New research from Iceland's deCode project reported in the Economist shows that the most children born come from people who are third and fourth cousins. Not too closely related, not too unrelated. So somewhere between birth defects and frigid feminism there's a balance:
The strong relationship between kinship and fertility was so unexpected that the researchers have not yet calculated exactly how much it contributes to the demographic transition. But even from the figures they present, it is clearly an important factor, and one that is likely to apply in other parts of the world where the records needed to prove it are not so good. Even in poor countries, birth rates are now falling fast. An important part of the explanation may simply be the additional choice of mates that development and urbanisation bring with them.Hmmm..... Kissing cousins is okay, I guess. Just don't kiss too close.