Speculation about the evolutionary origins of morality has yet to show how a biologically based capacity for morality might be connected to moral reasoning. Applying an evolutionary approach to three kinds of cases where partiality may or may not be morally reasonable, this paper explores a possible connection between a psychological capacity for morality and processes of wide reflective moral equilibrium. The central hypothesis is that while we might expect a capacity for morality to include aspects of partiality, we might (...) also expect these same aspects of the capacity to produce systemic forms of performance-based error. Understanding these errors helps point the way toward a theory of moral competence that includes aspects of both partiality and impartiality. (shrink)