Dehumanization is a pervasive force in society that can be observed in a range of behaviors from small interactions to large-scale, systematic violence during conflicts. Understanding the fundamental science behind our ability to dehumanize is essential in the practice of human rights law and the prosecution of mass atrocity crimes. Celia Guillard and Lasana Harris show how decades of empirical social psychology and neuroscience research has established that all people have the capacity to dehumanize as a consequence of social context. While emphasizing the cognitive role of language in the process of dehumanization, this chapter draws on previous research to establish how social contexts facilitate dehumanization, which neural pathways might allow atrocities to occur and how we can prevent this behavior in the future. The chapter examines the implications of these findings within the broader context of serious violations of human rights, including some of the specific cases documented through international criminal trials. Most importantly, the expert evidence offered by Guillard and Harris can assist all parties – the prosecution, the defense and the judges – engaged in fact- and truth-finding procedures as instituted through international criminal trials.