incest prohibition, n.All

The Incest prohibition (n.) is ⁠​‌​​​‌​​​‌​‌‌​​​​‌​‌​​‌‌​‌​​‌​​​​​‌‌​​‌​​​‌‌​‌‌​⁠universal, so one could call it natural; it is also a rule, so one could call it cultural.