From Scientific American:
Study analyzing more than 200 years of data finds that couples consisting of third cousins have the highest reproductive success. 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.