The Washington Post reports that because Pennsylvania prohibits marriages between first cousins, long-time lovers Donald W. Andrews Sr. and Eleanor Amrhein moved into Maryland to be wed in a civil ceremony. The Ethics Scoreboard wishes the couple well, and thanks them too. People always confuse morality and ethics, so it’s good to have an issue arise that clarifies the distinction. That issue is whether marriages between first cousins are “wrong.” Are they?

They are illegal in 30 states, though six of these permit such marriages under special circumstances. That makes them wrong in the judgement of those states, at least, and breaking a law, any law, is itself wrong except when the law causes harm or is objectively unjust. Those 30 states hold a marital union between cousins to be wrong because of ancient taboos against marriages between blood relatives, a taboo that developed long before there was any science of genetics to bolster it.

Taboos enter the realm of morality, which involves formal systems of right and wrong where certain behaviors are either dictated or forbidden by a religion, culture, or authority figure. In morality, the ethical process of evaluating conduct is completed; an act is wrong because that’s what the moral code says…argument over. So well ingrained is the moral prohibition of intimate relations between cousins that most people get an automatic twinge of revulsion when they think about it. It just seems uncomfortably close to incest, one of our strongest taboos.

But is it wrong?

There is strong evidence that cousin to cousin marriages got swept up in the cultural revulsion toward inter-familial marriages. The Bible, surprisingly, in its toughest Book, Leviticus, condemns marriages between parents and children, brother and sister, half-brother and half-sister, stepbrothers and stepsisters, step-parents and step-children (sorry, Woody!), nephews and uncles or aunts, fathers-in-law and daughter-in-law, brother-in-law and sister-in-law, and grandfather and granddaughter, but not marriages between cousins. There is a logical reason for the prohibition, however: it is a lot more effective to teach and enforce a broad taboo against all marriages among close blood relatives than to have a smattering of exceptions. But that’s primarily a practical argument, not an ethical one.

Legislators justify the state laws against cousin to cousin marriages with science: studies show that the children of such marriages have twice the likelihood of genetic defects. That seems a rather questionable rationale in this day and age, however. States do not forbid marriages between dwarves or couples who are both congenitally deaf; people with Down’s Syndrome can marry; so can carriers of the gene for the lethal Huntington’s Chorea, like folk-singing legend Woody Guthrie (and Arlo turned out just fine). The United States has officially foresworn eugenics, and even doubling the usual chances of a genetic anomaly only brings it to 6 or 7 percent. The anomaly might not even be a serious one. Heck, Charles Darwin married his first cousin; you’d think if anyone would be wary, he would.

If marriages between cousins are wrong, there has to be a reason. Do the marriages harm individuals or society? Do they threaten the social structure? Are they unhealthy or dangerous? If we doubled or tripled the number of cousins marrying each other in America, would anyone know the difference? If there’s harm, what is it?

But we can easily see the harm of forbidding such marriages, which brings us back to Donald W. Andrews Sr. and Eleanor Amrhein. They are both in their late 30s, and don’t plan on having children, so the genetic argument, weak as it is, doesn’t even apply. They love each other. A marriage will make them happier, and cause no tangible harm to anyone. Yes, many of their family members have shunned them since they revealed their intentions; some family members also shun those who marry outside their race, faith, or nationality, or reject a son or daughter or nephew or niece who is gay. But Donald and Eleanor aren’t doing the shunning, and they aren’t doing anything wrong, either.

This is an area, like gay marriage, where law and morality has lagged behind ethics. Is marriage between cousins wrong? No. Many people think it is, because that’s what they have been taught. “Taught,” however, isn’t what ethics is all about. Ethics is about thought. And once you think about it, there is no good reason to stop Donald and Eleanor from tying the knot.