Stationary distribution of moral strategies in a population

Evolutionary models show that human cooperation can arise through direct reciprocity relationships. However, it remains largely unclear which psychological mechanisms may proximally motivate an individual to reciprocate. Recent evidence demonstrates that psychological motives for reciprocal choices (i.e., moral strategies) differ between individuals, which raises the question whether these differences have a stationary distribution in a population or are rather an artifact of the experimental task. Here, we combine data from three independent studies and participant samples to find that the relative prevalence of different moral strategies is highly stable across these datasets. Furthermore, the distribution of moral strategies is relatively unaffected by changes to the salient features of the experimental paradigm. Finally, the moral strategy classification assigned by our computational modeling analysis corresponds to the participants’ own subjective experience of their psychological decision process, and no existing models of social preference can account for the observed individual differences in moral strategies. This research supports the view that social decision-making is not just regulated by individual differences in 'pro-social’ versus ‘pro-self’ tendencies, but also by trait-like differences across several alternative pro-social motives, whose distribution in a population is stationary.