Instrumental and reactive subtypes of aggression are likely to have different biopsychosocial roots. This has implications for research aimed at identifying aetiological mechanisms and for forensic practice. In this methodological note we describe the development of a unidimensional
measure of lifetime persistent instrumental aggression based on information collected from a random sample of 241 Scottish male prisoners. We applied modern psychometric methods – confirmatory factor analysis and item response theory methods – to the development of a short, reliable
and informative measure of this important forensic construct. Results indicated that information from files and interviews could be combined to develop a unidimensional measure of the degree of instrumentality, which provided a psychometrically robust measure of the construct of instrumental
aggression. Further, item response theory analyses indicated good measurement across the middle to high range of the trait. Identifying life-long patterns of instrumental aggressive behaviour, and distinguishing instrumental aggression from the broad pool of aggressive behaviour, has the potential
to improve understanding of underlying mechanisms, increase the accuracy of risk assessments, and contribute to more effective treatment and management strategies.