It’s a directly cited reason in a constitutional challenge to the law in March 2008.

• Not every country is Germany.
• Post-hoc rationalizations happen always anyway. No one is immune.

There’s dominant and recessive genes.

You seem to think that people missed high school biology.

Recessive genes causing defects are rare and most people don't carry them at all. No amount of incest will will them into existing if there are no carriers. That was the obvious point about folklore, where any offspring of incest is thought of as some disfigured hobo tard.

Some nation states may require prior testing for any pair who want to become a couple (this is not just theoretical btw), using the exact same reasoning, yet most people would agree that that's overstepping and authoritarian-adjacent. Yet, the same reasoning gets re-employed and reused to post-hoc justify continuing banning incest. Another data-point pointing to illogical hubris at play.

Feel free to site a specific study from the corpus on actual human populations, and let's check its date, specific claims and numbers, and check the validity of process and statistical analysis.

Here is a hint: the few flawed modern studies that tried hard to find a hint of correlation about something already understated (like some type of hereditary high blood pressure, nothing close to typical folklore) didn't get helped by the basic understanding genes like you learned them in high school. And had to speculate about theories that involve genetics topics you didn't actually get taught at high school.