  Social dilemmas are situations in which collective interests are at odds with
private interests: pollution, depletion of natural resources, and intergroup
conflicts, are at their core social dilemmas.
  Because of their multidisciplinarity and their importance, social dilemmas
have been studied by economists, biologists, psychologists, sociologists, and
political scientists. These studies typically explain tendency to cooperation
by dividing people in proself and prosocial types, or appealing to forms of
external control or, in iterated social dilemmas, to long-term strategies.
  But recent experiments have shown that cooperation is possible even in
one-shot social dilemmas without forms of external control and the rate of
cooperation typically depends on the payoffs. This makes impossible a
predictive division between proself and prosocial people and proves that people
have attitude to cooperation by nature.
  The key innovation of this article is in fact to postulate that humans have
attitude to cooperation by nature and consequently they do not act a priori as
single agents, as assumed by standard economic models, but they forecast how a
social dilemma would evolve if they formed coalitions and then they act
according to their most optimistic forecast. Formalizing this idea we propose
the first predictive model of human cooperation able to organize a number of
different experimental findings that are not explained by the standard model.
We show also that the model makes satisfactorily accurate quantitative
predictions of population average behavior in one-shot social dilemmas.
