In an important paper in this issue Murphy et al. (pp. 503–512) report on incidence rates for depression over 40 years in the Stirling County Study. Incidence rates remained stable, contrary to most evidence that is principally based on prevalence or retrospective lifetime prevalence. Incidence rates were only a little higher in women than in men.

Incidence rates are assuming increasing importance in psychiatry as they do in other areas of epidemiology. Prevalence rates are complex, depending not only on incidence of new cases of the disorder, but its persistence, and in a recurrent disorder, recurrence. Incidence rates are unbiased by these factors, a particularly valuable attribute when considering risk factors. In depression, development and widespread use of antidepressants, newer patterns of care, and in the opposite direction, increased urbanization with its accompanying social pressures, could have produced considerable changes in outcome in the last 40 years. Changes in enumerated prevalences alone could reflect changed length of episodes and rates of recurrence, so giving a misleading picture as to what has happened to the disorder.

The Stirling County Study is one of the classics of epidemiology. Its originator, Alexander Leighton, is an author of the present paper with his wife, Jane Murphy, who has directed the study since the mid-1970s. Representative community samples were studied cross-sectionally in 1952, 1969 and 1992, and the previously studied samples restudied on follow-up at the later points. The design enables a separation of period effects, involving all subjects at one time point, from cohort effects. In this study similar temporal stability of prevalence has been found (Murphy et al. 2000b) to that now reported for incidence.