Forbidden Relatives challenges the belief - widely held in the United States - that legislation against marriage between first cousins is based on a biological risk to offspring. In fact, its author maintains, the U.S. prohibition against such unions originated largely because of the belief that it would promote more rapid assimilation of immigrants. A social anthropologist, Martin Ottenheimer questioned U.S. laws against cousin marriage because his research into marriage patterns around the world showed no European countries prohibit such unions. He examines the historical development of U.S. laws governing marriage, contrasts them with European laws, and analyzes the genetic implications of first cousin marriage. Modern genetic evidence, Ottenheimer says, doesn't support the concept that children of these unions are at any special risk.