BioEd Online

No doubt about it: incest has a bad reputation. In Western societies, ‘kissing cousins’ are mocked, reviled and sometimes outlawed. Oddly, however, a new study suggests that a little bit of inbreeding might be beneficial to the family line.

Researchers combed through 165 years of genealogical records tracing more than 160,000 couples in Iceland. They found that the more closely related that couples were, the more children and grandchildren they tended to have. But for first and second cousins the outlook was gloomy — their offspring died younger and reproduced less.

Breeding outside of the family is considered beneficial because it provides a source of new genetic material. Outbreeding increases the chances that offspring will inherit at least one ‘good’ copy of a gene, potentially masking harmful mutations lurking in a family’s genetic background. Without that influx of fresh genetic material, the probability of birth defects rises: highly inbred animals and royal families are littered with the consequences. Inbreeding is also thought to decrease the reproductive success of offspring.

But the new results suggest that things don't always work that way, says Kári Stefánsson of deCODE Genetics in Reykjavik, and an author on the study. “It’s counterintuitive in many ways,” he says.

It's all relative

Inbreeding once ran rampant in the West, particularly in rural societies where finding a mate outside of the family could entail a long hike to the next village. And first-cousin marriages are common today in many eastern cultures, where marrying within the family is a way to avoid a dowry and consolidate familial resources.

“In Western society there is this impression that inbreeding is bad,” says Alan Bittles, a human geneticist at Edith Cowan University in Perth, Australia. “But that didn’t really commence until the middle of the nineteenth century.”

Researchers have studied the question of how inbreeding weakens the family line for decades. But they have generally struggled to tease out the effects of inbreeding from study groups with vast differences in social and economic status.

Stefánsson and his colleagues tackled the question using data from Iceland, an island slightly larger than the United Kingdom with a remarkably homogeneous population in terms of socioeconomic status. The researchers found that first and second cousins had more children than distantly related couples, but that those children also die at a younger age and bear fewer children. That fits with previous data showing that children of first-cousin marriages have a 3-4% higher chance of ill health or early death.

Third and fourth cousins also had more children than more distantly related couples, but their children tended to be more reproductive. For example, women born between 1925 and 1949 who partnered with a third cousin had an average of 3.3 children and 6.6 grandchildren. Women born during the same period who had a partner that was an eighth cousin or more distantly related, had only 2.5 children and 4.9 grandchildren on average.

Live long and prosper

Although the researchers don't have more subtle data on the health of these children, the number of children who go on to have children of their own is often taken as a general marker of 'genetic fitness': they get to pass on their DNA to future generations. In this regard, the third and fourth cousins were best off. The results are published today in Science¹.

The result may reflect the fact that genetic compatibility is important to breeding, says Bittles. For example, a dangerous condition called ‘Rh incompatibility’ can arise when one parent has an antigen called the Rh factor and the other doesn't. If the developing fetus inherits the father's Rh status, then it can trigger an immune response in the mother.

There might be a compromise to be struck between the benefits and problems of genetic similarity, says Bittles. “The idea that there could be an optimum balance makes sense,” he says.