

After three centuries of this, varying forms of insanity became so common in the family that it was said that every time a new Targaryen was born, the Gods would flip a coin to determine if he or she would grow to be insane.


Therefore generations of inbreeding increasingly produced insanity in some of them. Although there were plenty of times throughout the centuries of Targaryens marrying other people of noble houses of Westeros, they still continued to practice incest occasionally.
