A Cock-and-Bull StoryExplaining the huge rise in teen oral sex.

By Tim Harford

"Parents, brace yourselves." With those words, Oprah Winfrey introduced news of a teenage oral-sex craze in the United States. In the Atlantic Monthly, Caitlin Flanagan wrote, "The moms in my set are convinced—they're certain; they know for a fact—that all over the city, in the very best schools, in the nicest families, in the leafiest neighborhoods, 12- and 13-year-old girls are performing oral sex on as many boys as they can."

Are they right? National statistics on teen fellatio have only recently been collected, but the trend seems to be real. Johns Hopkins University Professor Jonathan Zenilman, an expert in sexually transmitted infections (and father of former Slate intern Avi Zenilman), reports that both the adults and the teenagers who come to his clinic are engaging in much more oral sex than in 1990. For men and boys as recipients it's up from about half to 75 to 80 percent; for women and girls, it's risen from about 25 percent to 75 to 80 percent.

In some quarters, that might be regarded as progress, but how you feel about it probably depends on whether you are a teenager or a parent of teenagers. I am more than a decade away from being either and so regard myself as a neutral in this debate. Moreover, as an economist, I feel uniquely qualified to opine on why it is happening.

Now, there is no shortage of explanations: Perhaps everyone just thought that if it was good enough for Bill Clinton and Monica Lewinsky, it was good enough for them. But an economic explanation would instead start with the premise that this is a response to changing incentives. What sort of incentives have changed?

Schoolchildren are now bombarded with information about the risks of sex, particularly HIV/AIDS. Oral sex can be safer than penetrative sex: It dramatically reduces the risk of contracting HIV and reduces the effects of some other sexually transmitted infections (although you can still pick up herpes, warts, and thrush). An infection that might have made a girl infertile instead gives her a sore throat.

The rest is basic economics. When the price of Coca-Cola rises, rational cola-lovers drink more Pepsi. When the price of penetrative sex rises, rational teenagers seek substitutes. Perhaps we shouldn't be surprised that even as the oral-sex epidemic rages, the Centers for Disease Control and Prevention reports that the percentage of teenage virgins has risen by more than 15 percent since the beginning of the 1990s. Those who are still having sex have switched to using birth-control methods that will also protect them from sexually transmitted infections. Use of the contraceptive pill is down by nearly a fifth, but use of condoms is up by more than a third. The oral-sex epidemic is a rational response to a rise in the price of the alternative.

Now, this is a glib explanation. A real economist would want a tighter hypothesis and serious data to back it up. That economist might well be Thomas Stratmann, who, with law professor Jonathan Klick, has pushed the idea of the rational teenage sex drive. Their hypothesis is that if teenagers really did think about the consequences of their actions, they would have less risky sex if the cost of risky sex went up. They discovered a very specific source of that higher risk: "In some states, there are abortion-notification or -consent laws, which mean that teenagers can't get an abortion without at least one parent being informed or giving consent." If teenagers are rational, such laws would discourage risky sex among teens, relative to adults.

Klick and Stratmann claim to have found evidence of exactly this. Wherever and whenever abortion-notification laws have been passed, gonorrhoea rates in the teenage and adult populations start to diverge. When it becomes more troublesome to get an abortion, teenagers seem to cut back on unprotected sex.

Economic nerds may be interested to know that the Klick-Stratmann statistical technique owes much to the one used by Steven "Freakonomics" Levitt and John Donohue to show a link between legalized abortion in the 1970s and lower crime in the 1990s.

The rest of us may be wondering what to make of it all. On the one hand, good news: Teenagers are finding safer ways to get their kicks. On the other, it suggests that teenagers believe one of the most serious consequences of an unwanted pregnancy is that their parents will find out. If teenagers are avoiding unsafe sex, it may not be for the best reasons.