After reviewing the literature pertaining to intellectual deterioration, Yates (1956) concluded that the major determinant of declining vocabulary in hospitalized persons, especially schizophrenic patients, was the length of stay in hospital. The studies on which Yates based his hypothesis were cross-sectional; that is, groups of patients admitted for relatively short periods of time were compared with patients admitted for longer periods. Two studies of schizophrenics showed that long-term patients did not perform as well as short-term patients on vocabulary subscales (Nelson, 1953; Rabin, King and Ehrmann, 1955). Similar decline was noted both in long-term epileptic patients (Capps, 1939) and in non-psychotic residents of old-age homes (Fox and Birren, 1949).