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'Kissing Cousins' Have More Babies

Study finds distantly-related couples may be more fertile

February 9, 2008
Keeping it in the family may have an added dimension: You may have more kids.

A study released in Iceland found that couples who are distantly related -- third or fourth cousins -- are likely to have more kids and grandkids than couples who are not related by blood.

The study may help dispel the popularly held belief that marrying within a family breeds hereditary diseases and mental disorders.

In fact, many sociologists blame this practice as the leading cause for the decline in the rate of population among Zoroastrians, or Parsis, a group of people who are descendants of Prophet Mohammad but fled Iran centuries ago to avoid persecution by the state.

Their race is said to be facing extinction and many say it is because of the practice of marrying within the family among Parsis that has caused them this harm.

But Kari Stefansson, a researcher in Iceland, released a report saying mating between "kissing cousins" makes good biological sense.

The findings are part of a study that seeks to shed light on how relatedness affects reproduction and ultimately the size of families.

The researchers suggest that marrying third and fourth cousins is so optimal for reproduction because they sort of have the "best of both worlds." While first-cousin couples could have inbreeding problems, couples who are far removed from each other could have genetic incompatibilities.

"The formation of densely populated urban regions that offer a large selection of distantly related potential spouses is a new situation for humans in evolutionary terms," the researchers write in the Feb. 8 issue of the journal Science.

During the past two centuries, the researchers point out, the average relatedness of Icelandic couples has widened from third and fourth cousins to the more recent couple relatedness of fifth cousins. (Children of siblings are cousins. Children of first cousins are second cousins, and their children are third cousins.)

Cousin couples

The results make sense from a biological perspective, the researchers say.

"Our definition of a species is a group of individuals who are closely enough related to each other to be able to have offspring," said Stefansson. "There is recognition in that definition of the fact that individuals have to be somewhat related to each other to be able to reproduce."

However, intermarriage between close cousins ups the chances of both partners carrying a recessive gene for some detrimental conditions. The resulting offspring would have a 25 percent of expressing that gene, meaning they'll have the disease.

On the flip side, coupling with a close cousin means it's more likely the mother and fetus will be genetically compatible. The mother won't have to worry so much about Rh incompatibility, which can be lethal to a fetus.

"It could be argued that in human populations there is a point of balance between the disadvantages associated with inbreeding versus those with outbreeding," said Alan Bittles, director of the Center for Human Genetics at Edith Cowan University in Western Australia. Bittles was not involved in the new study.

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 to the media. "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."

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