80% of all marriages in history have been between second cousins or closer.
According to Robin Fox, a professor of anthropology at Rutgers University, it's likely that 80% of all marriages in history have been between second cousins or closer. A very probable explanation is that until the past century, families tended to remain in the same area for generations, and men typically went courting no more than a 5-mile radius around their homes, which is basically the distance a 1 and a half-hour walk would take you. That makes more sense if you think about the number of ancestors a given person can have. Let's say each generation has children when they are 25 years old. If you go back 2 generations the number of ancestors you have will be your 4 grandparents. If you go back a thousand years to the start of the 11th century AD, that will be about 40 generations. If no inbreeding was taking place, the number of ancestors you would have on 1000AD would be over 1 trillion. That's 1,099,511,627,776 distinct ancestors to be exact. That's impossible since the population back then was just 310,000,000.