Whitehouse dissects the antecedents of a form of strong alignment with a group dubbed “identity fusion.” He offers many intriguing ideas regarding the causes of fusion in naturally occurring settings, including ones that bear upon topics ranging from how it evolved among our ancestors to the mechanisms through which it leads to terrorism and extreme behaviors today. By offering an anthropological spin on what has heretofore been viewed primarily as a psychological process, Whitehouse offers an exciting new perspective on identity fusion.

As impressed as we were with Whitehouse's insightful analysis, we were ambivalent regarding his assertion that his formulation represents a more general or more encompassing theory than its predecessors (sect. 1, para. 2). Our reservation is that, in several important respects, his analysis is narrower than the original statement of identity fusion theory (Swann et al. Reference Swann, Jetten, Gómez, Whitehouse and Bastian2012). For example, his article engages almost exclusively with one form of fusion (local fusion) and is relatively silent on another form of fusion (extended fusion). Similarly, he focuses on a single consequence of fusion – extreme violence – and the original theory considered additional consequences. True, he goes into greater detail regarding two causes of fusion (evolution and shared experiences/biology) and how these causes are linked to extreme violence. Yet, both of these causes and this outcome were discussed in the original article, and going into these processes in greater depth comes at the cost of diminishing the apparent scope and explanatory power of the theory. From this vantage point, Whitehouse goes deeper on specific aspects of the original formulation, but not broader.

With respect to causes of fusion, Whitehouse argues that the perceptions of shared essence that produce fusion are derived predominantly from either shared dysphoric experiences or similar genes. To his credit, Whitehouse does unpack these two causes in greater detail than previous analyses, thereby providing a deeper understanding of them. Nevertheless, evidence that shared values are independently linked to endorsement of extreme behavior among strongly fused persons (Swann et al. Reference Swann, Buhrmester, Gómez, Jetten, Bastian, Vázquez, Ariyanto, Besta, Christ, Cui, Finchilescu, González, Goto, Homsey, Sharma, Susianto and Zhang2014a) is absent from his analysis. Instead, it appears that shared values are reduced to a psychological marker of similar genes. This surely occurs on occasion, but we doubt that people routinely infer shared genes wherever they share values with someone. We believe that the perception of shared values can promote fusion quite independently of the perception of shared genes. This is an important point, because omitting key antecedents of fusion from one's formulation produces an overly narrow analysis.

We also believe that Whitehouse is surely correct in noting that shared dysphoric experiences can trigger fusion. However, he implies that fusion is the inevitable consequence of such experiences, overlooking the fact that such experiences can also have the opposite effect. Whereas natural disasters such as the San Francisco earthquake of 1909 and Hurricane Katrina in 2005 were widely known for bringing people together and encouraging acts of altruism, they were also known to have triggered violence and acts of barbarism against innocents (Solnit Reference Solnit2010). To understand when sharing extreme experiences will trigger fusion, one must consider the broader social context as well as intragroup and intergroup dynamics. For example, out-groups may try to systematically short-circuit the tendency for shared trauma to produce fusion (Kellezi & Reicher Reference Kellizi, Reicher, Jetten, Haslam and Haslam2012). Similarly, when societies have drifted into a state of normlessness or anomie, extreme events may amplify feelings of isolation rather than promote fusion (Teymoori et al. Reference Teymoori, Bastian and Jetten2017). In short, although Whitehouse has reviewed persuasive evidence that shared trauma can foster fusion among group members, this is only one of several possible outcomes of shared trauma. A general theory should broaden the scope of his analysis to understand the causes and consequences of each of these outcomes.

With respect to the consequences of fusion, Whitehouse's rather narrow focus on violence, terrorism, and extreme behavior overlooks the fact that identity fusion may manifest itself in many other ways. For example, fusion has been linked to charitable giving to members of the in-group (Buhrmester et al. Reference Buhrmester, Newson, Vázquez, Hattori and Whitehouse2018b; Misch et al. Reference Misch, Fergusson and Dunham2018; Swann et al. Reference Swann, Gómez, Huici, Morales and Hixon2010b), donations of time and money to the community following a catastrophic earthquake (Segal et al. Reference Segal, Jong and Halberstadt2018), adaptive coping in couples (Walsh & Neff Reference Walsh and Neff2018), endorsement of self-sacrifice in intergroup versions of the trolley dilemma (Gómez et al. Reference Gómez, Brooks, Buhrmester, Vázquez, Jetten and Swann2011a; Swann et al. Reference Swann, Gómez, Dovidio, Hart and Jetten2010a; Reference Swann, Gómez, Buhrmester, López-Rodríguez, Jiménez and Vázquez2014b), expected life satisfaction following an election defeat or victory (Buhrmester et al. Reference Buhrmester, Gómez, Brooks, Morales, Fernandez and Swann2012), plans to remain in the group (Gómez et al. Reference Gómez, Morales, Hart, Vázquez and Swann2011b), curtailing medical aid to an out-group (Fredman et al. Reference Fredman, Bastian and Swann2017), endorsement of granting favors to one's twin (Vázquez et al. Reference Vázquez, Gómez, Ordoñana, Swann and Whitehouse2017), undergoing sex reassignment surgery (Swann et al. Reference Swann, Gómez, Vázquez, Guillamón, Segovia and Carillo2015), and endorsement of having the group fight for the self (Heger & Gaertner Reference Heger and Gaertner2018b). One goal for a general theory of fusion would be to develop a common explanatory framework that could accommodate all of the diverse manifestations of identity fusion. More generally, a general theory should explain how these processes might galvanize pro-group behavior, not only among fighters in militias, but also among members of church groups, soccer clubs, and political parties (Swann & Talaifar Reference Swann and Talaifar2018). Answers to these questions will provide a broader, more comprehensive picture of the role of identity fusion in sacrifice for the group.

These quibbles aside, let us emphasize again that Whitehouse has advanced fusion theory by extending it into the world of anthropology and evolutionary psychology. His insights, together with the empirical contributions he has made to this area, will help inspire future efforts to develop an even fuller understanding of identity fusion and the behaviors it motivates.