According to comedian Jimmy Carr the main reason is because the Catholic Church banned cousin marriage in the 1200’s.
So Nelson Mandela Bay residents – if you have a weak chin you will be forgiven for blaming your ancestors for being just that teensy bit too close to one of their relations.
An intriguing hypotheses one would think but it does bear checking out.
According to Wikipedia, before 1215 the Catholic Church’s rules on marriage between blood relatives (consanguinity) were historically very strict.
For centuries, the Church calculated kinship differently than civil law, using a system based on “degrees” (the number of generations back to a common ancestor).
During the early and high Middle Ages, the Church had prohibitions extending up to the seventh degree of consanguinity (which would include marrying a fifth cousin). This meant marrying a fourth or fifth cousin was theoretically forbidden without a special dispensation.
EBSCO and Encyclopedia.pub say that during the Fourth Lateran Council the Catholic Church recognized that the rule extending to the seventh degree was extremely burdensome, difficult to track, and led to a high demand for dispensations.
In 1215, Pope Innocent III convened the Fourth Lateran Council.
This council issued a crucial decree that reduced the prohibited degrees of consanguinity from the seventh degree to the fourth degree.
What this meant for Cousin Marriage:
First Cousins: Marriage between first cousins (who are in the second degree of kinship) remained forbidden without a dispensation.
Second Cousins: Marriage between second cousins (who are in the third degree of kinship) remained forbidden without a dispensation.
Third Cousins and Beyond: Marriage between third cousins (in the fourth degree) was still forbidden, but marriage between fourth cousins and beyond was now permitted without needing any dispensation.
So, Jimmy, strictly speaking the Church did not impose a ban on cousin marriage in the 1200s; rather, it liberalized the rules by reducing the scope of the pre-existing ban, making it easier for distant cousins to marry.
Today, marriage between first cousins is still considered an impediment but is routinely dispensed by bishops.
