Traditional economic models assume that people are self-interested in the narrow sense. If “homo economicus” - the stereotypical rational actor in these models - finds a wallet on the sidewalk, he keeps the cash inside. He doesn’t leave tips after dining in restaurants that he will never visit again. And he would never vote in a presidential election, much less make an anonymous donation of money or time to a presidential campaign.

This posture stems from the so-called free-rider problem, a cornerstone of rational-choice theory. The problem, as described by Mancur Olson in his classic book, “The Logic of Collective Action,” is that even those who share a presidential candidate’s policy goals will reap no significant material advantage by donating their time or money. After all, with cash donations legally capped at $2,300, even donors who give the maximum have no realistic hope of influencing an election’s outcome. Nor can any individual volunteer — even one whose efforts resulted in hundreds of additional votes for his candidate — realistically hope to tip an election.

Although the logic of the free-rider problem may seem compelling, people’s behavior strikingly contradicts many of its predictions. Die-hard proponents of self-interest models sometimes counter that by becoming involved in campaigns, volunteers reap a variety of personal advantages. They often meet interesting people, for example, or they may learn about attractive employment opportunities. Major donors, for their part, are often rewarded with ambassadorships or other prominent positions when their candidate wins.

Researchers at the intersection of economics, psychology, sociology and other disciplines have had interesting things to say about the anomaly inherent in collective action. Albert O. Hirschman, an economist at the Institute for Advanced Study at Princeton, was one of the first to grapple seriously with it. In his 1982 book “Shifting Involvements,” he acknowledges that self-interest indeed appears to be the dominant human motive in some eras. But over time, he argues, many people begin to experience disappointment as they continue to accumulate material goods. When consumption standards escalate, people must work harder just to hold their place. Stress levels rise. People become less willing to devote resources to the public sphere, which begins to deteriorate. Against this backdrop, disenchanted consumers become increasingly receptive to appeals from the organizers of social movements.

Eventually, Mr. Hirschman argues, a tipping point is reached. In growing numbers, people peel away from their private rat race to devote energy to collective goals. The free-rider problem ceases to inhibit them, not only because they now assign less value to private consumption, but also because they find satisfaction in the very act of contributing to the common good. Activities viewed as costs by self-interest models are thus seen as benefits instead.

From an informal survey of 20th-century American social movements, Mr. Hirschman concluded that these cycles have an average duration of about 20 years. But sometimes patterns take much longer to recur.

Self-interest is surely an important human motive, perhaps even the most important motive much of the time. But it is never the only important motive. And during at least some moments in history, narrow self-interest models miss the essential story line completely. This may be one of those moments.

Read the full article at The New York Times.
Robert H. Frank is an economist at the Johnson School of Management at Cornell University. E-mail: rhfrank@nytimes.com.