  Over the years, numerous experiments have been accumulated to show that
cooperation is not casual and depends on the payoffs of the game. These
findings suggest that humans have attitude to cooperation by nature and the
same person may act more or less cooperatively depending on the particular
payoffs. In other words, people do not act a priori as single agents, but they
forecast how the game would be played if they formed coalitions and then they
play according to their best forecast. In this paper we formalize this idea and
we define a new solution concept for one-shot normal form games. We prove that
this \emph{cooperative equilibrium} exists for all finite games and it explains
a number of different experimental findings, such as (1) the rate of
cooperation in the Prisoner's dilemma depends on the cost-benefit ratio; (2)
the rate of cooperation in the Traveler's dilemma depends on the bonus/penalty;
(3) the rate of cooperation in the Publig Goods game depends on the pro-capite
marginal return and on the numbers of players; (4) the rate of cooperation in
the Bertrand competition depends on the number of players; (5) players tend to
be fair in the bargaining problem; (6) players tend to be fair in the Ultimatum
game; (7) players tend to be altruist in the Dictator game; (8) offers in the
Ultimatum game are larger than offers in the Dictator game.
