Reciprocity is the secret of our success, even though two unrelated individuals sometimes find it difficult to cooperate. They may mutually reciprocate help, if they know they will meet again. However, there is always the temptation not to return the help to the donor. Using a strategy such as Tit-for-Tat (see Chapter 4) can minimise the risk of being the sucker in the end, but there is no guarantee. To achieve cooperation seems hopeless when groups of three or more unrelated individuals need to cooperate in order to maintain a common resource: the resource is usually overused and collapses, as do fish populations as a consequence of over-fishing, and the global climate as a consequence of unrestricted use of fossil energy. The latter is regarded as the greatest challenge to humankind. The tragedy of the commons, as Hardin (1968) called this kind of social dilemma (see Chapter 6), appears inevitable – free access to a public resource brings ruin to all.

The so-called Public Goods game has been invented as a paradigm to study tragedy of the commons situations experimentally. For example, a group of four volunteers is asked to supply one euro each to a public pool, which is then doubled and redistributed among the four players irrespective of whether they have contributed. If all contribute, each has a net gain of one euro. However, a single defector has a net gain of 1.50 euro whereas each of the three contributors gains only 50 cents. Why should you cooperate?