Asking why people are sometimes willing to lay down their lives for the sake of their groups implies that extreme self-sacrifice is incomprehensible or perplexing. What, precisely, needs explaining? The perplexity seems to arise from a deep-rooted but usually unarticulated assumption that people are inherently selfish and invariably motivated to act in their individual self-interests. Within that implicit theory of human motivation, deeply embedded in individualistic societies such as the United States, extreme self-sacrifice is indeed inexplicable, but the theory itself is false and misleading.

The philosophical doctrine that we are all motivated solely to maximize our individual welfare is called psychological egoism (May Reference May2011; Slote Reference Slote1964). It asserts that everything we do, no matter how beneficial to others, is actually calculated to benefit ourselves. It allows the possibility of mistakes (acting in ways that fail to maximize one's welfare through errors or oversights) and akrasia (trying to do the best for oneself but failing through weakness of will), but it rules out altruism (benefiting another person at some cost to oneself) and leaves no room for collective rationality (acting in the interests of a group to which one belongs). Psychological egoism is either tautological or false: tautological if we interpret any deliberately chosen action as selfish by definition, or false if we adopt a more intuitive interpretation of selfishness. The economist and Nobel laureate Paul Samuelson (Reference Samuelson1993) poured scorn on the tautological interpretation:

There is overwhelming experimental evidence that altruistic behavior can be elicited reliably through experimental procedures designed to arouse empathy (Batson Reference Batson2011; Batson & Shaw Reference Batson and Shaw1991; Batson et al. Reference Batson, Dyck, Brandt, Batson and Powell1988). It is hard to escape the conclusion that empathic emotions evoke genuinely altruistic motives that have the ultimate goal of benefiting those who elicit the empathy and not those who feel it. Furthermore, there is evidence that people sometimes choose to act in the interests of their groups, rather than their individual selves (Bardsley & Ule Reference Bardsley and Ule2017; Bardsley et al. Reference Bardsley, Mehta, Starmer and Sugden2010; Butler Reference Butler2012; Colman et al. Reference Colman, Pulford and Rose2008; Reference Colman, Pulford and Lawrence2014). These findings confirm everyday observations of people acting in what they believe to be the interests of their families, companies, universities, or religious, ethnic, or national groups, even when those interests do not coincide with their own selfish interests. When these social motives are sufficiently powerful or fanatical, they can lead to extreme self-sacrifice.

The theory of social value orientation (SVO) was introduced by Messick and McClintock (Reference Messick and McClintock1968) and McClintock (Reference McClintock1972) to provide a rigorous conceptual framework for interpreting altruism, collective rationality, and other social motives. In its simplest (dyadic) interpretation, an SVO represents a person's preferences regarding the allocation of a resource between self and another individual. In terms of utility theory, suppose t _i and t _j are objective payoffs – for example, amounts of money – to two people i and j. Then i’s utility u _i(t _i, t _j) is a function of both objective payoffs. Under the individualistic SVO, i is motivated to maximize u _i = t _i, selfishly optimizing i’s own objective payoff without regard to j’s. This is the only motivation recognized under the doctrine of psychological egoism, although from a contemporary neurobiological perspective, it can be viewed as pathological (Sonne & Gas Reference Sonne and Gash2018). There is also persuasive experimental evidence for an altruistic SVO, when i is motivated to maximize u _i = t _j, hence being concerned solely to maximize another's welfare, and a cooperative SVO, when i is motivated to maximize u _i = t _i + t _j, the collective payoff of both individuals (for reviews of SVO theory and evidence, see Balliet et al. [Reference Balliet, Parks and Joireman2009], Bogaert et al. [Reference Bogaert, Boone and Declerck2008], and Murphy & Ackermann [Reference Murphy and Ackermann2014], who also discuss competitive and equality-seeking SVOs that do not concern us here).

Once we acknowledge that people are sometimes motivated to perform actions intended to benefit others, the puzzle of extreme self-sacrifice evaporates, and we do not need out-group threats to interpret social motivation. In a Stag Hunt game, for example, two hunters are motivated to act in the joint interest of the dyad in catching a stag, although each is tempted selfishly to chase a hare that can be caught without the other's help. No external threat is involved, but collective preferences need to be supplemented with team reasoning (Bacharach Reference Bacharach1999; Sugden Reference Sugden1993). Team reasoners first search for an outcome that they believe would be best for the dyad or group; if such an outcome exists and is unique, they then play their parts in the joint enterprise. In these theories, orthodox decision theory is a special case of team reasoning when the team is a singleton (for a comprehensive review of team reasoning theories and experimental evidence, see Colman & Gold [Reference Colman and Gold2017]).

People occasionally perform acts of extreme self-sacrifice to benefit others or to act in what they believe to be the interests of the groups to which they belong. Such self-destructive actions can arise from nonselfish motives of altruism or collective rationality. How people come to hold social motives so fervently is worth investigating; in many instances of self-sacrificial mass murder, religion evidently plays a key role. According to the physicist and Nobel laureate Steven Weinberg (Reference Weinberg1999), “With or without religion, good people can behave well and bad people can do evil; but for good people to do evil – that takes religion.” However, why people practice extreme self-sacrifice given their fervent altruistic or collectivistic social motives is hardly puzzling once we free ourselves from the debilitating misconception of psychological egoism.