Why do people cooperate? Why do they act in the common interest, for the public good? Why do they do their part in cooperative ventures, in promoting outcomes that will benefit everyone, or everyone in their society or group? For people do sometimes play their part, even when doing so is not without cost to themselves. They vote in elections – going well out of their way, in rain and snow, waiting in line, even (in some places) braving intimidation – though they surely know (most of them) that it is extremely unlikely that their individual votes will make any difference to the outcome. They devote unpaid hours to political campaigns. They mail checks to causes and organizations they support. They join protests, sometimes at considerable risk.

But not everyone does his or her part in mutually beneficial cooperative ventures. Many, for example, do not vote, though they have a preference for one candidate over the others and believe that competitive elections are a good thing. And of those who do vote, most do nothing more in the political sphere. Most people do not participate in protests, or join and support political and other organizations from whose efforts they will benefit. Most Americans claim (in surveys) that they want a healthier environment but continue to be wasteful and extravagant consumers and to use any number of products that harm the environment. They do not want fisheries to be destroyed but continue to kill or eat fish from rapidly dwindling stocks.