Humans cooperate because they remember their own fallibility, which is why cooperation increases with age.

Cooperation has been studied using evolutionary game theory.
The vectorial analysis used in evolutionary game theory is simplistic, because communications between humans have both magnitude and direction, and can occur at multiple levels simultaneously.
The study of evolutionary game theory is predominantly a male activity that predicts a reduced expectation of life, that shifts focus from the present to the future, and that requires conditions which limit spontaneity, and thus one of the main functions of human verbal communication, which is to be, through hearing the sound of one’s own voice, now.

‘Costly punishment’ emerged as an explanatory variable in the study of cooperation: as usual, in that Procrustean world, the passage of time revealed a narrow applicability, hardly surprising given that punishment could range from the mental effort of doing nothing to financial penalties. Cost, c, was one of the variables in Hamilton’s original formula, but it needs experimental quantification, not celebration.