, the Web site to which the NSGC study refers couples for support services, wants to keep Big Brother out of the bedroom. Advocates of cousin marriage say they’re against sex between siblings, but they can’t explain why. If you still won’t cry uncle, let’s look at hard-core incest. If first cousins can marry, why can’t Roger and Chelsea? Science says there’s no difference. First cousins have as many genes in common as a man and his half-brother’s child do. Would you rather restrict marriage to ordinary first cousins? That won’t get you out of the woods.

The greater the genetic similarity between your parents, the greater your chance of getting two copies of the bad gene. To get the disease, you have to get the bad gene from both parents. To let cousins marry, they argue, is “to play Russian roulette with genetics.” Many genetic diseases are caused by recessive genes. Instead, advocates of laws against cousin marriage appeal to science.

You can’t appeal to the Bible in the Bible, God commands marriages between first cousins. Is cousin marriage icky? Why? You can’t appeal to Victorian morality Queen Victoria married her first cousin. Often, you have to change your opinions on related issues in order to honor that principle, or you have to throw out the principle and change your mind about the original question.

You have to state a principle and think through its implications. You can’t just say the practice in question is icky. As Frame Game has argued before, topics such as sex with animals, dog-eating, and sex with cousins are never as simple as they’re made out to be.

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