The authors argue that, in the course of its history, social work has oscillated between two contrasting views of human nature: that which views people as essentially rational and that which views them as predominantly emotional. Concentrating on the area of children and families, they show how current legislation and practice guidance are premised on the notion of people as essentially rational beings who are capable of discharging their social obligations and working responsibly towards agreed objectives. On this view, clients should be judged–and treated–according to their external performance rather than their internal motivation. The authors contend that this view of human nature is over-restrictive and often self-defeating. They argue that human nature is inescapably emotional and irrational, and that true justice can only be done to the human ‘raw material’ of social work practice through a psychological understanding of clients in the context of their own unique developmental histories.