What is the "shudder test" in Steven Pinker's "The Moral Instinct"?

This is the paragraph that I do not understand:

“There are, of course, good reasons to regulate human cloning, but the shudder test is not one of them. People have shuddered at all kinds of morally irrelevant violations of purity in their culture: touching an untouchable, drinking from the same water fountain as a Negro, allowing Jewish blood to mix with Aryan blood, tolerating sodomy between consenting men”.

Then, what is the shudder test?? What do they mean by “people have shuddered”?