"2. We could reasonably travel more than 10 miles from home in a day." No, this is not the reason. Plenty of societies had very strong norms against cousin mar

"2. We could reasonably travel more than 10 miles from home in a day." No, this is not the reason. Plenty of societies had very strong norms against cousin marriage centuries ago -- from memory below 1 percent in colonial new england? Others did not. Sometimes because they were just far from the enforcers of such norms. But often for deliberate reasons of keeping family and property together. Saddam Hussein wasn't short of private jets, but was very concerned about losing power, and blood relatives are more reliable. And about 50 percent of his countrymen feel the same way.