Effect of case definition on affective disorder rates.

In a preliminary analysis by the NIMH-Clinical Research Branch Collaborative Program on the Psychobiology of Depression the lifetime rate of affective illness among 1,090 interviewed relatives of depressed and manic probands was considerably lower in Iowa than in the other four centers. Among various affective disorder diagnoses, only primary unipolar depression was significantly less frequent in Iowa. This rate difference decreased with increasingly restrictive case definitions. Possible determinants of the low depression rate in Iowa will be investigated; the present data illustrate the importance of case definition in the interpretation of future findings.