This paper focuses on ethnic, national, religious and ideological large groups. Large-group identity refers to persistent sameness of tens, hundreds of thousands or millions of individuals, most of whom will never meet during their lifetimes. Within the interactions of large groups, ‘otherness’ is very powerful and is both desirable and frightening. We can enjoy human diversity when we are not preoccupied with the pressures and anxieties associated with the repair and maintenance of our large-group identity. When certain elements of us and them psychology are combined, derivatives of massive aggression surface. Clinicians willing to take part in interdisciplinary efforts have much to offer for those who wish to resolve us and them conflicts.