What is the great "myth" that is commonly held in North America regarding the reason for the establishment of a taboo against first-cousin marriage in the United States?

Answer: The correct answer is : The myth is that a greater number of birth defects would be caused.

Explanation: Nancy Thornhill conducted a sample study of 129 societies where she discovered that the most common incest taboo was among cousins. Massachusetts Governor Briggs delegated a commission to study "idiots" in the state and one of the factors responsible for idiocy is cousin marriage, also a study between deaf and mute in Kentucky yielded the same results, adding blindness.