Humans are highly social animals, and, since our evolutionary beginnings, our fitness outcomes have been deeply interdependent with those around us. From parent–child relationships to war parties, to mating relationships, our ability to transfer our genes to the next generation is often tied to others’ success. Here, we suggest that the idea of identity fusion, which Whitehouse describes as “a visceral sense of oneness with the group” (abstract), may be a proximate mechanism for aligning human behavior with degrees of fitness interdependence.

Fitness interdependence can be defined as the degree to which two or more organisms influence each other's success in replicating their genes (Aktipis et al. Reference Aktipis, Cronk, Alcock, Ayers, Baciu, Balliet, Boddy, Curry, Krems, Munoz, Sullivan, Sznycer, Wilkinson and Winfrey2018; Brown Reference Brown1999; Brown & Brown Reference Brown and Brown2006; Kelley & Thibaut Reference Kelley and Thibaut1978; Roberts Reference Roberts2005). Fitness interdependence can arise for many reasons, including genetic relatedness (Hamilton Reference Hamilton1964a; Reference Hamilton1964b), mating, having common descendants (Dow Reference Dow1984; Hughes Reference Hughes1988), food sharing, risk pooling (Aktipis et al. Reference Aktipis, de Aguiar, Flaherty, Iyer, Sonkoi and Cronk2016; Cronk et al. Reference Cronk, Berbesque, Conte, Gervais, Iyer, McCarthy, Sonkoi, Townsend, Aktipis, Lozny and McGovern2018), and, perhaps most importantly for the concept of identity fusion, intergroup competition and conflict. The concept of fitness interdependence applies to human interactions including many that Whitehouse describes, but it also applies to other interactions in the biological world. Any organisms that have positive impacts on one another's survival and reproduction, for example, in biological mutualisms, can be considered to have fitness interdependence.

Whitehouse proposes that extreme self-sacrifice – for example, altruistic suicide – is possible because of the process of identity fusion. Extreme self-sacrifice of this kind is seen in many biological systems where there is an extremely high level of fitness interdependence. For example, cells in clonal multicellular bodies are highly fitness interdependent, and these cells are equipped with the ability to undergo apoptosis – cell suicide – if they are malfunctioning and potentially threatening the viability of the multicellular organism of which they are a part (Aktipis et al. Reference Aktipis, Boddy, Jansen, Hibner, Hochberg, Maley and Wilkinson2015). Perhaps even more relevant to the idea of identity fusion is the biology of the slime mold Dictyostelium discoideum, which has a unicellular life stage, but which can also join together into a multicellular slug when resources are low, moving along the forest floor and then turning into a stalk and spore (Strassmann et al. Reference Strassmann, Zhu and Queller2000). The cells that end up in the stalk are engaging in the kind of extreme self-sacrifice that Whitehouse describes, and the D. discoideum cells literally fuse together during the multicellular stage. Therefore, identity fusion is not a uniquely human phenomenon, but one that has parallels in biological systems where fitness interdependence is very high.

If identity fusion is indeed a mechanism for aligning motivation and behavior with fitness interdependence, then identity fusion would be expected to occur in those situations characterized by high fitness interdependence and, in particular, during transitions to situations in which fitness interdependence becomes so high that it is practically at unity (i.e., a one-to-one correlation of fitness between the involved parties). In looking at human relationships, we see that some physical interactions, such as sexual intercourse and nursing, can increase oxytocin and subsequent feelings of bonding. These are both situations where there is a potential shift from lower to higher fitness interdependence of the involved parties. Similarly, in situations of intergroup conflict, there can be a shift from lower to higher fitness interdependence, which may facilitate the process of identity fusion, as with the “band of brothers” phenomenon in combat units during wartime (Shakespeare, Henry V, Act IV, scene iii [Ambrose Reference Ambrose1992]).

Whitehouse also suggests that rituals may play an important role in the procession of local identity fusion. This idea corresponds well with thinking among cooperation theorists that rituals are important for social coordination more broadly. Rituals help create both the common knowledge and the common meta-knowledge (i.e., common knowledge that there is common knowledge), among both participants and observers, that is needed for solving coordination problems (Chwe Reference Chwe2003; Cronk & Leech Reference Cronk and Leech2013). In the small-scale, highly interdependent societies in which our ancestors lived, the process of local fusion, perhaps enhanced through the use of rituals to create common knowledge and the use of kin terms to indicate fitness interdependence, may have routinely led to behaviors that, although very costly to the individual actor, were worthwhile in an evolutionary sense, thanks to the underlying fitness interdependence among the individuals within the group.

Large-scale societies, on the other hand, create the possibility of mismatches between perceived and actual fitness interdependence. This possibility may sometimes be exploited by the process of extended identity fusion that Whitehouse describes. As with local identity fusion, kin terms as markers of fitness interdependence may sometimes play a role in this process. As Whitehouse notes, such terms as “brother,” “sister,” “fatherland,” and “motherland” are common in political rhetoric, and such usage increases the persuasiveness of such rhetoric (Salmon Reference Salmon1988). In addition, people are more tolerant of violence toward out-group members if those who act violently use kin terms such as “brothers” and “family” among in-group members (Abou-Abdallah et al. Reference Abou-Abdallah, Kashima and Harb2016). Kin terms are also routinely employed by groups that demand such extremely fitness-reducing behaviors from their members as celibacy and suicide. Consider, for example, Roman Catholics’ use of terms such as “mother,” “father,” “sister,” and “brother,” and the fact that organizations that train suicide bombers use kin terms to manipulate and motivate their recruits (Qirko Reference Qirko2004; Reference Qirko2009).

Identity fusion may be one of the proximate mechanisms that motivate us to behave in ways that are consistent with our fitness interdependence with others. In other words, we may have a “visceral sense of oneness” with others when our fitness interests are highly aligned, perhaps even completely aligned. The identity fusion mechanism may be designed to be triggered only in extreme circumstances, characterized by high degrees of fitness interdependence. As with the cells of the slime mold D. discoideum that join together as a self-sacrificial slug only when their ability to survive and thrive as single cells is compromised, humans might join together in a self-sacrificing group when their fitness literally depends on it.