Couples who are third or fourth cousins tend to have more kids and grandkids than other couples. And though considered somewhat of a cultural taboo, mating between "kissing cousins" makes good biological sense, say scientists.

The findings, which come from a recent study of Icelanders, shed light on how relatedness affects reproduction and ultimately the size of families.

The researchers suggest 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 study also has implications for population growth in a world that's becoming more and more urbanized. In Iceland, the dramatic demographic shift from a rural society to a highly urbanized one could slow population growth as individuals mingle with a bigger pool of distantly related mates and therefore have fewer kids. A similar urban shift is happening across the globe. In 2008, for the first time in history, more than half the world’s population will live in towns and cities, according to the United Nations.

"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. "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 lead author Kari Stefansson of the University of Iceland in Reykjavik. "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, shacking up between close cousins ups the chances of both partners carrying a recessive gene for some detrimental condition. The resulting kids 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 so-called 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. "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."