To assess consanguinity, researchers give relationships an inbreeding coefficient — the higher the number, the closer the two individuals are related. Anything at or above 0.0156, the coefficient for second cousins, is considered consanguineous; that includes relationships between people and their nephews and nieces.

Are cousins inbreeding?

Contrary to widely held beliefs and longstanding taboos in America, first cousins can have children together without a great risk of birth defects or genetic disease, scientists are reporting today. They say there is no biological reason to discourage cousins from marrying.