Modern explicit diagnostic criteria have greatly improved the reliability of diagnosis in genetic and other types of psychiatric research. However, there are several sets of different diagnostic criteria that can be used, particularly in the case of studies focusing on the major psychoses. These different criteria do not always agree well with each other and there is a further difficulty that reliability does not necessarily ensure validity. A polydiagnostic approach is advocated whereby research data are collected in such a way as to allow the application of competing definitions of disorder. This is accomplished in the OPCRIT system by "decomposing" diagnostic criteria into their component items. These can then be reassembled using algorithms based on the original criteria or used to generate novel categories or dimensions. This is necessary if psychiatric diagnoses are ultimately going to be refined and validated against biological criteria.