Steve Sailer has been on the cousin marriage "beat" for a while now, every since his 2003 piece on the practice in Iraq. Why is cousin marriage bad? Because large interrelated clans can create sets of societies within societies. Here's a Bedouin proverb: "I against my brother, my brothers and I against my cousins, then my cousins and I against strangers" Like polygyny hyper-endogamy as a normative practice is corrosive to the institutional and civic skeleton which a liberal democracies rest upon. Remember, these are societies where you don't need to look outside the family for friends or marriage partners. The incentive for nepotism and corruption becomes very strong, and every extended family unit is operationally a "firm," analogous to the mafia. But there's one issue about this narrative which has always made me hopeful: what happens to nepotism when you don't have nephews? This is what I'm talking about: As much of the world experiences demographic transition the "circle of cousins" begins to shrink. You can't have extended families in great quantity when you don't have many siblings. If Dunbar's number is true then in societies which have experienced demographic transition you can't just fill out your Facebook friends lists with your relatives. There aren't enough of them to go around.
