My thesis aims to offer an interdisciplinary, multi-level explanation of the process of radicalisation towards violence in intergroup conflict. The main novel contribution of this project is the integration of Social Identity Theory (Tajfel and Turner, 1978) and Parochial Altruism (Choi and Bowles, 2007) to form an individual-to-group level psychological explanation of altruistic/cooperative, and hostile intergroup and intragroup, interaction. This integration forms the Social Identity Theory of Radicalisation in Intergroup Conflict (SITRIC). SITRIC attempts to explain the individuals’ journey towards social identity (group membership) salience, and how this general process (i.e. it occurs in all people to some extent) may differ across individuals leading to differential behavioural outcomes under certain conditions. These salient social identities (e.g. family, tribe, clan, nationality, religion – the latter is a particularly powerful social identity) are manifestations of proximate psychological mechanisms that may have evolved to maintain the differentiated behavioural tendencies that humans, past and present, have tended to show towards groups they are members of, and those which they are not. Namely, individuals in group contexts tend to be more cooperative and altruistic towards members of their ingroup than towards outgroup members. The role of religion – which has many ingroup enhancing features (e.g. promotion of trust, cohesion, cooperation etc), is an important one in terms of a salient social identity. That is, the unique aspects of individual religions are less important here, than the fundamental provision of a salient group membership that thus defines individuals on the basis of their groups’ shared belief system and values, whilst distinguishing them from others.