, said the article.

A study last year found that brother and sister fish known as cichlids inbreed so they'll have cooperative relationships raising their young, scientists from the University of Bonn said.

In humans, unrelated parents can have different blood rhesus factors, which can complicate pregnancy and nursing, according to the Mayo Clinic's Web site. Parental kinship decreases the chance for that incompatibility, Stefansson said.

"I found this observation not particularly attractive," Stefansson said. "Though the definition of a species are individuals who are closely enough related to be able to reproduce, the idea that the individuals are related is inherent."