Eventually, after generations of it. But not all the time. Ancient Egypt produced several competant and great rulers from sister-brother coupling.Facial said:The thing is, incest IS harmful.
We're all humans, decended from the first human that evolved from Cro-magnon.
In a certain sense, all living things on earth are distantly related, since we all evolved from the same single-celled organism in the pre-cambrian seas 3 billion years ago.