Inbreeding is where cousins and other close relatives have children together. Most cultures have strong taboos against it, primarily because of the increased risk of birth defects. Here's why that risk isn't all it's cracked up to be.
It's fairly well-established that the children of older mothers are more likely to have genetic defects than those of their younger counterparts. Now we have some of the first solid evidence that older dads could carry a risk as well.