Whether upheld as heroic or reviled as terrorism, people have been willing to lay down their lives for the sake of their groups throughout history. Why? Previous theories of extreme self-sacrifice have highlighted a range of seemingly disparate factors, such as collective identity, outgroup hostility, and kin psychology. In this paper, I attempt to integrate many of these factors into a single overarching theory based on several decades of collaborative research with a range of special populations, from tribes in Papua New Guinea to Libyan insurgents and from Muslim fundamentalists in Indonesia to Brazilian football hooligans. These studies suggest that extreme self-sacrifice is motivated by identity fusion, a visceral sense of oneness with the group, resulting from intense collective experiences (e.g., painful rituals or the horrors of frontline combat) or from perceptions of shared biology. In ancient foraging societies, fusion would have enabled warlike bands to stand united despite strong temptations to scatter and flee. The fusion mechanism has often been exploited in cultural rituals, not only by tribal societies but also in specialized cells embedded in armies, cults, and terrorist organizations. With the rise of social complexity and the spread of states and empires, fusion has also been extended to much larger groups, including doctrinal religions, ethnicities, and ideological movements. Explaining extreme self-sacrifice is not only a scientific priority but also a practical challenge as we seek a collective response to suicide, terrorism, and other extreme expressions of outgroup hostility that continue to bedevil humanity today.

Terrorism, whether it is group-related or performed as lone actor terrorism, is a predominantly male phenomenon. Generally and throughout history, young males have been the main protagonists of criminal and political violence.

This article aims to contribute, from different perspecives, to the question of what makes young men violent. These include neurobiological aspects, such as sex differences in the brain that predispose males to physical aggression and violence; gender role aspects, with regard to aggression and violence being basic components for demonstrating and reconstructing masculinity; demographic aspects of male youth bulges as potential breeding grounds for terrorism; aspects of group dynamics and identity fusion in the process of radicalization; and psychosocial characteristics of lone actor terrorists, which differ from group-related terrorists.

It is concluded that in addition to ideological, political, economic, regional, demographic, or psychosocial causes, experiences of threatened masculinity may be an underlying factor and driving force for terrorism.

Group violence, despite much study, remains enigmatic. Its forms are numerous, its proximate causes myriad, and the interrelation of its forms and proximate causes poorly understood. We review its evolution, including preadaptations and selected propensities, and its putative environmental and psychological triggers. We then reconsider one of its forms, ethnoreligious violence, in light of recent discoveries in the behavioral and brain sciences. We find ethnoreligious violence to be characterized by identity fusion and by manipulation of religious traditions, symbols, and systems. We conclude by examining the confluence of causes and characteristics before and during Yugoslavia’s wars of disintegration.