DeCODE 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 define genealogical data on the population of Iceland, shows that couples related at the level of third cousins have the greatest number of offspring.

The findings hold for every 25-year interval studied, beginning with those born in the year 1800 up to the present day…

Perhaps most importantly, these new findings also 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.

If so, this could be of relevance to slowing population growth in the many other — and much more populous – societies around the world undergoing transition from closely-knit rural societies to more urbanized ones. Indeed, the UN estimates that in the 2007-2008 period the majority of the world’s population will, for the first time in human history, live in town and cities.

Interesting stuff. Comments from peers suggest that further elements of culture and economy may still be at play.