It might be considered a taboo in present society, but a new study has suggested that incest or mating with cousins has reproductive advantages.

Earlier research has shown that children of related couples are more likely to inherit two copies of disease-causing recessive genes.

However, scientists at deCODE Genetics, a biopharmaceutical company based in Reykjavik, Iceland, have found that inbreeding between 'kissing cousins' might produce more children and grandchildren.

K�ri Stef�nsson and colleagues suggest marrying third and fourth cousins is so optimal for reproduction because they sort of have the 'best of both worlds'.

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.

But the new results suggest that things don't always work that way, says Stef�nsson.

Stef�nsson 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 socio-economic 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 percent 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.

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.