From interdependence to a proportionality-based morality

Much of hominid life has been about achieving social integration in fluid groups by (1) reliably signaling one's willingness to respect others' welfare, while simultaneously (2) protecting oneself from exploitation. Evolutionary theorists (Alexander Reference Alexander1987; Baumard et al. Reference Baumard, André and Sperber2013; Frank Reference Frank1988; Trivers Reference Trivers1971) have argued that selection pressures resulting from competitive altruism (Barclay & Willer Reference Barclay and Willer2007), and increasingly refined abilities to sanction insufficient dedication in potential partners (e.g., gossip and shunning), have selected for moral preferences calibrated for securing conditional cooperation (in hunting, gathering, shared parenting, combat, etc.), by sharing its costs and benefits equally among stakeholders. As a result, humans evolved intuitions that individuals with whom they interact are persons with inalienable rights, and that one's commitment to their interests should be proportionate to the amount of benefits one can expect to secure (or losses one can hope to avoid) through them (Aktipis et al. Reference Aktipis, Cronk, Alcock, Ayers, Baciu, Balliet, Boddy, Curry, Krems, Muñoz, Sullivan, Sznycer, Wilkinson and Winfrey2018; Baumard et al. Reference Baumard, André and Sperber2013).

Moral rigidity as an error management “bias”

The logic of partner choice mutualism implies that to any type of joint venture corresponds a minimal level of cooperative engagement that each team member would obligatorily demand of his partners. As long as (i) groups cohabitated peacefully or natural resources abounded, for instance, one can expect within-group interdependence to have been moderate. Social selection pressures would have mainly been on individuals’ ability to demonstrate unconditional respect for their in-groups’ physical integrity and property, and the community's resources, to avoid being left out of everyday ventures like hunting and gathering. However, (ii) ancestral hominins also faced recurrent intergroup competition and warfare (LeBlanc & Register Reference LeBlanc and Register2003; Tooby & Cosmides Reference Tooby, Cosmides and Høgh-Olesen2010). In such circumstances, lethal threats coming from outside the group and the possibility of losing reproductive resources (such as territory or women) to rival coalitions would have driven costs of insufficient commitment to skyrocket, thereby dramatically elevating the level of prosocial engagement demanded of each group member.

Insufficient dedication to one's partners’ interests, whether in situation (i) or (ii), would on average have led to reproductive impasses. In terms of error management theory (Haselton et al. Reference Haselton, Nettle, Andrews and Buss2015), evolution should therefore have selected for moral heuristics that are “biased” in favor of the most adaptive strategy: (1) making absolutely sure the individual will display the minimal level of prosocial commitment demanded by the contextual level of interdependence, while (2) showing automatic aversion to acts that could endanger the coalition's cooperative potential and combativeness in intergroup conflict. In this respect, intuitively sacralizing and objectifying core interpersonal obligations may constitute the most cost-effective computational way of completely isolating them from the temptation to trade them off with immediate self-interest (Marie & Fitouchi, Reference Marie and Fitouchiin preparation).

In line with this hypothesis, experiments find that potential partners expressing deontological moral judgments in everyday settings – a proxy of underlying moral rigidity – are judged more trustworthy and are more likely to be chosen (Everett et al. Reference Everett, Pizarro and Crockett2016). Furthermore, the anthropology and psychology of contemporary parochial altruism in military, insurgent, and terrorist behavior suggest that moral rigidity may have been shaped by ancestral warfare to bolster defense of in-groups, territory, and communal resources. The values and goals for which contemporary attackers and defenders fight (e.g., the Reich, the Caliphate) often undergo ritualistic processes of sacralization and objectification (Atran Reference Atran2010; Reference Atran2016), up to a point where activists end up representing them as absolute coordinates of reality, and become blinded to exit strategies. By turning fighters into “devoted actors,” activation of high levels of moral rigidity – often complemented by “identity fusion” (Whitehouse Reference Whitehouse2018) – predicts individuals’ willingness to engage in costly sacrifice, including death, for their comrades and cause and ceteris paribus significantly increases fighters’ chances to prevail in combat (Atran Reference Atran2016; Gómez et al. Reference Gómez, López-Rodríguez, Sheikh, Ginges, Wilson, Waziri, Vázquez, Davis and Atran2017).