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, 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.

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.