The existence of a link between mental disorder and violence continues to be debated. Whilst some have argued against a clear link, Hodgins draws on three types of study (birth cohort follow-ups, discharged inpatient studies and studies of convicted offenders) to argue that there is a clear and consistent association. The relationship between mental disorder and offending behaviour varies between individuals. Hodgins argues that there is a need to identify subgroups of violent behaviour in order to understand the dynamic interactions between biological, psychological and social factors, and thereby develop the most effective interventions. Schizophrenia is the most common mental illness, affecting about 1% of the general population. Some people develop organically based conditions such as tumours that can result in violent or antisocial behaviour. It is now generally accepted that treatment programmes for mentally disordered offenders should address the principles of 'risk', 'need' and 'responsivity'.