All societies regulate the sexual activities of their members to some degree.

That statement is true

and it differ from one society to another. For example in China or south East Asia, it is very common for first cousins to marry each other, meanwhile in the United States, it kinda taboo to do that

another example is in the US laws, some states will throw those who engage in incestuous activities to the Jail, meanwhile some states still legalized it as long as they're not under the age of 18

Hair follicles, blood, etc.