>>513616878
There are four types of social organization. Bands, Tribes, Chiefdoms, and States.
Most people lived in Bands prior to the neolithic. A Band is about 40 people. Tribes are basically loose confederations of Bands which depend wholly on voluntary support. Chiefdoms are the first form of social organization that has any kind of hierarchical authority.
Clans are not the same as Bands and Tribes because Clans are formed within tribes/states. Although clans are a product of tribal behavior(see nepotism).
States generally hate clannish/tribal behavior because it creates conflict. Although monarchy itself was the product of one clan(faction) coming to dominate the rest. To ease tensions between clans/houses/tribes, states promote exogamy. People are more likely to cooperate with people who are related to them. The more closely they are related, the greater the cooperation.
This is why both states and religions began to discourage incest. To increase social integration/cooperation, not because they recognized a tiny increase in the odds of inheritable diseases.
In any case, even in Europe, pretty much everyone lived in tiny villages(manorial system) up until only a few hundred years ago(or less). The only exceptions were soldiers, traders, artisans, prostitutes, and the church. And of course, colonists moving to the new world.
If incest was the problem many people imagine, you wouldn't be here.