More sex is safer sex

MORE SEX IS SAFER SEX



It's true: AIDS is nature's awful retribution for our tolerance of immoderate and socially irresponsible sexual behavior. The epidemic is the price of our permissive attitudes toward monogamy, chastity, and other forms of extreme sexual conservatism.

You've read elsewhere about the sin of promiscuity. Let me tell you about the sin of self-restraint.

Suppose you walk into a bar and find four potential sex partners. Two are highly promiscuous; the others venture out only once a year. The promiscuous ones are, of course, more likely to be HIV-positive. That gives you a 50-50 chance of finding a relatively safe match.

But what if all once-a-year revelers could be transformed into twice-a-year revelers? Then, on any given night, you'd run into twice as many of them. Those two promiscuous bar patrons would now be outnumbered by four of their more cautious rivals. Your odds of a relatively safe match just went up from 50-50 to four out of six.

It's true that loosening up will make those ``relatively safe'' matches a little less safe then they used to be. But that's easily outweighed by their increased availability.1 1A quick example: Suppose the bar contains two people with infection probability 5% and two with infection probability 85%. Then your chance of a bad draw is 45% (the average of 5%, 5%, 85%, and 85%). But if you replace the two 5's with four 10's, your chance of a bad draw goes down to 35% (the average of 10%, 10%, 10%, 10%, 85% and 85%).

Or consider Martin, a charming and generally prudent young man with a limited sexual history, who has been gently flirting with his co-worker Joan. As last week's office party approached, both Joan and Martin silently and separately entertained the prospect that they just might be going home together. Unfortunately, Fate, through its agents at the Centers for Disease Control, intervened. The morning of the party, Martin happened to notice one of those CDC-sponsored subway ads touting the virtues of abstinence. Chastened, he decided to stay home. In Martin's absence, Joan hooked up with the equally charming but considerably less prudent Maxwell---and Joan got AIDS.

When the cautious Martin withdraws from the mating game, he makes it easier for the reckless Maxwell to prey on the hapless Joan. If those subway ads are more effective against Martin than against Maxwell, they are a threat to Joan's safety. This is especially so when they displace Calvin Klein ads, which might have put Martin in a more socially beneficent mood.

If the Martins of the world would loosen up a little, we could slow the spread of AIDS. Of course, we wouldn't want to push this too far: If Martin loosens up too much, he becomes as dangerous as Maxwell. But when sexual conservatives increase their activity by moderate amounts, they do the rest of us a lot of good. Professor Michael Kremer of MIT's economics department has estimated that the spread of AIDS in England could plausibly be retarded if everyone with fewer than about 2.25 partners per year were to take additional partners more frequently. That would apply to three-quarters of all British heterosexuals between the ages of 18 and 45.


* * * * * * * * * * *

Now let me tell you about Bruno. Bruno has had more partners in the past month than Martin has had in a lifetime. Martin prefers partners who share trust and intimacy; Bruno prefers partners who don't share their first names. Bruno might or might not be infected, but if you're playing the odds, you (or your sister) would be a lot safer with Martin.

But Bruno has decided to change his ways. It doesn't matter why---it might be fear of disease, or a hankering to settle down, or a new passion for tennis that consumes the bulk of his energy---but for whatever reason, Bruno expects his sex life to be a lot like Martin's from now on.

Despite Bruno's radical reform, he'll never be as safe as Martin. He can change the way he behaves in the present and the future, but he can't change the way he behaved in the past. And when you're sizing up a partner, it's the past that's relevant. Martin's occasional flings can improve the partner pool and make everyone else safer, but Bruno's flings do just the opposite.

You might be tempted to conclude that the best thing Bruno can do to slow the epidemic is to stay home. But surprisingly, that's not the case. It's true that Bruno can't improve the partner pool as Martin does, but he can still provide a different---if more macabre---social service. To visualize that service, imagine you're an AIDS virus and you're making a list of the worst fates that could befall you. One of the first entries on your list would be getting passed on to a guy like Bruno, whose future plans don't include a lot of partying. If Bruno goes out tonight and picks up the virus, he does his part to slow the epidemic by serving as a ``dead end'' for a chain of infection.

If you or someone you care about plans to be trolling for partners tonight, you should hope that Martin is hitting the bars and Bruno is home with a video. But if you plan to troll for partners tomorrow night, you should hope that Bruno's in the bars tonight. Better the virus should be passed to Bruno, who won't be back for six months, than to Promiscuous Pete, who is sure to be around every night.

Sexual conservatives come in two varieties---those with moderate sexual pasts (like Martin), and those who expect moderate sexual futures (like Bruno). (Of course it's perfectly possible for one person to be both kinds of conservative at once.) Conservatives of either variety can slow the epidemic by slightly loosening their standards. Martin slows the epidemic whenever he takes an uninfected partner and (at least temporarily) saves that partner from mating with someone more dangerous. Bruno slows the epidemic whenever he takes an infected partner and (at least temporarily) prevents that partner from infecting someone more promiscuous than Bruno.

Bruno and Martin are equally interesting, each in his own way. In what follows, I'll confine my attention to Martin, though much of what I'll say applies to Bruno as well.


* * * * * * * * * * *

If multiple partnerships save lives, then monogamy can be deadly. Imagine a country where almost all women are monogamous, while all men demand two female partners per year. Under those circumstances, a few prostitutes end up servicing all the men. Before long, the prostitutes are infected; they pass the disease on to the men; and the men bring it home to their monogamous wives. But if each of those monogamous wives were willing to take on one extramarital partner, the market for prostitution would die out, and the virus, unable to spread fast enough to maintain itself, might well die out along with it.

The parable of the monogamous wives has a more profound moral than the legend of Martin and Joan. Here's why: If Martin mates with Joan---or if a once-a-year barroom reveler graduates to twice a year---there is both bad news and good news for the war against AIDS. The obvious bad news is that each new coupling gives the virus an extra chance to spread. The less obvious good news is that Joan is safer with Martin than with Maxwell. The story of Martin---like the story of the bar patrons---proves only that at least there is some good news. But those stories don't prove that the good news can outweigh the bad.

The ``monogamous wives'' story fills that gap. If the wives abandon their monogamy, there is again both bad news and good news. The bad news is that some wives might be infected by their neighbors; the good news is that the prostitutes are driven out of business. But in this story it's easy to see that the good news really does outweigh the bad. That doesn't prove the good news must outweigh the bad in every story, but it at least proves that it's possible.


* * * * * * * * * * *

Now you might begin to suspect that I'm concocting stories to illustrate a point, and that you could just as easily concoct counter-stories to illustrate the opposite. I say that Martin's dalliance with Joan keeps her safe from Maxwell; you might respond that it thereby leaves Maxwell free to infect someone else. To this there are several replies.

First, we don't know that Maxwell finds another partner. Perhaps he just strikes out. Second, even if Maxwell does find another partner, he might have to settle for someone more promiscuous---and more likely to be already infected---than the relatively demure Joan. And that can be good, because passing the virus to someone who's already infected does no harm.

So Martin's behavior could have many different consequences, depending on innumerable details of the mating game--- details like whether Maxwell's second choice partner is likely to be more or less promiscuous than Joan. Change your assumptions about those details and you'll change the story; change the story and you might change the moral.

That raises two questions: First, do any assumptions imply that, after we've accounted for all effects in all directions, more sex retards the epidemic? And second, do we still get the same answer if we insist that our assumptions be realistic?

The answer to both questions is yes. The ``yes'' answer to the first question is supplied by the story of the monogamous wives, which (unlikely the story of Martin and Joan) completely spells out the details of everyone's mating habits, and so allows us to account for all the complicated chains of causation like ``A mates with B, who therefore doesn't mate with C, who therefore mates with D, who....'' In the ``monogamous wives'' story, it is quite easy to see that a slight increase in promiscuity (with each wife taking one extramarital partner) can defeat the virus entirely. That disposes of the first question and entitles to move on to the second: What happens in the real world?

Here it's not so easy to jump to a conclusion, but that's where Professor Kremer's research comes in. With plausibly realistic assumptions about how people choose partners, his work shows that the moral remains essentially the same as in the much simpler story of the monogamous wives. When your relatively demure neighbor experiences a rare moment of rakishness, he really is doing his part to combat the deadly scourge.


* * * * * * * * * * *

That's one reason why you should root for Martin to have sex with Joan. Here's another: They'll probably enjoy it.

Enjoyment should never be lightly dismissed. After all, reducing the rate of HIV infection is not the only goal worth pursuing; if it were, we'd outlaw sex entirely. What we really want is to minimize the number of infections resulting from any given number of sexual encounters. That's the same as maximizing the number of (consensual) sexual encounters leading up to any given number of infections. Even if Martin fails to deny Maxwell a conquest, he can at least make someone happy.

If you are a monomaniac whose goal is to minimize the prevalence of AIDS, then you should encourage Martin to have more sex.2 But if you are a sensible person whose goal is to maximize the difference between the benefits of sex and the costs of AIDS---then you should encourage Martin to have even more sex.

2 Actually, if you are a monomaniac who wants to minimize the prevalence of AIDS and can control everyone's behavior, then, as I said earlier, you should outlaw sex entirely. But if you are a monomaniac who wants to minimize the prevalence of AIDS and can control only Martin's behavior while taking Maxwell's as given, then you should encourage Martin to have more sex, not less.

To an economist, it's crystal clear why people with limited sexual pasts choose to supply too little sex in the present: Their services are underpriced. If sexual conservatives could effectively advertise their histories, HIV-conscious suitors would compete to lavish them with attention. But that doesn't happen, because conservatives are hard to identify. Insufficiently rewarded for relaxing their standards, they relax their standards insufficiently.

The same problem crops up elsewhere. The reason factory owners don't do enough to protect the environment is that they're insufficiently rewarded for environmental protection (or insufficiently punished for neglecting it). They reap some rewards (even factory owners like clean air), but most of the benefits go to total strangers. Likewise, the reason Martin might not do enough to fight the scourge of AIDS (by sleeping with Joan) is that, while he certainly reaps some rewards (such as sexual pleasure), many of the benefits go to Joan's future partners, and their future partners, all of whom owe their health to Martin's willingness to distract Joan from Maxwell. Unless Martin is implausibly altruistic toward those total strangers, he ignores their interests when he decides whether to consummate his relationship with Joan---just as the factory owner ignores his neighbors' interests when he decides whether to install a filter on his smokestack.

The flip side of the analogy is that Martin's chastity is a form of pollution---chastity pollutes the sexual environment by reducing the fraction of relatively safe partners in the dating pool. Factory owners pollute too much because they have to breathe only a fraction of their own pollution; Martin stays home alone too much because he bears only a fraction of the consequences.

The pollution analogy is so powerful that it dictates the moral of virtually any story you could tell. To conclude that Martin's coupling with Joan slows the epidemic, you have to make some assumptions about what Joan and Maxwell and all of their potential partners would be doing if Martin stayed home. But to conclude that Martin's coupling with Joan makes the world a better place (where ``better'' accounts for both the costs of disease and the benefits of sex), you don't need any of those assumptions. It is a quite general principle that when goods (such as Martin's sexual services) are underpriced, they are undersupplied, and by a strict cost-benefit criterion, there ought to be more of them.

If Martin chose his sexual activity level at random, then he might choose either too much or too little; random activity levels could as easily be too high as too low. But he doesn't choose randomly; he chooses after accounting for all the risks to himself (as well as the offsetting pleasures), but without accounting for the benefits to others. Therefore his choice is inevitably biased toward excessive conservatism.


* * * * * * * * * * *

We've met several distinct but interlocking arguments about several distinct but interlocking questions. Let me disentangle some threads by listing the main ideas separately:

1) If sexual conservatives other than yourself increase their activity levels, your chance of infection goes down. That's because there are more of them in the dating pool at any given time, which increases the chance you'll go home with one of them instead of someone more dangerous. (The only exception is if both you and your partner are entirely monogamous, in which case others' behavior is irrelevant.)

2) If you increase your activity level, your chance of infection goes up. That's because every sexual encounter has some chance of leading to an infection.

3) If sexual conservatives generally, including yourself, increase their activity levels, there are effects in opposite directions. According to point 1), your chance of infection goes down, while according to point 2), your chance of infection goes up. To determine which effect is bigger---and to determine whether the net effect is to speed or slow the epidemic---we must tell a story about how people find partners. The answer might depend on the details of the story.

4) There are at least some stories where the answer is the surprising one---that increased activity by sexual conservatives tends, on net, to retard the epidemic. The story of the monogamous wives is one example.

5) If you insist on a more realistic story, you are still led to the same surprising conclusion. In fact, Professor Kremer of MIT has shown that under realistic assumptions, the epidemic could be slowed if 3/4 of adult British heterosexuals became more sexually active.

6) If more sex retards the epidemic, then surely more sex is a good thing. But even if more sex fails to retard the epidemic, it could still be a good thing, because the right measure of ``good'' is not the prevalence of AIDS, but the difference between the benefits of sex and the costs of AIDS.

7) Not only could more sex by conservatives be a good thing in the sense of point 6); it is virtually certain to be a good thing in that sense, no matter what story you tell. That's because lovers don't bear all the costs of their own chastity (the only cost they bear is that they forgo the joy of sex; the health consequences are borne by others); thus they're sure to be overly chaste, just as factory owners who don't bear all the costs of their own pollution are sure to overpollute.


* * * * * * * * * * *

So---how do we encourage Martin (and others like him) to have more sex?

I wish this book could nudge him in the right direction, but sadly, there's no reason why it should---even if he reads and understands it completely. (Don't let that stop you from buying him a copy, though.) Martin has already chosen the activity level that's right for him. He's not likely to adjust that level just because he learns that a bunch of strangers---namely Joan's future partners and their future partners---would appreciate it.

Martin, being human, tends to concentrate on what's good for Martin, not what's good for the society he lives in. You can make a polluting factory owner understand that he's hurting his neighbors, but that's not the same as convincing him to stop.

So we need something more effective than mere education. Extrapolating from their usual response to environmental issues, I assume that liberals will want to attack the problem of excessive sexual restraint through coercive legislation. But as a devotee of the price system, I'd prefer to encourage good behavior through a well-designed system of subsidies.

In other words, we could pay people to have more sex with more partners. But that's not ideal, because we don't want everyone to have more sex with more partners. Maxwell, for example, is quite oversexed enough as it is. The problem is to subsidize Martin's sexual awakening without simultaneously subsidizing Maxwell's genuine excess.

So a better idea is to pay people for having sex only if they are relatively inexperienced. Unfortunately, that doesn't work very well either---not as long as Maxwell can lie about his past and keep a straight face long enough to collect his handout.

A still better idea, then, is to invent a reward that is valuable to the Martins of the world but not the Maxwells--- or at least one that's not worth Maxwell's trouble to collect. For example, Martin, who spends more evenings at home, probably values his library privileges a lot more than Maxwell does. So we could make library privileges a reward for sexual escapades. The prospect of earning the right to renew his library card might be just enough to nudge Martin into Joan's bed, while having no discernible impact on Maxwell.

Using library privileges instead of cash is a definite improvement, but now a new problem arises: When Martin arrives at the circulation desk looking appropriately smug and disheveled, how can the librarian know whether he's really fulfilled his coital obligations or just putting on a good show? If Martin's goal is to use the library, he doesn't have to have sex; it's enough for him to be a good actor.

To solve that problem, we need a different reward---one that is of no value to Martin unless he actually has sex. And as before, it has to be a reward that Martin values more than Maxwell does.

I can think of only one reward that fits both criteria: free (or heavily subsidized) condoms. To reap the benefits of a free condom, Martin has to have sex. And Martin probably values a free condom considerably more than Maxwell does. Here's why: Martin's almost surely not infected yet, so a condom has a good chance to save his life. Maxwell, by contrast, knows he might have the virus already, so for Maxwell, a condom at this point is less likely to make a difference.3,4 Subsidized condoms could be just the ticket for luring Martin out of his shell without stirring Maxwell to a new frenzy of activity. 3Suppose, for example, that Martin currently has a 1% chance of being infected and Maxwell has a 5% chance. Then in a single encounter with an infected partner, a condom has a 99% chance of protecting Martin from a potentially deadly exposure, but only a 95% chance of protecting Maxwell. 4 You might object that Maxwell, who takes a lot of risks, has good reason to value condoms more than Martin does. That observation is correct but not relevant. Maxwell values his condom collection more than Martin does, but Martin values each individual condom more than Maxwell does. It's the value per individual condom that's relevant to the subsidy issue.

* * * * * * * * * * *

As it happens, there is another reason to subsidize condoms. Condom use itself is under-rewarded. When you use a condom, you protect both yourself and your future partners (and your future partners' future partners), but you are rewarded (with a lower chance of infection) only for protecting yourself. Your future partners can't observe your past condom use and therefore can't reward it with extravagant courtship. That means you fail to capture all the benefits you're conferring, and as a result, condoms are underused.

In other words, people use too few condoms for the same reason they have too little sex. When Martin has sex with Joan, that's good for Joan's future partners. When Martin uses a condom, that's good for Martin's future partners. In neither case do the future partners get a fair opportunity to influence Martin's behavior.

It's frequently argued that subsidized (or free) condoms have an upside and a downside. The upside is that they reduce the risk from a given encounter, and the downside is that they encourage more encounters. But it's quite plausible that in reality, that's not an upside and a downside---it's two upsides. Without the subsidies, people don't use enough condoms, and without the subsidies, the sort of people who most value condoms don't have enough sex partners.

The big drawback to subsidizing condoms is that they're not very expensive to begin with. You can reduce the price of a condom all the way down from a dollar to zero without having much impact on people's sexual choices.

Our goal, then, should be to drive the price of condoms below zero, by rewarding people who use them. In other words, we should pay a bounty for used condoms. The best of all possible bounties would be one that is more valuable to abstemious Martins than to promiscuous Maxwells. With that in mind, the journalist Oliver Morton has made the marvelous suggestion that if at least some abstemiousness is due to shyness and the inability to find partners (while the promiscuous have relatively trouble in this regard), then the answer might be to establish a government-funded dating service: Bring us a used condom and we'll get you a date.5 5 When I expressed concern about the ease of fakery in this context, Mr. Morton responded ``Yes, I worried about the faking problem. But anyone who's willing to go to that kind of trouble should probably be encouraged on the dating market anyway.''

That we are driven to consider such extreme measures suggests that there really is a deep and perhaps unsolvable problem here. Yet the entire problem---along with the entire case for subsidies---would vanish if our sexual pasts could somehow be made visible, so that future partners could reward past prudence and thereby provide appropriate incentives. Perhaps technology can ultimately make that solution feasible. (I imagine the pornography of the future: ``Her skirt slid to the floor and his gaze came to rest on her thigh, where the imbedded monitor read `This site has been accessed 314 times'.'') Until then, the best we can probably do is to make condoms inexpensive---and get rid of those subway ads.




ADDENDUM


In 1996, Slate magazine published an abbreviated version of this chapter which generated over 400 email responses. Quite a few of those responses were both thoughtful and interesting, and helped me to improve the presentation you've just read. Others contained nothing but a line or two of invective. To those, I usually responded with a short note that read ``I'm sorry, but from the email you sent me I was not able to ascertain at exactly which point you stopped following the argument. If you can be more precise about where you got lost, I'll do my best to make it clearer.'' In a remarkable number of cases, I got responses that were both thoughtful and apologetic, and a few of those led to multi-round correspondences that taught me something.

Other readers seemed bound and determined to miss the point by miles. One, brandishing his credentials as a medical doctor, termed the column ``particularly unfortunate'' and---in a letter that was published in a subsequent issue of Slate---explained why:

We are at a stage in the HIV epidemic in which heterosexual spread is becoming increasingly significant. Casual readers...may justify increasing their sexual-risk-taking behavior. Unfortunately, failure, lasting in a shortened lifetime, can result from a sexually successful one-night stand.

For an appropriate sequel, the editor of Slate might solicit an article...defending Russian roulette as statistically OK but cautioning that three loaded chambers is too risky.
One of the great discoveries of 19th century economics was the principle of comparative advantage, according to which people are most successful when they stick to the things they're good at. (It's actually quite a bit subtler than that, but this oversimplified version suffices for the application I'm about to make.) The principle of comparative advantage explains why some people become medical doctors, while other, different, people go into fields (such as economics) that require at least a minimal ability to reason logically.

There is nothing---not one word---in the chapter you've just read or in the original Slate article that could provoke any reader to increased sexual-risk-taking behavior. Indeed, the whole point is that that the relatively chaste have too little sex because it is not in their interest to behave otherwise. If you and your spouse are monogamous, you won't get AIDS. If I point out that your continued monogamy is potentially deadly to your neighbors, I don't expect you'll rush to risk your life for theirs.

Imagine this scenario: I write an article explaining that when firms put filters on their smokestacks, they perform a positive social service. Unfortunately, installing filters cuts into firms' profits, so they install fewer smokestacks than the rest of us prefer. Therefore we might want to consider subsidizing such installations.

Along comes our medical doctor to argue that: a) filters reduce profits and are therefore a bad thing; b) my article is ``particularly unfortunate'' because ``casual readers who own factories may increase their anti-pollution efforts'' and c) if we're going to argue for anti-pollution equipment, we might as well solicit an article advising firms to convert all their assets into rowboats and then sink them.

Points a) and b) are both flat wrong (though if casual readers were so foolish---or so uncommonly altruistic---as to increase their anti-pollution efforts on the basis of an article that provides no justification for doing so, we could all be grateful for their foolishness, and would consider the article the very opposite of ``particularly unfortunate'').6 Point c) is a non sequitur worthy of the good doctor's comments about Russian roulette; it confers no benefits on the neighbors and thus is completely off-topic.

6 To be entirely explicit about the analogy: Installing filters is like becoming more promiscuous; it hurts you and helps your neighbors. The fact that something hurts you does not make it a bad thing, and the fact that it helps your neighbors does not make you want to go out and do it. On the other hand, if a few of my readers (medical students, perhaps?) are so easily confused that they go out and have more sex because of these arguments, that's probably something the rest of us can be thankful for. I've given this much space to my physician-correspondent because his comments were echoed by several others who expressed concern that naive readers would misunderstand the argument so completely that they'd all become highly promiscuous Maxwells and ultimately extinguish the human species. A few even urged me to publish a retraction for precisely that reason. In other words, they argued that ideas should be suppressed because somebody might misunderstand them. That's a position with a long and sordid history of which I'd rather not become a part.


* * * * * * * * * * *

Here are some more questions that came up often enough to make it worth recording the answers:


Question 1: You say that a bit more promiscuity would lead to less AIDS. If that were true, would it not follow that an enormous increase in promiscuity could defeat the disease altogether? And is that conclusion not manifestly absurd?

Answer: The ``conclusion'' is indeed manifestly absurd, but it is not a legitimate conclusion. Large changes and small changes don't always have similar consequences. I believe that if I ate a bit less, I would live a bit longer. But I do not believe that if I stopped eating entirely, I would live forever.


Question 2: In the words of one reader, ``a spoonful of promiscuity will only slow the disease; self-restraint can stop it.'' In view of that, is it not irresponsible to tout the merits of promiscuity without also emphasizing the merits of self-restraint?

Answer: This is like arguing that traffic lights can only reduce the number of auto accidents, while banning cars can stop auto accidents; therefore, it would be irresponsible to tout the merits of traffic lights.

The problem with such reasoning is that banning cars, like banning sex outside of longterm relationships, is neither realistic nor clearly desirable---it's not going to happen, and if it did happen, we'd probably be less happy, despite the attendant decrease in mortality.

In any event, everybody already knows that a perfectly monogamous society would have not have an AIDS problem. I prefer to write about things that are both true and surprising. As a writer, I dare to hope that there are readers who are actually interested in learning something.


Question 3: Okay, there are benefits to increased promiscuity. But there can also benefits to increased chastity. Isn't it inconsistent to subsidize one without subsidizing the other?

Answer: No, because there is a critical difference between the two kinds of benefit. The benefits of your promiscuity go to others; the benefits of your chastity go to you. Thus you already have sufficient incentives on the pro- chastity side.


Question 4: You disparage the goals of a monomaniac who wants to minimize the prevalence of AIDS, while endorsing the goals of a monomaniac who wants to ``make the world a better place'' after accounting for both the costs of AIDS and the benefits of sex. Does that mean that your according to your preferred goals, the prevalence of AIDS ought to increase?

Answer: Not necessarily, but perhaps. Suppose your favorite restaurant cuts all its prices in half. Then it's certainly a mistake to jump to the conclusion that you'll now spend half as much on dinner. In fact, you might now spend more than you used to, because at the new prices you might order steak instead of hamburger. Nevertheless, whether your expenditures increase or decrease, you're certainly now better off, because you're choosing from a better menu.

Choosing your level of sexual activity is like choosing from a menu. The price of having one partner is a particular probability of infection; the price of having two partners is a particular (higher) probability, and so forth. To ``minimize the number of infections resulting from any given number of sexual encounters'' is to cut all the prices on that menu. Once we've done that, you might or might not choose to ``spend'' more (which, in this context, means increasing your risk of infection), but I can be sure you're better off---again, because you're choosing from a better menu.

There are three relevant levels of activity for sexual conservatives: Level A is where we are today. Level B (which is higher) is where we would go if we wanted to minimize AIDS prevalence (taking the behavior of the very promiscuous as given). Level C (which is higher still) is where we would choose to be if we had the best of all possible menus. Level B, by definition, has the lowest AIDS prevalence. I do not know whether the AIDS prevalence at level C is higher or lower than at level A.

For a society, going to level B is like choosing the restaurant where you'll spend the least. Going to level C is like going to the restaurant where you'll be happiest (after fully accounting for the pain of paying the bill). The second goal is the sane one.


Question 5: Didn't you leave out some things that might be important?

Answer: Absolutely. For one thing, a change in human behavior could trigger a burst of evolution on the part of the virus. I doubt that consideration is important in this context (though it's surely important in others), but maybe I'm wrong. For another, at least one reader contended that slight increases in promiscuity are impossible because they trigger cultural changes that lead to large changes in promiscuity. I doubt that he's right, but I can't prove he's wrong.


* * * * * * * * * * *

Some of my most delightful email came from readers who had invented creative solutions to the problem of getting the right kind of people to have more sex. Solutions fell into two categories. First, there were mechanisms for making our sexual pasts visible to potential partners, who can then provide appropriate incentives. Second, there were ideas for new kinds of subsidies that would be particularly valuable to the sexually reticent.

One reader proposed an online service to record negative HIV test results. You'd type in your prospective partner's name number and get a response like ``Last negative test result 7/04/99''. Or, to protect privacy, you'd type in not a name but an ID number that the partner provides. The output would then consist of both a test result and a picture of the person in question to avoid fake ID's. Actually, this strikes me as such a good idea that I can't figure out why nobody's doing it.

Another reader argued that sexual conservatism is already highly rewarded, at least in the market for prostitutes, where virgins go for a tremendous premium. I objected that if the premium is large, there must be prostitutes who have sold their ``virginity'' hundreds of times, but he responded quite sensibly that the market seems to take age as a substitute for virginity. If he's right, then the premium for youth should have increased substantially since the advent of AIDS. That might be something worth collecting data on.

Finally, Oliver Morton sent me his marvelous proposal that we pay a bounty (in the form of free dating services) for used condoms. I've already addressed the problem of fakery; let me add one more thought to that discussion. Suppose there are three classes of people: those who did not have sex last night, those who had sex with a condom, and those who had sex without a condom. Ideally, we'd like the biggest rewards to go to the second class; but we'd be happy if we could reward the first and second classes at the expense of the third. And fortunately, it's the third class that will have the greatest difficulty generating deceptive ``used'' condoms. That's good news. I'm not sure how hard we want to think about it, though.