In Chapters 13 and 14 you will be introduced to sociological explanations of crime. In the present chapter, you will be introduced to psychological explanations of crime. Sociological and psychological explanations are not necessarily in competition with one another. While the relative merits of different criminological theories are open to informed debate, sociological and psychological theories simply approach the problem from different perspectives. The key distinction lies in their respective units of analysis. For sociology, the unit of analysis is society; for psychology, the unit of analysis is the individual. Thus, while the focus of sociology is on understanding how broader social structures and systems influence human behaviour, the focus of psychology is on understanding how individuals themselves come to behave in the way they do. Psychological theories are sometimes mistakenly assumed to locate the causes of behaviour, including criminal behaviour, exclusively within individuals. But for the last 40 years or so, there has been a broad consensus in psychology that human behaviour is best understood as an interaction between more or less stable individual characteristics and the immediate circumstances in which behaviour occurs. This is known as the person-situation interaction model (Mischel 1968). Applied to understanding criminal behaviour, the person-situation interaction model reminds us that while some people are clearly more predisposed than are others to engaging in criminal behaviour, it is also the case that some situations are more likely than others to evoke criminal behaviour. In this chapter you will be introduced to two rather different offenders, Mark and Dave (see Case studies 12.1 and 12.2). You will then be introduced to five kinds of psychological theory, or levels of explanation: biological, behavioural, social learning, social ecological and situational explanations. Just as sociological and psychological theories can be complementary, so too can the different levels of psychological explanation complement one another by allowing us to understand different parts of the problem. To illustrate this point, this chapter will show how each of the five kinds of psychological theory might help us to understand different aspects of Mark's and Dave's offending behaviour.