He has previously published research on differences between human and animal operant conditioning and on the treatment of chronic fatigue syndrome. However, he is best known for his work in psychosis, especially the psychological processes responsible for delusions and hallucinations and has published extensively in these areas. His research on persecutory (paranoid) delusions has explored the idea that these arise from dysfunctional attempts to regulate self-esteem, so that the paranoid patient attributes negative experiences to the deliberate actions of other people. His research on hallucinations has identified a failure of source monitoring (the process by which events are attributed to either the self or external sources) as responsible for hallucinating patients' inability to recognise that their inner speech (verbal thought) belongs to themselves. Along with many other British researchers, he has used these discoveries to inform the development of new psychological interventions for psychosis, based on cognitive behavioural therapy (CBT). This work has included randomised controlled trials of CBT for first episode patients and patients experiencing an at risk mental state for psychosis.