The above consideration leads again to more speculation on the mental state of children born to such close blood relatives, in terms of these illnesses being more prevalent (sp) in cases where close blood relatives had children. Here, I'm going from memory of some show/book that said close relatives (i.e. brothers, sisters, cousins, second cousins, etc) who have children are more likely to have children with mental problems because of the close bloodlines. This is why it is, or at least was, common practice for couples getting married to have blood tests done to check for incompatibility. I may be wrong in the close bloodline thing though, like I said I'm going from memory from a number of years ago.
However, the question of incest as mentioned in the first paragraph is still valid.