Foulds' hierarchy: validation of predictions in psychiatric and dermatological patients

Foulds' inclusive non-reflexive law of symptom formation has been hitherto confirmed only on psychiatric or non-clinical persons. Given that respectively a yes-bias and a high frequency of non-classified patients may have inflated the rate of confirming protocols in these groups, a validation study was conducted with the Delusions-Symptoms-States Inventory (DSSI) on 188 psychiatric and 295 dermatological patients. Although non-classified patients were not included in the study (thus lowering the number of conforming patterns), both samples showed percentages of patterns conforming to Foulds' law which were above 85 per cent. No significant intergroup difference was found. The rate of conforming patterns in both groups was lower for members of the two psychotic classes (61-77 per cent). Foulds further hypothesized that the number of pathological sets within a given class grows if there is allocation into a superordinate class. This assumption was confirmed in both groups and for most of the inter-class comparisons. Suggestions were advanced to clarify the issue of the time span to be covered by Foulds' pyramid, to refine the instructions and the response format of the DSSI, and to include into the pyramid further relevant disturbances.