Type of symptomatology as a form of volunteer bias.

We sought to explore the relationship between type of psychopathology and consent bias. Using the Brief Psychiatric Rating Scale we assessed a group of 48 forensic psychiatric inpatients. These patients were later independently approached by a researcher who attempted to get their consent for a study on the validity of self-reported criminal behavior. Thirty agreed to participate and 18 did not. The consenting patients were significantly younger and had significantly more negative symptoms than the nonconsenters. The difference in negative symptoms remained significant after age and medication dose, in chlorpromazine equivalents, were covaried out. The authors discuss the implications of these findings for forensic research.