[dead]

I've read that even first cousins have very little additional risk of having a baby with birth defects. Second cousins, none at all.

Used to be people never moved more than ten miles away from their small isolated villages. Lots of cousin marrying going on for millenia.

[Yaelle]

First cousins only share 1/8 of their genes. Second - is that the children of first cousins marrying? - would share 1/32.

You can have dangerous mismatching of genetics between total strangers. Certain genes, if shared by both parts of a couple, will make it hard for them to have children or have healthy children. Yet they are not related in any recent generation.