Revenge appears among a “suite” of social interactions that includes competition, alliance building (a prerequisite for tribal revenge raids), and so forth. Rather than a modular “system” directly reflecting evolutionary fitness constraints, revenge may be (another) social cost-benefit calculation involving potential or actual aggression and proximately controlled by individual personality characteristics and beliefs that can work against fitness.

McCullough et al. propose an evolved cognitive revenge system which imposes retaliatory costs on aggressors. They distinguish between this and other forms of punishment (e.g., those administered by judges) which are not underpinned by a specifically designed evolutionary mechanism. Here we outline mechanisms and circumstances through which the revenge system might nonetheless infiltrate decision-making within the criminal justice system.

McCullough et al. recognize that revenge and forgiveness jointly constitute a functional strategic complex. However, they model the halves of the complex as outputs of modules selected for regulating dyadic relationships. This is backwards. Forgiveness is a culturally evolved institution that can be exapted for use in dyadic contexts; it would be cheap talk among dyads were it not for the shadow of society.

McCullough et al. present a compelling case that anger-based revenge is designed to disincentivize the target from imposing costs on the vengeful individual. Here I present a contrast between revenge motivated by anger (as discussed in the target article) and revenge motivated by hatred, which remains largely unexplored in the literature.

Insofar as South Africa underwent a rapid transformation from apartheid to democracy, it may provide a unique laboratory for investigating aspects of revenge and forgiveness. Here we suggest that observations and data from South Africa are partially consistent with the hypotheses generated by MCullough and colleagues. At the same time, the rich range of revenge and forgiveness phenomena in real-life settings is likely to require explanatory concepts other than specialized modules and their computational outputs.

McCullough et al.'s functionalist model of revenge is highly compatible with the person-centered approach to moral judgment, which emphasizes the adaptive manner in which social perceivers derive character information from moral acts. Evidence includes act–person dissociations in which an act is seen as less immoral than a comparison act, yet as a clearer indicator of poor moral character.

Applying a non-developmental evolutionary metatheory to understanding the evolution of psychological capacities leads to the creation of models that mischaracterize developmental processes, misattribute genes as the source of developmental information, and ignore the myriad developmental and contextual factors involved in human decision-making. Using an evolutionary systems perspective, we argue that revenge and forgiveness cannot be understood apart from the development of foundational human psychological capacities and the contexts under which they develop.

McCullough et al. argue that revenge has a future-oriented function, that is, to deter future harms by changing other individuals' incentives toward the self. Recent research has shown that people seek revenge even when harms are unintentional. This commentary reports these results and proposes that revenge may also serve to reduce the immediate psychological pain resulting from unfair treatment.

In this response, we address eight issues concerning our proposal that human minds contain adaptations for revenge and forgiveness. Specifically, we discuss (a) the inferences that are and are not licensed by patterns of contemporary behavioral data in the context of the adaptationist approach; (b) the theoretical pitfalls of conflating proximate and ultimate causation; (c) the role of development in the production of adaptations; (d) the implications of proposing that the brain's cognitive systems are fundamentally computational in nature; (e) our preferred method for considering the role of individual differences in computational systems; (f) applications of our proposal to understanding conflicts between groups; (g) the possible implications of our views for understanding the operation of contemporary criminal justice systems; and (h) the question of whether people ever “genuinely” forgive.

What makes humans moral beings? This question can be understood either as a proximate “how” question or as an ultimate “why” question. The “how” question is about the mental and social mechanisms that produce moral judgments and interactions, and has been investigated by psychologists and social scientists. The “why” question is about the fitness consequences that explain why humans have morality, and has been discussed by evolutionary biologists in the context of the evolution of cooperation. Our goal here is to contribute to a fruitful articulation of such proximate and ultimate explanations of human morality. We develop an approach to morality as an adaptation to an environment in which individuals were in competition to be chosen and recruited in mutually advantageous cooperative interactions. In this environment, the best strategy is to treat others with impartiality and to share the costs and benefits of cooperation equally. Those who offer less than others will be left out of cooperation; conversely, those who offer more will be exploited by their partners. In line with this mutualistic approach, the study of a range of economic games involving property rights, collective actions, mutual help and punishment shows that participants' distributions aim at sharing the costs and benefits of interactions in an impartial way. In particular, the distribution of resources is influenced by effort and talent, and the perception of each participant's rights on the resources to be distributed.

To the extent that acting fairly is in an individual's long-term interest, short-term impulses to cheat present a self-control problem. The only effective solution is to interpret the problem as a variant of repeated prisoner's dilemma, with each choice as a test case predicting future choices. Moral choice appears to be the product of a contract because it comes from self-enforcing intertemporal cooperation.

Any evolved disposition for fairness and cooperation would not replace but merely compete with selfish and other antisocial impulses. Therefore, we propose that human cooperation and fairness depend on self-regulation. Evidence shows reductions in fairness and other prosocial tendencies when self-regulation fails.

The process of partner selection reflects ethnographic realities where cooperative rewards obtain that would otherwise be lost to loners. Baumard et al. neglect frequency-dependent processes exemplified by games of coordination. Such games can produce multiple equilibria that may or may not include fair outcomes. Additional, group-selection processes are required to produce the outcomes predicted by the models.

Baumard et al. attribute morality to a naturally selected propensity to share costs and benefits of cooperation fairly. But how does mundane mutualism relate to transcendent notions of morality critical to creating cultures and civilizations? Humans often make their greatest exertions for an idea they form of their group. Primary social identity is bounded by sacred values, which drive individuals to promote their group through non-rational commitment to actions independently of likely risks and rewards.

Among other things, Baumard et al.'s “A Mutualistic Approach to Morality” considers the enforcement and establishment of moral norms, the interpersonal comparison of welfare, and the structure of fairness norms. This commentary draws attention to the relevance of the game theory literature to the first and second topic, and the social psychology literature to the third topic.

An individual obtains an unfair benefit and faces the dilemma of either hiding it (to avoid being excluded from future interactions) or disclosing it (to avoid being discovered as a deceiver). In line with the target article, we expect that this dilemma will be solved by a fixed individual strategy rather than a case-by-case rational calculation.

Baumard et al. propose a functional explanation for the evolution of a sense of fairness in humans: Fairness preferences are advantageous in an environment where individuals are in strong competition to be chosen for social interactions. Such conditions also exist in nonhuman animals. Therefore, it remains unclear why fairness (equated with morality) appears to be properly present only in humans.

This commentary argues that the observed heterogeneity in fairness views, documented in many economic experiments, poses a challenge to the partner choice theory developed by Baumard et al. It also discusses the extent to which their theory can explain how people consider inequalities due to pure luck.

Baumard et al.'s definition of morality is narrow and their review of empirical work on human cooperation is limited, focusing only on economic games, almost always involving strangers. We suggest that theorizing about mutualisms will benefit from considering extant empirical behavioral research far more broadly and especially from taking relational context into account.

The evolutionary hypothesis advanced by Baumard et al. makes precise predictions on which emotions should play the main role in our moral lives: morality should be more closely linked to “avoidance” emotions (like contempt and disgust) than to “punitive” emotions (like anger). Here, we argue that these predictions run contrary to most psychological evidence.