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.
"It's an obvious form of genetic discrimination," she said of laws banning cousins from marrying or that allow marriages only on the condition that the husband and wife undergo genetic counseling or won't have children. "We don't forbid other people with a high risk of other genetic problems from having a child."
In parts of the Middle East, Africa and Asia, marriages between cousins are commonplace.
Several recent highly publicized cases put the spotlight on so-called genetic sexual attraction. "There is a consistent and necessary need to be close to each other physically," Cameron told "Good Morning America." "To actually feel each other close."
Cameron and Heaney share the same mother but grew up apart. Cameron was put into foster care when he was just a toddler while Heaney was raised by their birth mother.
They met only once during childhood, a brief meeting arranged by social services. Then, two years ago, they reunited at a family reunion.
"I think the first time I saw Danielle I found her very attractive," Cameron said. "But I also thought, 'Hang on a second, this is your sister you're talking about.'"
These cases are rarely considered by the courts, which have not been sympathetic to siblings who want to marry. Earlier this year Germany's highest court upheld a law that makes incest a criminal offense, rejecting an appeal by a man who was sentenced to prison after fathering four children with his sister. The court cited the need to protect the family order and avoid serious genetic illness.
A U.S. federal appeals court rejected a similar appeal in 2005 by a brother and sister who had several children together.
That doesn't necessarily mean cousins should be banned from marrying or that a court would uphold a ban on marriages between cousins, said Joanna Grossman, a law professor at Hofstra University, who has written about incest. "I don't think there is a justification for the cousin marriage ban," she said.
The best justification against relationships between family members, she argued, is to prevent the corruption of families and protect children from becoming victims of abuse. That justification doesn't necessarily apply to marriages between consenting adult cousins who grew up apart, she said.
Still, she said, don't expect major overhauls to anti-incest laws anytime soon.
"There would be more variation [between laws] if anyone spent any time thinking about it, but the truth is no one cares that much," she said. "There's no incest lobby. Most states have never gone back to the table to see if they want the law to say something different than it did 200 years ago."