The trick is eligible women are rare and valued. Thus when looking for a wife you end up using the good old boy network to find a wife. So you end up talking to your family and people in your village and find a wife in the village next door. Well because of previous wife swaps that woman is your 2nd cousin. Its proximity plus familiarity. The fact that an eligible woman is a rare thing complicates matters and makes this type of thing more likely. They can't just go find a woman anywhere. They have to take whatever they can get.
The same with Christian communities. They can only marry other Christians. Thus their are fewer of them. Thus they end up eventually being with a distant relative. The mass exterminations etc don't help.
The trick is eligible women are rare and valued. Thus when looking for a wife you end up using the good old boy network to find a wife. So you end up talking to your family and people in your village and find a wife in the village next door. Well because of previous wife swaps that woman is your 2nd cousin. Its proximity plus familiarity. The fact that an eligible woman is a rare thing complicates matters and makes this type of thing more likely. They can't just go find a woman anywhere. They have to take whatever they can get.
The same with Christian communities. They can only marry other Christians. Thus their are fewer of them. Thus they end up eventually being with a distant relative. The mass exterminations etc don't help.