More Sex Is Safer Sex: The Unconventional Wisdom of Economics by Steven E. Landsburg

From the Publisher

Steven Landsburg's writings are living proof that economics need not be "the dismal science." Readers of The Armchair Economist and his columns in Slate magazine know that he can make economics not only fun but fascinating, as he searches for the reasons behind the odd facts we face in our daily lives. In More Sex Is Safer Sex, he brings his witty and razor-sharp analysis to the many ways that our individually rational decisions can combine into some truly weird collective results -- and he proposes hilarious and serious ways to fix just about everything.

When you stand up at the ballpark in order to see better, you make a rational decision. When everyone else does it too, the results, of course, are lousy. But this is just the tip of the iceberg of individual sanity and collective madness. Did you know that some people may actually increase the spread of sexually transmitted diseases when they avoid casual sex? Do you know why tall people earn more money than shorter competitors? (Hint: it isn't just unfair, unconscious prejudice.) Do you know why it makes no sense for you to give charitable donations to more than one organization?

Landsburg's solutions to the many ways that modern life is unfair or inefficient are both jaw-dropping and maddeningly defensible. We should encourage people to cut in line at water fountains on hot days. We should let firefighters keep any property they rescue from burning houses. We should encourage more people to act like Scrooge, because misers are just as generous as philanthropists.

Best of all are Landsburg's commonsense solutions to the political problems that plague our democracy. We should charge penalties tojurors if they convict a felon who is later exonerated. We should let everyone vote in two congressional districts: their own, and any other one of their choice. While we're at it, we should redraw the districts according to the alphabetical lists of all voters, rather than by geography. We should pay FDA commissioners with shares of pharmaceutical company stocks, and pay our president with a diversified portfolio of real estate from across the country.

Why do parents of sons stay married more often than parents who have only daughters? Why does early motherhood not only correlate with lower income, but actually cause it? Why do we execute murderers but not the authors of vicious computer viruses? The lesson of this fascinating, fun, and endlessly provocative book is twofold: many apparently very odd behaviors have logical explanations, and many apparently logical behaviors make no sense whatsoever.

Publishers Weekly

Economics books full of "uncommon sense" are more common after the success of Freakonomics, but this rambling survey of hot-button and quotidian issues viewed from a libertarian economic perspective doesn't measure up. Landsburg (The Armchair Economist) is sometimes pleasantly counterintuitive, but too often simply contentious. In using cost/benefit calculations to argue in favor of racial profiling or why we shouldn't care about the looting of Baghdad's museums, he strains to celebrate "all that is counter, original, spare and strange." While positing multiple solutions to interesting problems, he forces logical readers to confront uncomfortable positions--as in the title essay, urging chaste citizens to sleep around, thereby diluting the pool of potential sex partners with AIDS. But the chapters typically conclude without resolution--at one point, the author shrugs: "It's not easy to sort out causes from effects." One suspects that a rival economist could swiftly debunk many of Landsburg's arguments--for instance, his chapter praising misers (who produce but don't consume) depends on the assumption that all resources are fixed and finite. By the time he makes the head-scratching case that "it's always an occasion for joy when other people have more children," the reader may be in the mood for some plain old common sense. (Apr.)

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Table of Contents

Read an Excerpt

PREFACE:

UNCONVENTIONAL WISDOM

Common sense tells you that promiscuity spreads AIDS, population growth threatens prosperity, and misers make bad neighbors. I wrote this book to assault your common sense.

My weapons are evidence and logic, especially the logic of economics. Logic is most enlightening -- and surely most fun -- when it challenges us to see the world in a whole new way. This book is about that kind of logic.

Daughters cause divorce. A thirst for revenge is healthier than a thirst for gold. A ban on elephant hunting is bad news for elephants, and disaster assistance is bad news for the people who receive it. Malicious computer hackers should be executed. The most charitable people support the fewest charities. Writing books is socially irresponsible; elbowing your way to the front of the water-fountain line is not. The tall, the slim, and the beautiful earn higher wages -- but not for the reasons you think.

Each of those statements is closer to the truth than you might imagine. If your common sense tells you otherwise, remember that common sense also tells you the earth is flat.

What you're about to read is a celebration of all that is counter, original, spare, and strange. I mean every word seriously and every word in fun. These are carefully considered arguments about important issues. But they're also surprising arguments, and surprises are fun. This book will give you new insights about how the world works. Sometimes it might outrage you. I hope it also makes you smile.

Copyright © 2007 by Steven E. Landsburg

PART I

THE COMMUNAL STREAM

Come out to my suburbanneighborhood on any crisp October Saturday, and I will show you a minor tragedy: on every lawn a man with a leaf blower, blowing his leaves onto the next man's lawn. Eventually, they all go inside to recover from a hard and thoroughly unproductive morning's work.

That's a bad way to spend a Saturday. If we all ditched our leaf blowers and stayed inside to watch football, we'd all be happier. Unfortunately, human beings are too rational for that. Blowing leaves always makes sense, whatever your neighbors do. If everyone else blows leaves, you'd better blow them also to avoid a double cover. Or, if everyone agrees not to blow leaves, your best strategy is to cheat and have the only clean lawn in the neighborhood.

Economics is largely about the surprising and sometimes tragic consequences of rational behavior. When there's an exciting moment at the ballpark, everybody stands up trying to see better, and therefore nobody succeeds. At parties with a lot of simultaneous conversations, everyone speaks loudly to be heard over everyone else, and everyone goes home with a sore throat. Still, it's rational to stand at the ballpark, and to yell at the party. We stand and we yell for the same reason we blow leaves -- from exquisite (and entirely rational) concern for our own interests and none for the harm that spills over onto our neighbors.

It's a general principle of economics that things tend to work out best when people have to live with the consequences of their own behavior, or, to put it another way, things tend to work out poorly when the consequences of our actions spill over onto other people. Simple and obvious as that general principle might sound, it has the power to undermine vast deposits of conventional wisdom. It suggests that the world has too few people, too few misers, and not enough casual sex, but just the right amounts of secondhand smoke and child labor. It implies that a thirst for gold is socially ruinous, but a taste for revenge can be a social godsend. It casts light on why the tall, the thin, and the beautiful earn higher wages. It suggests sweeping reforms of the legal system, the political system, the tax code, and the rule against jumping to the front of the water-fountain line. And it explains why automobile insurance in Philadelphia is so damn expensive.

More mundanely, it tells us there's too much litter on the streets. That's less obvious than you might suppose. Sure, there's a lot of litter, but a lot isn't always the same as too much. After all, some litter ought to be there because the alternatives are worse. That half-eaten sandwich you just stepped on? Maybe someone dropped it to avoid being stung by a hornet. The newspaper wrapped around your ankles? Maybe the wind took it while someone chased down the tax returns that had just fallen out of his briefcase. And if you have a heart attack while you're walking down the street with a Popsicle, nobody thinks you should make a beeline for the nearest trash receptacle before collapsing to the ground.

In principle, all the litter on all the sidewalks in all the cities of the world could be there for good reasons. But in fact, I'm sure there is too much litter on the streets, and here's how I know: the person who drops a banana peel and the person who slips on it are not usually the same person. That pretty much guarantees that people sometimes drop banana peels even when the cost (to passersby) exceeds the benefit (to the litterbug). Each time that happens, the world becomes a poorer place -- and that's what I mean when I say there's too much litter.

"Too much," in other words, is no mere value judgment. It means precisely that in a world with less littering, we could all be happier -- just as we could all be happier in a world with fewer leaf blowers and a prohibition on standing at the ballpark.

Whether you're blowing leaves or discarding litter, having children or having sex, saving or spending, smoking or drinking, setting fires or reporting them -- your actions have costs and benefits. As long as you feel all the costs and benefits, you'll tend to get the quantity right. You'll drop the right number of banana peels, or have the right number of children, or choose the right number of sex partners. But if you feel only the benefits while someone else feels the costs, you'll tend to overindulge. And conversely, if you feel only the costs while someone else feels the benefits, you'll underindulge.

When you're splitting the dinner check, ordering dessert can be a lot like littering -- you get the benefits and the costs spill over onto your friends. If the $10 double chocolate mousse is worth only $4 to you, you really shouldn't order it -- and you won't, if you're paying your own way. But when you split the check ten ways, that mousse starts to look (to you) like a bargain. You place your order, the group pays $10 to buy you a $4 dessert, and the group (including you) is collectively $6 poorer. That's what I call a bad outcome.

Spillovers cause bad outcomes. That much, I think, is clear, at least in theory. The art is in figuring out what counts as a spillover. Take, for example, the problem of secondhand smoke in restaurants. It's called secondhand smoke precisely because it spills over from one table to another (or from a table into the kitchen). But that doesn't make it a spillover in the relevant sense. It counts as a spillover only if the decision maker ignores it. In this case, there's no spillover because the restaurant owner -- the fellow who decides to allow smoking in the first place -- is unlikely to ignore something that offends his customers.

Of course, he'll offend some customers no matter what he does. A permissive smoking policy offends nonsmoking customers and employees; a restrictive policy offends the smokers. But it's in the owner's financial interest to keep the offense to a minimum. He bans smoking if the benefits of the ban exceed its costs, and allows it otherwise -- for the simple reason that every cost and every benefit hits him directly in the pocketbook, via customers' willingness to spend money in his establishment. He's got all the right incentives so he makes all the right decisions. That's why most economists agree that second-guessing the owner -- say, by passing a law that overrides his choices -- is a bad idea.

Call it, then, the communal-stream principle: Feel free to pollute your own swimming pool, but if your sludge spills over into the stream we all share, you should pay for the damage. Conversely, if you volunteer for cleanup duty, you should get a reward. Otherwise we end up with too much pollution and too few volunteers.

A simple and obvious principle, no? But the consequences can be astonishing.

ONE

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.

Consider Martin, a charming and generally prudent young man with a limited sexual history, who has been gently flirting with his coworker 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. Harvard professor Michael Kremer estimates 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-fourths of all British heterosexuals between the ages of 18 and 45.

A cautious guy like Martin does the world a favor every time he hits the bars. In fact, he does the world two favors. First he improves the odds for everyone who's out there seeking a safe match. The second favor is more macabre, but probably also more significant: If Martin picks up a new partner tonight, he just might pick up an infection as well. That's great. Because then Martin goes home, wastes away in solitude, and eventually dies -- taking the virus with him.

If someone has to get infected tonight, I want it to be Martin rather than Promiscuous Pete, who would probably infect another twenty people before finally dying.

I'm always glad to see guys like Martin in the bars. When he takes home an uninfected partner, he diverts that partner from a potentially more dangerous liaison. When he takes home an infected partner, he diverts that partner from giving the virus to someone who might spread it far and wide. Either way, I sure hope he gets lucky tonight.

Sadly, none of this makes for a good pickup line. You're unlikely to get very far with an approach like "You should sleep with me so you can get infected, die, and take the virus with you." That would be like saying "You should sell your leaf blower so your neighbors' lawns stay cleaner" or "You should stay seated at the ballpark so everyone else can see." The whole point is that what's good for the group can be bad for the individual, and that's why we get bad outcomes.

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; 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, because it shows that even on a society-wide level, increased promiscuity could retard the epidemic -- at least in principle. But what about practice? 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. 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. 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.

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.

When you take a new sex partner, you bear some costs and you reap some benefits. Those are your business. You also impose costs and benefits on others, and those are everyone else's business. If you have a history of reckless promiscuity, that's a cost. Everyone's fishing for partners in a great communal stream and you've polluted that stream just by entering it.

But if you've always been cautious and selective, you're likely to raise the average quality of the partner pool. Just by jumping into the stream, you make it purer. Thanks to you, everyone who goes fishing for a partner tonight has a better chance of catching a safe one.

Like any other communal stream, the stream of partners has too many polluters and too few volunteers to clean it up. 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 water and 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 would reap some rewards (such as sexual pleasure), many of the benefits would go to Joan's future partners, and their future partners.

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.

Here, then, is what we know:

When sexual conservatives relax their standards, benefits spill over onto their neighbors. That alone is enough to tell us that the world would be a better place if we could loosen these people up a little.

There is, however, more than one way for the world to become a better place. Maybe the epidemic slows down. Maybe people enjoy more sex. Maybe the epidemic speeds up, but people enjoy so much more sex that it's worth it.

Pure theory -- in the guise of the communal-stream principle -- tells us that at least one of those good things must happen. Professor Kremer's research suggests that both good things happen: we get more sex and less illness.

If all you want to do is slow down the epidemic, Professor Kremer's research says that more sex is a good thing. But if you want to maximize the excess of benefits over costs, then even more sex is an even better thing.

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 would attack the problem of excessive sexual temperance 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 we should 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.

What we need is a reward that Martin values and Maxwell doesn't -- like, say, a library card. I'm guessing that Maxwell, with his busy social life, doesn't spend a lot of time at the library.

That's a definite improvement, but it's still imperfect. 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 is just putting on a good show?

Let's try again: We need a reward that's of no value to Martin unless he actually has sex. And as before, it should be something the cautious Martin values more than the promiscuous 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 a condom at this point is less likely to make a difference. Subsidized condoms could be just the ticket for luring Martin out of his shell without stirring Maxwell to a new frenzy of activity.

As it happens, there is another reason to subsidize condoms. Condom use itself is underrewarded. 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. 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 alleged downside is that they encourage more encounters. But 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 main drawback to subsidizing condoms is that they're not very expensive to begin with. You can reduce the price of a condom from a dollar all the way down 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 little 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.

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.'")

Or, as one of my Slate readers suggested, we could have an online service to record negative HIV test results. You'd type in your prospective partner's name and get a response like "Last negative test result 7/4/2006." Or, to protect privacy, you'd type in not a name but an ID number provided by the partner. Your screen could show both a test result and a photo to avoid fake IDs. This strikes me as such a good idea that I can't figure out why nobody's doing it yet. Until then, the best we can probably do is to make condoms inexpensive -- and get ride of those subway ads.

Addendum

In 1996, Slate magazine published an abbreviated version of this chapter that generated hundreds of 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 multiround 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 nineteenth-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 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 likely won't get any sexually transmitted diseases. 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 filters 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"). Point (c) is a non sequitur perfectly analogous to the good doctor's comments about Russian roulette; such a strategy confers no benefits on the neighbors and thus is completely off-topic.

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 long-term 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 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 be 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: 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 increases in promiscuity. I doubt that he's right, but I can't prove he's wrong.

Copyright © 2007 by Steven E. Landsburg

Read a Sample Chapter

More Sex Is Safer Sex

Free Press

PREFACE:

UNCONVENTIONAL WISDOM

Common sense tells you that promiscuity spreads AIDS, population growth threatens prosperity, and misers make bad neighbors. I wrote this book to assault your common sense.

My weapons are evidence and logic, especially the logic of economics. Logic is most enlightening -- and surely most fun -- when it challenges us to see the world in a whole new way. This book is about that kind of logic.

Daughters cause divorce. A thirst for revenge is healthier than a thirst for gold. A ban on elephant hunting is bad news for elephants, and disaster assistance is bad news for the people who receive it. Malicious computer hackers should be executed. The most charitable people support the fewest charities. Writing books is socially irresponsible; elbowing your way to the front of the water-fountain line is not. The tall, the slim, and the beautiful earn higher wages -- but not for the reasons you think.

Each of those statements is closer to the truth than you might imagine. If your common sense tells you otherwise, remember that common sense also tells you the earth is flat.

What you're about to read is a celebration of all that is counter, original, spare, and strange. I meanevery word seriously and every word in fun. These are carefully considered arguments about important issues. But they're also surprising arguments, and surprises are fun. This book will give you new insights about how the world works. Sometimes it might outrage you. I hope it also makes you smile.

Copyright © 2007 by Steven E. Landsburg

PART I

THE COMMUNAL STREAM

Come out to my suburban neighborhood on any crisp October Saturday, and I will show you a minor tragedy: on every lawn a man with a leaf blower, blowing his leaves onto the next man's lawn. Eventually, they all go inside to recover from a hard and thoroughly unproductive morning's work.

That's a bad way to spend a Saturday. If we all ditched our leaf blowers and stayed inside to watch football, we'd all be happier. Unfortunately, human beings are too rational for that. Blowing leaves always makes sense, whatever your neighbors do. If everyone else blows leaves, you'd better blow them also to avoid a double cover. Or, if everyone agrees not to blow leaves, your best strategy is to cheat and have the only clean lawn in the neighborhood.

Economics is largely about the surprising and sometimes tragic consequences of rational behavior. When there's an exciting moment at the ballpark, everybody stands up trying to see better, and therefore nobody succeeds. At parties with a lot of simultaneous conversations, everyone speaks loudly to be heard over everyone else, and everyone goes home with a sore throat. Still, it's rational to stand at the ballpark, and to yell at the party. We stand and we yell for the same reason we blow leaves -- from exquisite (and entirely rational) concern for our own interests and none for the harm that spills over onto our neighbors.

It's a general principle of economics that things tend to work out best when people have to live with the consequences of their own behavior, or, to put it another way, things tend to work out poorly when the consequences of our actions spill over onto other people. Simple and obvious as that general principle might sound, it has the power to undermine vast deposits of conventional wisdom. It suggests that the world has too few people, too few misers, and not enough casual sex, but just the right amounts of secondhand smoke and child labor. It implies that a thirst for gold is socially ruinous, but a taste for revenge can be a social godsend. It casts light on why the tall, the thin, and the beautiful earn higher wages. It suggests sweeping reforms of the legal system, the political system, the tax code, and the rule against jumping to the front of the water-fountain line. And it explains why automobile insurance in Philadelphia is so damn expensive.

More mundanely, it tells us there's too much litter on the streets. That's less obvious than you might suppose. Sure, there's a lot of litter, but a lot isn't always the same as too much. After all, some litter ought to be there because the alternatives are worse. That half-eaten sandwich you just stepped on? Maybe someone dropped it to avoid being stung by a hornet. The newspaper wrapped around your ankles? Maybe the wind took it while someone chased down the tax returns that had just fallen out of his briefcase. And if you have a heart attack while you're walking down the street with a Popsicle, nobody thinks you should make a beeline for the nearest trash receptacle before collapsing to the ground.

In principle, all the litter on all the sidewalks in all the cities of the world could be there for good reasons. But in fact, I'm sure there is too much litter on the streets, and here's how I know: the person who drops a banana peel and the person who slips on it are not usually the same person. That pretty much guarantees that people sometimes drop banana peels even when the cost (to passersby) exceeds the benefit (to the litterbug). Each time that happens, the world becomes a poorer place -- and that's what I mean when I say there's too much litter.

"Too much," in other words, is no mere value judgment. It means precisely that in a world with less littering, we could all be happier -- just as we could all be happier in a world with fewer leaf blowers and a prohibition on standing at the ballpark.

Whether you're blowing leaves or discarding litter, having children or having sex, saving or spending, smoking or drinking, setting fires or reporting them -- your actions have costs and benefits. As long as you feel all the costs and benefits, you'll tend to get the quantity right. You'll drop the right number of banana peels, or have the right number of children, or choose the right number of sex partners. But if you feel only the benefits while someone else feels the costs, you'll tend to overindulge. And conversely, if you feel only the costs while someone else feels the benefits, you'll underindulge.

When you're splitting the dinner check, ordering dessert can be a lot like littering -- you get the benefits and the costs spill over onto your friends. If the $10 double chocolate mousse is worth only $4 to you, you really shouldn't order it -- and you won't, if you're paying your own way. But when you split the check ten ways, that mousse starts to look (to you) like a bargain. You place your order, the group pays $10 to buy you a $4 dessert, and the group (including you) is collectively $6 poorer. That's what I call a bad outcome.

Spillovers cause bad outcomes. That much, I think, is clear, at least in theory. The art is in figuring out what counts as a spillover. Take, for example, the problem of secondhand smoke in restaurants. It's called secondhand smoke precisely because it spills over from one table to another (or from a table into the kitchen). But that doesn't make it a spillover in the relevant sense. It counts as a spillover only if the decision maker ignores it. In this case, there's no spillover because the restaurant owner -- the fellow who decides to allow smoking in the first place -- is unlikely to ignore something that offends his customers.

Of course, he'll offend some customers no matter what he does. A permissive smoking policy offends nonsmoking customers and employees; a restrictive policy offends the smokers. But it's in the owner's financial interest to keep the offense to a minimum. He bans smoking if the benefits of the ban exceed its costs, and allows it otherwise -- for the simple reason that every cost and every benefit hits him directly in the pocketbook, via customers' willingness to spend money in his establishment. He's got all the right incentives so he makes all the right decisions. That's why most economists agree that second-guessing the owner -- say, by passing a law that overrides his choices -- is a bad idea.

Call it, then, the communal-stream principle: Feel free to pollute your own swimming pool, but if your sludge spills over into the stream we all share, you should pay for the damage. Conversely, if you volunteer for cleanup duty, you should get a reward. Otherwise we end up with too much pollution and too few volunteers.

A simple and obvious principle, no? But the consequences can be astonishing.

ONE

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.

Consider Martin, a charming and generally prudent young man with a limited sexual history, who has been gently flirting with his coworker 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. Harvard professor Michael Kremer estimates 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-fourths of all British heterosexuals between the ages of 18 and 45.

A cautious guy like Martin does the world a favor every time he hits the bars. In fact, he does the world two favors. First he improves the odds for everyone who's out there seeking a safe match. The second favor is more macabre, but probably also more significant: If Martin picks up a new partner tonight, he just might pick up an infection as well. That's great. Because then Martin goes home, wastes away in solitude, and eventually dies -- taking the virus with him.

If someone has to get infected tonight, I want it to be Martin rather than Promiscuous Pete, who would probably infect another twenty people before finally dying.

I'm always glad to see guys like Martin in the bars. When he takes home an uninfected partner, he diverts that partner from a potentially more dangerous liaison. When he takes home an infected partner, he diverts that partner from giving the virus to someone who might spread it far and wide. Either way, I sure hope he gets lucky tonight.

Sadly, none of this makes for a good pickup line. You're unlikely to get very far with an approach like "You should sleep with me so you can get infected, die, and take the virus with you." That would be like saying "You should sell your leaf blower so your neighbors' lawns stay cleaner" or "You should stay seated at the ballpark so everyone else can see." The whole point is that what's good for the group can be bad for the individual, and that's why we get bad outcomes.

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; 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, because it shows that even on a society-wide level, increased promiscuity could retard the epidemic -- at least in principle. But what about practice? 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. 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. 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.

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.

When you take a new sex partner, you bear some costs and you reap some benefits. Those are your business. You also impose costs and benefits on others, and those are everyone else's business. If you have a history of reckless promiscuity, that's a cost. Everyone's fishing for partners in a great communal stream and you've polluted that stream just by entering it.

But if you've always been cautious and selective, you're likely to raise the average quality of the partner pool. Just by jumping into the stream, you make it purer. Thanks to you, everyone who goes fishing for a partner tonight has a better chance of catching a safe one.

Like any other communal stream, the stream of partners has too many polluters and too few volunteers to clean it up. 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 water and 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 would reap some rewards (such as sexual pleasure), many of the benefits would go to Joan's future partners, and their future partners.

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.

Here, then, is what we know:

When sexual conservatives relax their standards, benefits spill over onto their neighbors. That alone is enough to tell us that the world would be a better place if we could loosen these people up a little.

There is, however, more than one way for the world to become a better place. Maybe the epidemic slows down. Maybe people enjoy more sex. Maybe the epidemic speeds up, but people enjoy so much more sex that it's worth it.

Pure theory -- in the guise of the communal-stream principle -- tells us that at least one of those good things must happen. Professor Kremer's research suggests that both good things happen: we get more sex and less illness.

If all you want to do is slow down the epidemic, Professor Kremer's research says that more sex is a good thing. But if you want to maximize the excess of benefits over costs, then even more sex is an even better thing.

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 would attack the problem of excessive sexual temperance 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 we should 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.

What we need is a reward that Martin values and Maxwell doesn't -- like, say, a library card. I'm guessing that Maxwell, with his busy social life, doesn't spend a lot of time at the library.

That's a definite improvement, but it's still imperfect. 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 is just putting on a good show?

Let's try again: We need a reward that's of no value to Martin unless he actually has sex. And as before, it should be something the cautious Martin values more than the promiscuous 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 a condom at this point is less likely to make a difference. Subsidized condoms could be just the ticket for luring Martin out of his shell without stirring Maxwell to a new frenzy of activity.

As it happens, there is another reason to subsidize condoms. Condom use itself is underrewarded. 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. 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 alleged downside is that they encourage more encounters. But 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 main drawback to subsidizing condoms is that they're not very expensive to begin with. You can reduce the price of a condom from a dollar all the way down 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 little 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.

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.'")

Or, as one of my Slate readers suggested, we could have an online service to record negative HIV test results. You'd type in your prospective partner's name and get a response like "Last negative test result 7/4/2006." Or, to protect privacy, you'd type in not a name but an ID number provided by the partner. Your screen could show both a test result and a photo to avoid fake IDs. This strikes me as such a good idea that I can't figure out why nobody's doing it yet. Until then, the best we can probably do is to make condoms inexpensive -- and get ride of those subway ads.

Addendum

In 1996, Slate magazine published an abbreviated version of this chapter that generated hundreds of 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 multiround 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 nineteenth-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 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 likely won't get any sexually transmitted diseases. 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 filters 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"). Point (c) is a non sequitur perfectly analogous to the good doctor's comments about Russian roulette; such a strategy confers no benefits on the neighbors and thus is completely off-topic.

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 long-term 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 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 be 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: 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 increases in promiscuity. I doubt that he's right, but I can't prove he's wrong.

Copyright © 2007 by Steven E. Landsburg

Continues...

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Excerpted from More Sex Is Safer Sex by Steven E. Landsburg Copyright © 2007 by Steven E. Landsburg. Excerpted by permission.
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