I’d always thought marrying a blood relative as close as a cousin was immoral, and certainly risky if you plan to have kids. Conventional wisdom says only primitive people who live in isolated places marry cousins. It leads to stupid children. But that’s a myth.

It’s the sort of myth that leads to stupid laws. Half the states in America have banned cousin marriage, but there’s no good reason for it. You can marry your cousin and have perfectly intelligent kids.

Take Albert Einstein — was he intelligent enough for you? His parents were cousins, and he married his cousin. So did Charles Darwin and Queen Victoria. Worldwide, 20% of all married couples are cousins.

A lot of “experts,” politicians and clergymen, are dead set against cousin marriage, and they’ve convinced a lot of people, including many lawmakers, that marrying a cousin is a bad thing to do.

As with many of our laws, there is little reason for the ban. The laws date back hundreds of years to a time when the Catholic Church campaigned against cousin marriages, because in the Bible, in Leviticus, it says,”None of you shall approach to any that is near of kin.”

But a cousin isn’t terribly “near.” Just ask Brian and Caren Wagner. Mr. Wagner’s dad and Mrs. Wagner’s mom are brother and sister, so the Wagners spent a lot of time together at family functions. Eventually, they fell in love and decided to marry. This did not go over well with either of their parents.

“There was a phone call from my mother, to Brian’s father, of, ‘What are we gonna do about this?'” Caren told me.

But Mr. Wagner’s father, Dennis, knew their options were limited. “We said,‘Well, we’ve got a couple of choices. Either we can say no, we don’t want this to happen’ — which, you know it wasn’t our choice if this is what they were going to do. They’re both over 21. I said, hey, we’re not gonna lose you.”

The parents blessed the marriage. Then the Wagners decided to have kids. They’d heard stories about birth defects and worried that their kids would be stupid. But they had kids anyway — two sons — each of whom went on to be at the top of his class in school.

That confounds the conventional wisdom. Novels like James Dickey’s “Deliverance” and movies like “Brighton Beach Memoirs” reinforce the notion that cousin marriage will produce retarded children. (“You’ll have a baby with nine heads!”) But a study funded by the National Society of Genetic Counselors revealed that assumptions about cousin marriage are unfounded. The risks of birth defects or mental retardation are 2 or 3% higher among married cousins, but other parental risk factors are higher. Age, for example, increases the risk much more: There’s a 6 to 8% chance that a woman over 40 will give birth to a child with birth defects.

It would be ridiculous, however, to prohibit middle-aged women from having children. It’s equally wrong to prohibit cousins from marrying. There are risks and challenges in any marriage, but it should not be for politicians to decide such intimate matters as whether you get to marry the person you love. Love, marriage and procreation are personal choices better not left to “experts” who are often repeating myths.

There is one real risk, however, to cousin marriage. Pat Bradfield, Mrs. Wagner’s mother, had a warning about divorce: “You could divorce your husband,” she told Mrs. Wagner, “but you can’t divorce the whole family. Your father-in-law and your mother-in-law would still be your uncle and aunt.”

Now that’s expert advice worth considering.

Mr. Stossel is co-anchor of ABC News’ “20/20” and the author of “Myth, Lies, and Downright Stupidity: Get Out the Shovel — Why Everything You Know is Wrong.”