The P scale and psychosis.

Neuropsychology Unit, Department of NeurologyChurchill Hospital, OxfordH. Eysenck and S. Eysenck's psychoticism (P) scale was appraised. Their claimsof continuity between psychosis and normality are based on a shifting conceptof continuity. At times, Eysenck and Eysenck appear to be arguing for a con-tinuum between psychosis and normality at the level of clinical symptoms. Onother occasions they seem to say that the P scale measures the underlying con-dition responsible for the psychotic symptoms. The evidence is not strong ineither case. If it were, then the P scale would be a useful diagnostic instrument.However, in their most recent work, Eysenck and Eysenck clearly state thatthe P scale measures the predisposition to psychosis, and that only a verysmall proportion of people with high P scores are likely to develop a psychosis(p. S). In this case, the P scale cannot be used alone as a diagnostic tool, sinceit does not discriminate the mentally ill from the healthy. The value of the Pscale therefore rests on its validity as a measure of predisposition to psychosis.It is shown that validation data presented by Eysenck and Eysenck are at bestunconvincing and, at worst, contrary to their hypothesis.