Previous studies have reported that related human couples tend to produce more children than unrelated couples but have been unable to determine whether this difference is biological or stems from socioeconomic variables. Our results, drawn from all known couples of the Icelandic population born between 1800 and 1965, show a significant positive association between kinship and fertility, with the greatest reproductive success observed for couples related at the level of third and fourth cousins. Owing to the relative socioeconomic homogeneity of Icelanders, and the observation of highly significant differences in the fertility of couples separated by very fine intervals of kinship, we conclude that this association is likely to have a biological basis.
I actually missed this back in 2008. It got attention from bloggers (John Hawks, Razib Khan) and the mainstream press. The Economist writes that, if outbreeding infertility is a thing, we should conclude that
The demographic transition is thus, in part, a pure accident.
This blog suspects that the story is more interesting.