Cause and course of psychopathology: some lessons from longitudinal data.

Case-control studies of clinic samples constitute the usual research method for the investigation of the causes and course of psychiatric disorder. However, they carry substantial disadvantages and if causal hypotheses are to be tested in rigorous fashion, it is necessary to use longitudinal research strategies applied to epidemiologically based samples. Their advantages are reviewed with respect to the use of 'experiments of nature' for testing causal mechanisms through the study of within-individual change over time in relation to some prospectively measured alteration in the risk variable. Attention is drawn to the value of longitudinal data in studying the processes involved in 'escape' from risk; in examining differential vulnerability to risk experiences; in validating diagnostic categories; in investigating the timing of disorders; and in evaluating the role of variables that cannot be recalled. The data from a range of longitudinal studies are used to note some of the key implications for developmental and psychopathological concepts.