I teach a seminar titled "Ideas of Human Nature" in which I restrict enrollment to 15 students. Others typically want to get in, and yet much of the class's popularity stems from the benefits that come from keeping it small. Most students understand the advantages of small classes, and they wouldn't want everyone who wishes admittance to get in; just themselves! If everyone gets in, the class becomes too big and discussion is inhibited, to everyone's disadvantage. And so, each year I find myself in the difficult position of telling a number of students that there simply isn't room for them. Each student turned away from this class understands the logic, but nonetheless, each would like the limit to be expanded -- by just one.

This is what game theoreticians call a social dilemma: a "prisoner's dilemma" writ large, in which individuals "play" against the larger collectivity. Such dilemmas have a lot to teach us.

On one hand, the class as a whole is somewhat worse off for every student above a given number (arbitrarily set in this case at 15) who is admitted. On the other, each student who wants admission would be better off -- or expects to be -- as the 16th. Individuals seeking admission are playing against the rest of the class, and most are willing to "defect," in game-theory terms, by getting in, all the while assuming that the instructor will not allow everyone who wants entry to succeed.

In such cases, the downside for the group is generally distributed across many individuals, so the personal cost to the defector is likely to be low, whereas the benefit, for that person alone, is likely to be high enough that, on balance, he or she is best off being selfish. The dilemma is that if everyone is selfish, then all are worse off, although each individual is tempted to try to get away with it nonetheless.

Let's take a leap in scale, to global warming. Instead of "add yourself to David Barash's class," make it "add extra carbon dioxide to the earth's atmosphere." The accumulation of greenhouse gases in the atmosphere is having negative effects on the world's climate; this is obvious to all but industry lobbyists, a very small number of contrarian -- and/or financially compromised -- scientists, and free-market-worshiping, Bush-administration ideologues. Technical solutions to global warming exist; the real dilemma is social. It is often easier and, in the short run, cheaper to use polluting, carbon-dioxide-spewing devices than to refrain, even though if everyone does it, we're all worse off. So, at the individual level, most people would prefer to drive their private automobiles rather than take public transportation, all the while bemoaning the resulting traffic, not to mention the buildup of carbon dioxide and its effects. Companies are reluctant to cut back on their generation of greenhouse gases because it may place them at a competitive disadvantage. And the United States under George W. Bush refuses to abide by the Kyoto Treaty, complaining that it is not in the "national interest," even though it is clearly in the interest of the planet. The United States, in this case, has elected to defect, "playing against" other countries, and now it appears that Russia may follow suit. In such situations, the cost to each entity -- person, corporation, country -- of cooperating appears to be high, whereas the benefit seems low. Defection threatens to become the rule, whereupon everyone loses. That is precisely what happens in many cases.

Consider a water shortage. Individuals should cooperate and conserve, but each is inclined to cheat. After all, many people like a lush lawn, soapy showers, frequent flushes. People readily understand that the entire public, including themselves, would be in trouble if everyone indulged his or her private desires. But it is awfully tempting to cheat because the cost of each defection, to the group as a whole, is small, whereas the benefit to the cheating individual is potentially large, so long as only a few others do the same thing.

Defection happens. For example, in 2000 Stephen King announced that he would make his next book available online. The New York Times reported that "Mr. King is trusting his readers to send him a dollar after each download. ... If he does not receive payments for at least 75 percent of the downloads, he says, he will stop writing after two chapters, and readers won't learn the end." It transpired that Mr. King's electronic novel, The Plant, generated more than 120,000 downloads when the first chapter appeared. However, the mid-November installment generated only 40,000 downloads, and of those, only 46 percent were paid for. As a result, King stopped writing the novel. "If you pay, the story rolls. If you don't, the story folds," he had written on his Web site. It folded.

Nearly forty years ago, the ecologist Garrett Hardin wrote a now-classic article, "The Tragedy of the Commons," in which he pointed to the history of land abuse in England. In that case, livestock owners allowed land held in common to be overgrazed even though they realized that by doing so, the commons was diminished -- to everyone's detriment. They grazed their own animals there, out of fear that if they refrained, others would take advantage of the resource, and it would be ruined anyhow.

During the period of communist rule in the Soviet Union and Eastern Europe, the environment was no better protected there than in the capitalist West; indeed, by most measures, it was worse. Socialist production goals were accorded the highest priority, and, as a result, environmental values suffered greatly. International meetings of environmentalists were notable for the hopes of the participants, often pinned unrealistically on the opposing system. Thus, Western environmentalists expected that state control and authority would generate models of more reliable environmental protection, while environmentalists from the Soviet bloc had an equally idealistic -- and unrequited -- hope that private ownership might lead the way toward rational ecological stewardship.

It has been said that under capitalism, people exploit people, whereas under communism, it's the other way around. Either way, the environment has been dangerously abused, and to a large extent, social dilemmas are to blame.

Comparable situations are evidently felt by animals. Reproduction, in a strictly Darwinian perspective, is selfish. After all, baby-making is the primary way living things project their genes into the future, thereby receiving an evolutionary payoff. Consider, for example, African elephants: Increasingly restricted to game parks, they are often fenced in and, as a result, locally overcrowded. Desperate to satisfy their huge appetites, hungry elephants strip the bark from trees, eventually killing them and thus ultimately destroying their habitat, to their own long-term detriment. But try explaining that to the elephants. For eons, natural selection has rewarded those that -- selfishly and successfully -- reproduced. Once again, it's a kind of social dilemma, whereby every elephant who becomes a parent gains a payoff measured in evolutionary fitness, while in the process imposing a substantial long-term cost on their habitat and thus on elephants as a whole.

Biologists were intrigued when Robert Trivers, at the time a Harvard University graduate student, pointed out that "reciprocal altruism" can be expected among animals. But despite more than three decades of serious effort to document self-sacrificing behavior among nonrelatives, remarkably few such examples have been discovered. We must conclude that the temptation to defect is not uniquely human. Birds do it. Bees do it. Even monkeys in their trees do it. They give in to temptation and typically refrain from giving to the group, or at least from giving more than they have to. In fact, it may be that, if anything, animals are less generous -- more inclined to be selfish -- than their human counterparts because they don't have powerful ethical precepts (religion, morality, not to mention the IRS and the criminal courts) to remind them of their social obligations.

Social dilemmas are unavoidable. After all, individuals rarely exist in isolation. Nearly always, we interact with the rest of society, expected to cooperate, yet tempted to cheat, relying on the cooperation of others, yet vulnerable to their defection. The basic concept of "society" assumes give and take, a "social contract" whereby individuals make what is essentially a deal with society at large: Each will forgo certain selfish, personal opportunities in exchange for profiting from the cooperation of others.

Theories of social contract in relationship to social dilemmas have occupied many of the great thinkers in political philosophy. The question, in short, is simply this: How do you reconcile personal selfishness with public benefit?

Thomas Hobbes argued forcefully for the necessity of regulating selfish, nasty human impulses for the good of the larger whole. Although social dilemmas had not been identified as such in Hobbes's time, he clearly saw that the seductive power of social defection was dangerously strong. In Leviathan, Hobbes wrote that "during the time men live without a common Power to keep them all in awe, they are in that condition which is called Warre; and such a warre, as if of every man, against every man."

The role of the political sovereign, in game-theory terms, was to punish noncooperators and keep everyone in line, preventing each from defecting. Hobbes envisioned that without such control, we would all inhabit a state of nature in which people were incapable of arranging for such cooperative endeavors as agriculture, industry, arts, or even society itself, "and which is worst of all, continual feare, and danger of violent death; And the life of man, solitary, poore, nasty, brutish and short."

With his idealization of the "noble savage," Jean-Jacques Rousseau seems poles apart from Hobbes. Yet Rousseau, too, in his best-known work, The Social Contract, pointed in a similar direction. Rousseau made an important distinction, between the "will of all" (the sum of individual desires) and the "general will" (the good of society, taken as a whole), emphasizing that the social contract is a way of making sure that pursuit of the former doesn't destroy the latter. Rousseau begins his famous book, "Man is born free, and is everywhere in chains." Although he often inveighs against those chains, even Rousseau recognizes that they are necessary:

"In order that the social contract should not be a vain formula, it tacitly includes an undertaking, which alone can give force to the others, that whoever refuses to obey the general will shall be constrained by the whole body. ... The undertakings that bind us to the social body are obligatory only because they are mutual."

In short, the peculiar genius of society is that it forces people to abide by their social contracts, and allows them to bypass the siren call of social defection, by precisely the kind of restraints and restrictions that Hobbes recommended and that we might expect Rousseau -- given his adulation of the "noble savage," untrammeled by rules and regulations, conventions, and compulsions -- to have opposed. But even Rousseau, apostle of natural human inclinations, recognized the need to say no to the temptations posed by our many social dilemmas. Two centuries later, in "The Tragedy of the Commons," Garrett Hardin found himself arguing similarly, coming down in favor of "mutual coercion mutually arrived at."

There is, however, a problem; namely, the mandating of restraints and enforcement mechanisms to prevent selfish, socially irresponsible behavior (the recommendation of Hobbes, Rousseau, and Hardin) runs counter to what is probably the basic precept of free-market capitalism. Thus, in the most famous paragraph in his masterpiece, The Wealth of Nations, Adam Smith introduced the notion of the "invisible hand," whereby the common good is achieved most efficiently when each individual succumbs to private greed:

"It is not from the benevolence of the butcher, the brewer, or the baker that we expect our dinner, but from their regard to their own self-interest. We address ourselves, not to their humanity but to their self-love, and never talk to them of our own necessities but of their advantages. ... It is his own advantage, indeed, and not that of society which he has in view. ... He intends only his own gain, and he is in this, as in many other cases, led by an invisible hand to promote an end which was no part of his intention. ... By pursuing his own interest he frequently promotes that of the society more effectually than when he really intends to promote it."

Political conservatives love that sort of stuff, which gives them permission to extol personal selfishness with the claim that by pursuing private gain, individuals are also promoting the public good. Game theory in general and social dilemmas in particular help point out that this is self-serving nonsense.

Faced with a choice between private gain and public good, most people opt for the former, and yet such a response to social dilemmas -- itself consistent with conservative political philosophy insofar as it emphasizes the "fallen" aspect of human nature -- suggests that, if anything, the all-too-visible hand of personal defection is likely to result in disaster rather than benefit to society as a whole.

I am not going to suggest a way out because as far as I am concerned, the solution is apparent and available, even as it is abhorrent to free-market fundamentalists: the urgings, requirements, and restraints of society as a whole, that is, the benevolent intervention of government, demanding at least a modicum of cooperation on behalf of society and the greater good. After all, we accept any number of impositions on our personal freedom: It may be in my selfish interest to rob a bank, if I could get away with it. But it isn't in society's interest for there to be lots of bank robbers, so we agree that police, courts, and jails are necessary, to make it disadvantageous for individuals to defect in that particular social dilemma. As we have seen, Hobbes -- generally considered a political conservative, incidentally -- saw this situation as requiring that individuals hobble their freedom in their own, collective, long-term interest, just as I find myself required to act as benevolent doorman to students seeking admission to my seminar. Doesn't the same argument apply to the worst excesses of unfettered greed in the public arena? If free-market fundamentalists disagree, it's up to them to point out alternatives.

David P. Barash is a professor of psychology at the University of Washington. His most recent book is The Survival Game: How Game Theory Explains the Biology of Cooperation and Competition (Times Books, 2003).
The Chronicle of Higher Education, 4.4.16