range in individual ability is reflected in a variability in performance which is marked in the later years.
The degree of retention of intellectual powers apparently depends largely on factors other than physical ones--such as the degree of exercise or continuous use of mental faculties, previous experience, and emotional pressures. Motivation and the desire to learn play a very important part in the acquisition of new learning. One company making fine precision engineering instruments reports50 that "older men can learn rapidly when the incentive exists and instruction is good. We have found the older men can learn virtually any job in the plant in about two weeks' time." Twenty per cent of this company's employees are over age 60, and more than 10 per cent over age 70. The experience of adult education instructors shows decisively that older people given sufficient motivation can and do learn in a wide variety of fields.
One of the real difficulties in measuring intellectual performance in old age is the inadequacy of the tests used. Many of these tests penalize the subject in the areas of visual and auditory acuity, speed of reaction, educational background, motivation, and recency of practice. They are not devised to test special knowledge and experience nor high adult intellectual ability. Neither have they been administered to enough persons in the older ages to constitute an adequate sampling of that group. Not until we have longitudinal studies on a representative sample of the population based on testing at repeated intervals from early life to old age will we be able to reach sound conclusions about the occurrence of intellectual deterioration in old age.51
In conclusion, it should be emphasized that no account of the changes of senescence is valid which does not stress the wide range of individual variations to be found. Psychologically as well as____________________