The inverse relationship between social class and rates of mental disorders was first documented in early mental hygiene studies at the beginning of this century and similar findings have been demonstrated in numerous contemporary studies. Interpretations of this persistent relationship include a downward drift explanation that posits that a decline in social status occurs as a result of precipitating mental illness, and a social causation model that emphasizes the importance of psychosocial stressors in the onset of mental disorders. A wealth of evidence supporting the social causation model is reviewed, drawing on mental health epidemiology, women's mental health, unemployment and physical health research. Political and economic origins of the downward drift hypothesis are also discussed.