William Saletan defends cousin marriage on the grounds that you aren't that likely to have a baby with a birth defect. I think he's got the problem wrong: it's not an individual risk, but a population risk. Yes, an individual cousin marriage has a fairly low probability of birth defects. But if cousins keep marrying each other, they will reinforce some nasty recessives. That's why small populations that don't outmarry--the Amish, for example--have problems with birth defects, even when they aren't practicing cousin marriage. Whittle your mating population down to a thousand people and you're asking for trouble.