Paranoid delusions are the most common type of delusion found in psychotic patients. They are found not only in people with schizophrenia, but also in people with bipolar disorder and major depression. Milder forms of paranoid thinking are also found in people without mental-health problems. A study of 237 people in London and North-West England looked into the factors that could lead to paranoid thinking. Participants in the study included people with schizophrenia, psychosis and major depression and 64 people without mental-health problems who formed a control group. The study found that paranoid delusions were associated with a combination of pessimistic thinking style (low self-esteem, a pessimistic way of explaining events and negative emotion) and impaired cognitive performance such as poor decision making, a tendency to jump to conclusions and an inability to reason about the mental state of others. Both factors independently increased the risk of paranoid delusions even after the other one was taken into account.
Bentall, Richard P. ... [et al] - The cognitive and affective structure of paranoid delusions Archives of General Psychiatry March 2009, 66(3), 236-247