Joseph Henrich in his books The Secret of Our Success and The WEIRDest People in the World, traces how the Catholic Church in the Middle Ages increasingly banned cousin marriages, beginning with Pope Gregory I. Henrich argues that these bans had tremendous effects on western society. By forcing people to marry outside their villages, they expanded trade networks, created new social ties, and had greater cultural exchange and prosperity.
It is interesting then, to note how many cousin marriages appear in the Bible. Caleb’s daughter Achsah married her cousin Othniel after he captured Kiriath-sepher (Joshua 15:17). Isaac married Rebekah, who was the granddaughter of Nahor, Abraham’s brother (Genesis 22:20-23). Jacob married his cousins Leah and Rachel, daughters of his uncle Laban (Genesis: 29:10, 16). Even in Leviticus 18, where many close relative marriages are banned, it does not ban cousin marriage.
The Church’s own justifications at the time are less clear. Some argue it was to weaken local clan power and redirect loyalty toward the Church. Later on, others pointed out the unintended economic and genetic effects. It’s interesting how the medieval Church, arguably going beyond scripture, reshaped the economics of the west.