We argue that Whitehouse's group-based model neglects two key contextual variables: environmental threats and ideology. Environmental threats lead to extremism outside of group settings and predispose individuals toward joining ideologically zealous groups. Ideologies and environmental threats can also explain why certain groups adopt norms that encourage violent self-sacrifice.

We question whether altruistic motivation links identity fusion and extreme self-sacrifice. We review two lines of research suggesting that the underlying motivation is plausibly egoistic.

The role of ideas and beliefs is generally underplayed in Whitehouse's account. However, just as people may feel that their identity is fused with a collective, they may also feel that their identity is fused with an idea (god, history, justice), which can motivate the same type of behaviors that Whitehouse seeks to explain.

Fighting and dying, or what Whitehouse calls “out-group hostility” and “extreme self-sacrifice,” are not conceptually overlapping, but in fact are highly distinguishable, both theoretically and empirically. I present empirical evidence from a reanalysis of Ginges et al. (2009, Study 4), demonstrating the potentially inverse relationship between “parochial hostility” – fighting and “sacrificial altruism” – “and” dying.

This comment questions Whitehouse's theory. This comment proffers that people first choose their identity and later employ shared facts from the past to cement it. This is true with respect to two kinds of group identity: politics- and community-based identities. Contrary to Whitehouse, neither shared biology nor shared experience from the past is necessary for the constitution of either kind.

We argue that limiting the theory of extreme self-sacrifice to two determinants, namely, identity fusion and group threats, results in logical and conceptual difficulties. To strengthen Whitehouse's theory, we encourage a more holistic approach. In particular, we suggest that the theory include exogenous sociopolitical factors and constituents of the religious system as additional predictors of extreme self-sacrifice.

We believe that Whitehouse's model could be extended in a way that can help us make sense of self-radicalised individuals who are not active in cliques. We believe that conceptual ties may be important to this process and present a brief analysis of a database collected by the national consortium for the Study of Terrorism and Responses to Terrorism (START), to suggest future research to complement Whitehouse's proposal.

We seek strength in numbers as a survival strategy, so it seems unlikely that social bonds would make us want to intentionally die. However, our deep desire to be protected may explain our attraction to exaggerated notions of intentional self-sacrifice – even though research on suicide terrorists, kamikaze pilots, and cult members suggests they were not actually dying for their group.

Whitehouse adapts insights from evolutionary anthropology to interpret extreme self-sacrifice through the concept of identity fusion. The model neglects the role of normative systems in shaping behaviors, especially in relation to violent extremism. In peaceful groups, increasing fusion will actually decrease extremism. Groups collectively appraise threats and opportunities, actively debate action options, and rarely choose violence toward self or others.

Whitehouse makes no room for evolutionary approaches to extreme behaviors based on partner choice and mutualism, which have been convincingly invoked to make sense of ordinary morality. Extended to intergroup warfare, these evolutionary mechanisms may play a pivotal role in explaining the existence of extreme – though not functionally sacrificial – behaviors, benefiting non-kin fellow fighters, together with the distinctive phenomenology those behaviors display.

Physical enclavement, away from out-group members, may determine when identify fusion leads to self-sacrifice. When people surround themselves with ideologically similar others, their attitudes may polarize and become moralized, leading to more violence and hostility toward people who do not share those attitudes. We discuss how this segregation may increase the amount of political violence in typically nonviolent systems.

Many extreme forms of self-sacrifice occur without identity fusion or out-group threats. For example, people willingly participate in extreme charity fundraisers – exerting effort, enduring pain, and risking bodily injury – to advance desired causes. Therefore, it seems plausible that the motivation to engage in extreme self-sacrifice reflects a basic cognitive bias, rather than an evolutionary-cultural adaption specifically designed for intergroup conflict.

Whitehouse's article posits several plausible hypotheses, but suffers from an unwarranted reliance on the importance of distinct social groups in the causation of self-sacrificing behavior. A focus on relationships between individual kin is better able to account for both the evolution of self-sacrifice and present forms of self-sacrifice. The practical importance of this point is discussed.

Extreme self-sacrifice in intergroup conflict may be driven not only by situational factors generating “fusion,” but also by interindividual differences. Social value orientation is discussed as a potential contributor to self-harming behavior outside of intergroup conflicts and to the general propensity to participate in intergroup conflict. Social value orientation may therefore also be a person-specific determinant of extreme self-sacrifice in intergroup conflict.

Do instances of extreme self-sacrifice represent a valid paradigm to capture what makes typical individuals fuse with others? Probably not, because they can be viewed as aberrant phenomena. To understand the origins and mechanisms of human social fusion, one should first look at the development of babies and young children. Typical development represents the best alternative to Whitehouse's extreme model of social fusion.

We discuss a disconnect between the predictions of Whitehouse's model regarding the accumulative nature of fusion and real-world data regarding the age at which people generally engage in self-sacrifice. We argue that incorporating the link between age and identity development into Whitehouse's theoretical framework is central to understanding when and why people engage in self-sacrifice on behalf of the group.

Whitehouse's model explains when people engage in self-sacrifice, but not who is most likely to do so. We propose incorporating individual differences, such as cognitive style (one's inclination toward intuition versus deliberation), and argue that individuals who rely on intuition may be more likely to (1) develop group identity fusion after an emotional experience and (2) engage in pro-social self-sacrifice.

Whitehouse's focus on two causes of identity fusion (evolution and shared experiences/biology) deepens understanding of these specific topics. While we applaud his analysis, in his efforts to elaborate these two causes, he has – perhaps unavoidably – produced a narrower conceptualization of identity fusion. This is unfortunate because it undermines his stated goal of developing a more general, encompassing theory.

Identity fusion is remarkably similar to the extensively validated construct of communal sharing, proposed in 1991. Both posit that notions of oneness/unity/equivalence with others underpin altruism. However, we argue that oneness/equivalence instantiates an evolved, innate relational form, marked and constituted by cultural practices making participants’ bodies substantially the same. It is intuitive from earliest development, often encompasses persons whom one has never met, and results mostly in caring.

Whitehouse's view that our ultrasocial species evolved in small, warring bands is questioned, and alternative social selection pressures for the evolution of identity fusion and self-sacrifice in small-scale societies are proposed. Short durations of states of fusion allow for re-evaluation of risks; the consolidation of episodic memories into collective oral traditions elicits cooperation. Dysmorphic memories may be more powerful in generating identity fusion when followed by euphoric ones, as in “rites of torture” in male initiations.