It is not quite incest. And though it will increase your chances of birthing a healthy baby, it is a bit unorthodox, to say the least. Still, scientists at Icelandic biotechnology company deCODE genetics say that when third and fourth cousins procreate, they generally have scads of kids and grandkids (relative to everyone else).

It has long been wondered exactly how kinship influences reproductive success. Previous studies have uncovered positive correlations, but the biological data has been clouded by socioeconomic factors (such as average marrying age and family size) in those populations in which consanguineous marriage is commonplace, such as in India, Pakistan and the Middle East. The new study, however, was able to shed light on the biological reason for the earlier findings.

Scientists came to their conclusions after studying the records of more than 160,000 Icelandic couples with members born between 1800 and 1965. "The advantage of using the Icelandic data set lies in this population being small and one of the most socioeconomically and culturally homogenous societies in the world," the researchers report in Science, "with little variation in family size [and] use of contraceptives and marriage practices, in contrast with most previously studied populations."

The results of the exhaustive study are constant throughout the generations analyzed. Women born between 1800 and 1824 who mated with a third cousin had significantly more children and grandchildren (4.04 and 9.17, respectively) than women who hooked up with someone no closer than an eighth cousin (3.34 and 7.31). Those proportions held up among women born more than a century later when couples were, on average, having fewer children.

Despite the general pattern for reproductive success favoring close kinship, couples that were second cousins or more closely related did not have as many children. The most likely reason, scientists say: offspring of such close relatives were likely to have much shorter life spans, because of the chance of inheriting harmful genetic mutations.

"With close inbreeding—between first cousins—there is a significant increase in the probability that both partners will share one or more detrimental recessive genes, leading to a 25 percent chance that these genes will be expressed in each pregnancy," says Alan Bittles, director of the Center for Human Genetics at Edith Cowan University in Joondalup, Australia, who was not involved in the study.

Interestingly, one evolutionary argument for mating with a relative is that it might reduce a woman's chance of having a miscarriage caused by immunological incompatibility between a mother and her child. Some individuals have an antigen (a protein that can launch an immune response) on the surface of their red blood cells called a rhesus factor—commonly abbreviated "Rh." In some cases—typically during a second pregnancy—when a woman gets pregnant, she and her fetus may have incompatible blood cells, which could trigger the mother's immune system to treat the fetus as a foreign intruder, causing a miscarriage. This occurrence is less probable if the parents are closely related, because their blood makeup is more likely to match.

"It may well be that the enhanced reproductive success observed in the Iceland study at the level of third [and] fourth cousins, who on average would be expected to have inherited 0.8 percent to 0.2 percent of their genes from a common ancestor," Bittles says, "represents this point of balance between the competing advantages and disadvantages of inbreeding and outbreeding."