From googling, I understand that out-of-date research in the US blamed various birth defects on cousin marriage, but that this is now pretty well discredited - I'm NOT an expert, however!
It seems that the genetic risk is slightly higher for children whose parents are cousins, but that only applies if one (or both) of the parents is unhealthy. My husband is a biologist, but - nach - is now asleep, so don't think this is the moment to ask him for an explanation!
However, he has frequently pointed out to me that it's normal for our kids to be healthy because they're hybrids...
(He's Swiss, I'm English!)
I don't know how common cousin marriage was before the Reformation, but my uncle (who researched Mum's family) has found at least one case among my Irish Catholic ancestors in the 1800's.
I'll try and find out more on the biological pros and cons tomorrow, but I'm about to keel over and go to bed - 17th wedding anniversary today, and far too much champagne is starting to have its effect....
Rambler (Or perhaps Staggerer ismore accurate right now!)