Inbreeding was standard accepted practice pretty much everywhere until 1500 years ago, and still is in many parts of the world. (In many Arab countries, for example, marriages between first cousins make up 15+% of all marriages, and marriages between first or second cousins make up 30+% of all marriages.)

It’s especially in Western Europe where cousin marriages out to 2nd, 3rd, or further cousins were taboo, and universal “outbreeding” was/is the enforced norm. (Primarily because the Catholic church wanted to break the political power of extended families, and divert more inheritances to the church instead of to people’s kin. In similar vein they discouraged/disallowed divorce, adoptions, marriage of widows, etc.) I did a google search to look for quick sources just now. This page has a summary: http://www.collective-action.info/_DEB_MAP_RelationMarriageP...