Kissing Cousins
Incest taboos have become prevalent in our interlinked, modern societies because promoting exogamy acts as a great mechanism for wealth redistribution. Societies that facilitate familial endogamy tend not to be relatively successful in a post-medieval economic situation.
In a very great way you can trace the evolution of the pre-Renaissance economic conditions to the inadvertent creation of a pan-European merchant class by the Roman Church throughout Europe in the late medieval period following its proclamation of extreme cosanguinity marriage decrees . These defined (and constrained) marriage between relatives out to the 7th degree (later relaxed after several decades to the 4th decree) and had a powerful disruptive effect on the tight kinship economic blocs that had characterised medieval European society and mitigated against the emergence and growth of strong City and State power structures.
Basically, it took the Catholic Church a long time to decide on its preferred balance between Roman and German techniques of kinship evaluation. It was noted by many observers at the time that under the Roman system (~4th degree) pan-European trade and tax systems had propsered, but that under the Germanic systems (~7+ degrees) the ability of the King/State to tax widely and promote wide trade networks had foundered.
Someone said in response to this argument:
it’s just utterly unlikely that a council of elders convened somewhere and decided to somehow train youngsters that incest was wrong.
Why do people believe that some of the greatest minds in some of the most powerful organisations of their time were not capable of planning and executing social engineering projects on a huge scale? The 1st through the 4th Lateran Councils of the 12th Century were that period’s equivalent of the G7 Powwows. And they concerned themselves tremendously with describing and codifying the shape of the new incest decrees, and with establishing methods of education and enforcement throughout Europe.
In recent times the Roman Church even engaged in a little bit of incest redefinition. This nicely illustrates pre- and post-Vatican II Canon Law incest taboos.
Pre-Vatican II
Post-Vatican II
Women whom Ego cannot marry are shaded in red