Blood, sex, personality, power, and altruism: factors influencing the validity of strong reciprocity

It is argued that the generality of strong reciprocity theory (SRT) is limited by the existence of anonymous spontaneous cooperation, maintained in the absence of punishment, despite free-riding. We highlight how individual differences, status, sex, and the legitimacy of non-cooperation need to be examined to increase the internal and ecological validity of SRT experiments and, ultimately, SRT's external validity.