The Logic of Life: The Rational Economics of an Irrational World. Cet article n’est pas disponible.The Logic of Life: The Rational Economics of an Irrational World

Book by Harford Tim

Chapter One  
Introducing the Logic of Life  
The Economics of Sex, Crime, and Minnie Mouse  
Harpo Studios, Chicago  

Parents, brace yourselves." With those words, Oprah Winfrey introduced America to the shocking news of the teenage oral sex craze. In The Atlantic, 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, twelve- and thirteen-year-old girls are performing oral sex on as many boys as they can." Flanagan poked a bit of fun, but she wasn't really laughing: She was convinced that the fears were largely justified. Indeed, the American "blow job epidemic" has now been addressed everywhere from PBS documentaries to the editorial page of The New York Times, sometimes with giddy and slightly voyeuristic horror, sometimes with calm reassurance that the epidemic is simply a myth.  

The so-called epidemic is often exaggerated, but it's no myth. One recent study, conducted by researchers at Johns Hopkins Bayview Medical Center in Baltimore, found that between 1994 and 2004, young people between ages twelve and twenty-four became more than twice as likely to report that they'd recently had oral sex. (For boys the rate climbed from 16 percent to 32 percent; for girls, from 14 percent to 38 percent.) Anecdotal evidence from experts suggests that the true increase may be even higher. I sought advice from Professor Jonathan   Zenilman, an expert at Johns Hopkins University on sexually transmitted diseases. He explained to me that in 1990, perhaps half the women and a quarter of the men who came to his clinic (both teenagers and adults) sometimes performed oral sex on their partners. He believes that oral sex is now much more common: "Now it's seventy-five to eighty percent." And while it's the blow jobs that predictably have caused the panic, oral sex is now much more equitably distributed between boys and girls than in 1990. "Epidemic" might be putting it too strongly, but oral sex is definitely in vogue.  

The question few people seem to have asked is "Why?" Are kids really becoming more depraved-or are they just being smart? Might there not be such a thing as a rational blow job?   I'll say more about exactly what rational means later in this chapter, after we've dealt with those libidinous teenagers. But the basic idea is not complicated: Rational people respond to trade-offs and to incentives. When the costs or benefits of something change, people change their behavior. Rational people think-not always consciously-about the future as well as the present as they try to anticipate likely consequences of their actions in an uncertain world.  

Armed with this basic definition of rationality, then, we can ask: What are the costs, benefits, and likely consequences of a blow job? Okay, perhaps the benefits are too obvious to be stated, particularly for the recipient. But it should also be obvious that the cost of a close relative of oral sex has risen: Regular sex is more costly than it used to be because of the spread of HIV/AIDS. HIV is much more likely to be spread by regular sex than oral sex. Many teenagers know that: One recent study of sex education concluded that it was more common for U.S. kids to be taught about HIV/AIDS than about preventing pregnancy. Teenagers may also know of other sexually transmitted diseases such as gonorrhea, an infection that might make a girl infertile if transmitted through penetrative sex, but when transmitted by oral sex may have much milder symptoms, such as a sore throat. The costs of oral sex are, quite simply, lower than the costs of regular sex.  

If teenage girls really do weigh those costs and benefits before going down on their boyfriends, this is a straightforward explanation for the growing popularity of oral sex. Since regular sex is riskier than it used to be, and since teenagers are unlikely to have given up on the idea of having sex, the rest is basic economics. When the price of Coca-Cola rises, rational people drink more Pepsi. When the price of an apartment in the city goes up, rational people move out to the suburbs. And when the price of penetrative sex rises, rational teenagers have more oral sex instead.  

Certainly, the evidence suggests that teenagers are moving toward less risky sexual behavior. The U.S. Centers for Disease Control and Prevention reports that since the beginning of the 1990s, the number of teenage virgins has risen by over 15 percent. There are still a few million teenagers who haven't given up on sex, of course, but since the early 1990s they've 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.  

Perhaps Oprah shouldn't be quite so worried. Oral sex isn't a symptom of more promiscuous teenagers. In fact, it's a sign that teenagers are behaving more responsibly, in enthusiastically-and rationally-choosing an alternative to riskier sex.  

This is all very cute-or horrifying, depending on your tastes. But it is also a glib explanation. Before blithely claiming that oral sex is more popular because rational teenagers know that regular sex is riskier, a real economist would want a tighter hypothesis and serious data to back it up.  

That real economist might well be Thomas Stratmann, who with the law professor Jonathan Klick has pinned down the rationality of the teenage sex drive rather precisely. Rational teenagers would have less- risky sex if the cost of risky sex went up, so the question is how to work out whether that is how teenagers behave. That requires some precisely measurable source of increased cost, something more quantifiable than a general increase in the amount of education about AIDS.  

The U.S. Constitution has duly obliged, by providing a federal structure that allows states to determine their rules governing teenage abortion; some permit teenagers to have abortions without the notification or consent of their parents, and some do not. Such laws provide plenty of fodder for political controversy, but they also provide a natural experiment for researchers. Since abortion notification laws make it more difficult for teenagers-but not for adults-to get an abortion, they should discourage risky sex among teens, relative to adults. If, that is, teenagers are in fact rational.  

It is not hard to see that abortion notification laws raise the cost of getting pregnant, at least for those teenagers who, given the choice, would have terminated an accidental pregnancy without telling their parents. If teenagers look ahead and work all this out, they should also take extra steps to prevent that accidental pregnancy-steps which, besides that of choosing oral sex over regular sex, are likely to include more use of condoms, or perhaps no sexual activity at all.  

Sex is not a calculated act, and so that degree of foresight may sound implausible, but Klick and Stratmann found persuasive evidence that abortion notification laws really do discourage teenagers from having risky sex. Looking at statistics from sexual health clinics, they found that wherever and whenever parental notification laws are passed, gonorrhea rates start to fall in the teenage population relative to the adult population-to whom, of course, the new laws do not apply. The only explanation for this would seem to be that an abortion notification law significantly raises the risk of unprotected sex, and that the teenagers rationally respond to that risk.  

Sex, then, has a cost. The risk of AIDS-along with intensive education about that risk-has probably encouraged teenagers to switch to a lower-cost substitute, oral sex. The threat to careless or unlucky girls that they will have to tell Mom or Dad that they accidentally got pregnant has done something similar.  

A young economist named Andrew Francis has gone still further. If oral sex is a substitute for regular sex, he reasoned, isn't it at least possible that heterosexual sex is a substitute for homosexual sex? The rise of AIDS has made it more risky than it used to be to have sex with men, making homosexuality more dangerous for men and heterosexuality more dangerous for women. If the cost of one's sexual orientation is perceived to have gone up, wouldn't we expect rational people to respond to that?  

Andrew Francis stumbled upon the possibility-it remains speculative-while trawling through a survey from the early 1990s in which nearly 3,500 people were asked intimate questions about their sexual preference and sexual history. The survey also asked people whether they knew anyone with AIDS. Francis then concentrated on people whose relatives suffered from AIDS, because you can choose your friends but not your relatives: It would not be surprising, or informative, to discover that gay men knew more people with AIDS than straight men.  

Francis discovered that both men and women with a relative who had AIDS were less likely to have sex with men, and less likely to say they were attracted to men. At first, that didn't seem to make much sense-the unfortunate relatives were quite likely to be gay men, but if anything, genetic theories suggest that people with gay relatives should be more likely to be gay, not less. Then he realized what was going on: "Oh my God, they were scared of AIDS!" he told Stephen Dubner and Steven Levitt for The New York Times Magazine.  

With that insight, everything fits. People with a relative who had AIDS were more likely to be aware of how terrible it is, especially back in the early 1990s, when treatments for AIDS were very limited and the disease killed many people within two years. Then what? Men who had a relative with AIDS were less likely to say that they found the idea of sex with men appeali...