In the search for environmental causes of mental disorder, the epidemiologist looks for an association between the disorder and some environmental factor. The significance of such an association can only be determined by a comparison of the patient population with a control population. Ideally, die control population will be the general population from which the patients have been drawn, but because this often presents great technical difficulties an alternative is to compare one diagnostic group with another. Sound conclusions cannot, however, be drawn from such a comparison unless it is known how far the diagnostic groups differ for factors which influence, or are thought likely to influence, the association being studied. Such factors, of which age- and sex-distribution are the most obvious, will often include social and cultural status. Cultural status—at least in so far as this may be equated with religion and country of birth—can be allowed for by confining the comparison to patients of a particular religion and place of birth. As regards social status, it is the patient's background rather than his present condition which seems important; for on the one hand present social status will be varyingly influenced by the type and severity of disorder, and on the other hand a person's general social behaviour and attitude (in as far, at least, as these are associated with the sort of social factors which psychiatrists have commonly studied) are more likely to stem from the type of upbringing he had than from his recent experiences.