All in the Family: Where Does Incest Begin?

May 7, 2008— -- Danielle Heaney and Nick Cameron were convicted of incest, and the half brother and sister were sentenced to nine months' probation. As a compromise of sorts, the courts in their native Scotland allow the couple to live together but not sleep together.

If the couple moved to France, where Napoleon abolished incest laws nearly 200 years ago, their relationship wouldn't be criminal. If they lived in Idaho, however, they could face up to 10 years in prison.

While there is a widespread cultural taboo against incest, there appears to be no modern consensus on whether all such relationships should be banned or how close is too close when it comes to familial relationships. All U.S. states and most countries bar marriages within the nuclear family, but American cultural mores aside, there is far less agreement about half siblings or first cousins.

"In many parts of the world, it's a legal and even preferred form of marriage," said Robin Bennett, a genetics counselor at the University of Washington and the former president of the National Society of Genetics Counselors, referring to marriages between cousins.

There have been a number of justifications for anti-incest laws, which historians trace at least as far back as the Bible, the book of Leviticus, which bars sexual relationships between certain family members.

The anthropologist Claude Levi-Strauss theorized that forcing men to look outside their families for a mate would foster the exchange of members among groups, the building of alliances and, ideally, the avoidance of conflict. And, there is the well-known propensity toward genetic disorders among children of incest.

Some geneticists and sociologists, however, question whether these are reasons to ban all relationships among relatives, even half siblings like Heaney and Cameron.

Bennett said that while there is a higher risk of birth defects from marrying half siblings, that does not mean the risk is so high that those marriages should always be banned.

Bennett is co-author of a 2002 study that showed that first cousins can have children together without a great risk of genetic defects. Children of first cousin marriages -- banned in about half of the U.S. states -- have serious genetic disorders or mental retardation about 1.7 to 2.8 percent more often than children of unrelated parents, the study found.