Introduction

Aggression is a term used to describe broad classes of behaviour, although it is most typically used within the context of a social interaction. Many definitions of aggressive behaviour have been offered (see Kaufmann 1965), but one of the most widely cited definitions states that an aggressive response is ‘a response that delivers noxious stimuli to another organism’ (Buss 1961). This definition does not distinguish between different functional classes of behaviour, such as responding maintained by money or harm produced to others (i.e. positive reinforcers) or responding maintained by escape from aversive situations (i.e. negative reinforcers), but it encompasses the most salient feature of aggressive responding, and equally important, provides an operational definition of aggressive responding which is essential for aggression research.

Although overall rates of violent behaviour have remained fairly constant across the last few decades, incidents of violence among younger people have shown a significant increase (Potter and Mercy 1997). The costs of such antisocial behaviour to the aggressive individual, to his or her family and to society can be severe and include the disruption of the offender's social relations, the harm to the recipient and the enormous monetary costs associated with social services and the judicial system (see Kazdin 1987). Identifying the environmental and biological variables that influence aggressive responding thus remains an important task for researchers and clinicians, as these variables will provide the framework for the reduction and prevention of harmful aggressive behaviour.