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.

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

"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."