Radicalization is an inherently relational concept. One can only be radical in relation to someone who is not. Similarly, one cannot be extreme without an accepted center norm. But the center is not a fixed state. It shifts and changes across time, place, circumstance, and culture. The same could be said of radicalization. Radicalization is a social and psychological process of increasing acceptance and engagement, attitudinally and behaviourally, with any group associated with conflict.1 It need not necessarily involve violence. However, taking the perspective that radicalization is associated with extremism, particularly certain types of extremism, may help policy-makers, law enforcement, and increasingly, social media platforms, to identify people at risk of becoming involved in what we currently think of as terrorism. But it certainly doesn’t help us to understand what it is or why it happens. Rather, it is often the social process and political aspect of radicalization that is emphasized. This makes sense for certain agendas. Policy-makers, law enforcers, etc. need to be able to identify a threat and inhibit/prevent/neutralize such threats. A definition reflecting the notion of a static center provides a way to pinpoint who or what is threatening according to current thinking on what is extreme.
However, this stance inhibits our capacity to understand the radicalization process because it exceptionalizes people on the basis of what can be, admittedly, a set of rather exceptional behaviors (i.e. suicide terrorism), though often also increasingly includes unexceptional behaviors (i.e. providing funding, logistics, or even just online support for certain groups). Radicalization and extremism are thus little more than labels. The ‘in the eye of the beholder’ philosophy is a luxury that some cannot afford, and perhaps many are unable to stomach. But it leaves us better equipped to understand why (some) people may engage in what we currently think of as extremism and violent extremism because it looks to normal psychological processes and mechanisms that are involved in the radicalization process, rather focusing on the qualities that we have labeled as exceptional.
One such psychological mechanism is Parochial Altruism. Parochial altruism is the propensity for humans to engage in costly-to-self behavior to protect group members from non-group members.2 One (of many) causes of death in ancestral times was outgroups. Whether due to resource encroachment, the spread of disease and parasites, or overt aggression, the mere presence of outgroups would have been enough to trigger parochial altruism. Parochial altruistic responses include fear, withdrawal or fleeing, withholding benefits/resources, and overt hostility and aggression. Presuming that an individual belongs to a sufficiently important group, perceptions of threat to that group will stir parochial altruism in modern humans, despite these conditions being unlikely to manifest in the potential existential threat that may have occurred during ancestral times. This is known as mismatch.3
Parochial altruism gives us hints about the conditions that threatened ancestral individuals/groups, many of which are reflected in terrorist grievances.4 Thus, engagement with what can appear to be increasingly extreme thoughts, attitudes, and behaviors, may be expected responses to perceptions of threat to one’s ingroup/self from an outgroup. Indeed, experimental evidence suggests that there are certain features associated with sensitivity to parochial altruism that may also explain individual variation in radicalization.5 Behavioral parochial altruism is associated with higher levels of personal vulnerability, extrinsic religiosity, sex (males), higher levels of social dominance orientation, or preference for hierarchy and dominance of the ingroup, and importantly, perceptions of threatening outgroups.
In effect, the drivers of parochial altruism are rooted in groupishness, status protection/enhancement, and perceptions of threat from other groups, with perceptions of threat being particularly important in driving behavioral parochial altruism. Given that terrorist grievances typically tap into these types of factors, it is suggested that terrorist grievances, therefore, are really tapping into sensitivity to parochial altruism. Individuals more sensitive to parochial altruism may be more likely to perceive outgroup threat, thereby triggering parochial altruistic responses. Thus, cognitive mechanisms like parochial altruism may underlie the radicalization process by enabling perceptions and responses that make engagement and participation in terrorism more likely to occur, all else being equal.
From this perspective, radicalization and engagement with (perceived) extremism, are a set of expected, albeit mismatched parochially altruistic responses triggered largely by perceptions of threatening outgroups. Perceiving extremism is akin to perceiving a threat from outgroups – it is parochially altruistic providing individuals/groups with a detection-response system to protect the ingroup from the potential harms posed by others who are not ‘us’. Whilst terrorist organizations may try to harness and direct parochial altruism with ideology, indoctrination, socialization, and propaganda, these social and political factors are not necessary for parochial altruistic responses to be triggered. Therefore, looking to normal psychological mechanisms allows us to consider how and why certain contexts and conditions might be particularly triggering for those sensitive to parochial altruism. On one hand, there may be individuals who are more sensitive to parochial altruism and cues indicating threatening outgroups, and on the other, exposure to the extremist message may make perceptions of those threats to the ingroup more prominent, and guide parochially altruistic responses.
 Clark McCauley and Sophia Moskalenko, Friction: How Radicalization Happens to Them and Us (Oxford: Oxford University Press, 2011).
 Jung-Kyoo Choi and Samuel Bowles, “The Coevolution of Parochial Altruism and War.,” Science (New York, N.Y.) 318, no. 5850 (October 2007): 636–40, doi:10.1126/science.1144237.
 Mark van Vugt et al., “Evolution and the Social Psychology of Leadership: The Mismatch Hypothesis,” in Evolution and the Social Psychology and Leadership, ed. C Hoyt, D Forsyth, and A Geothals (New York: Praeger Perspectives, 2008).
 Zoey Reeve, “Islamist Terrorism as Parochial Altruism,” Terrorism and Political Violence, 2017, doi:10.1080/09546553.2017.1346505.
 Zoey Reeve, “Terrorism as Parochial Altruism: Experimental Evidence,” Terrorism and Political Violence, 2019, doi:10.1080/09546553.2019.1635121.