While reading about cousins and degree of genetic relationship, I came across this:
Scientists have established a substantial and consistent positive correlation between the kinship of couples and the number of children and grandchildren they have. The study, which analyzes more than 200 years of deCODE's comprehensive genalogical data on the population of Iceland, shows that couples related at the level of third cousins have the greatest number of offspring. These new findings suggest that the recent and dramatic demographic shift experienced in Iceland -- from a rural society to a highly urbanized one -- may serve to slow population growth, as individuals are exposed to a much broader range of distantly related potential mates.
Third cousins share a great-great grandparent.