Chronological Age Differences

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A person's score on an intelligence test is not a fixed number that remains invariant from year to year and test to test. The fact that test scores are not perfectly reliable means that a person's score will change somewhat with time, test, and conditions of administration. For example, IQ scores on the Stanford-Binet Intelligence Scale vary on the average about 5 points from testing to testing, though variations as large as 20 points can occur with dramatic changes in physical health, emotional adjustment, or living circumstances.

Of particular interest to developmental psychologists have been age-related changes in intelligence test scores. The results of earlier cross-sectional studies (e.g., Jones & Conrad, 1933; Yerkes, 1921) suggested that, on the average, test scores decline steadily after late adolescence. For example, Yerkes found that mean scores on the Army Examination Alpha administered to large groups of American soldiers during and shortly after World War I declined from the late teens through the sixth decade of life. The general form of the curve drawn from data obtained from Jones and Conrad's classic study of 1,200 New Englanders between 10 and 60 years of age was an increase in mean scores on the Army Alpha from age 10 to 16 and a gradual decline to the age-14 level by age 55. These findings of a decline in intelligence test scores during middle and later adulthood were supported by Wechsler's (1958) analysis of scores on the Wechsler-Bellevue Form I and, more recently, by standardization data from the WAIS-R. As shown in Figure 4-1, the mean sum of scaled scores on the Verbal, Performance, and Full Scales of the WAIS-R reach a peak at a somewhat later age than that found in earlier studies: the

Verbal Scale M Performance Scale H Fui Scale

Verbal Scale M Performance Scale H Fui Scale

16-17 16-19 20-24 25-34 35-44 45-54 55-64 65-69 70-74

Age Interval (Years)

Figure 4-1 Mean sum of scaled scores on the Verbal, Performance, and Full Scales of the WAIS-R at various age levels. (Based on data in the Manual for the Wechsler Adult Intelligence Scale-Revised. Copyright © 1981 by The Psychological Corporation. Reproduced by permission. All rights reserved.)

16-17 16-19 20-24 25-34 35-44 45-54 55-64 65-69 70-74

Age Interval (Years)

Figure 4-1 Mean sum of scaled scores on the Verbal, Performance, and Full Scales of the WAIS-R at various age levels. (Based on data in the Manual for the Wechsler Adult Intelligence Scale-Revised. Copyright © 1981 by The Psychological Corporation. Reproduced by permission. All rights reserved.)

scores increase from the late teens to the late twenties or early thirties and then gradually decline throughout middle and late adulthood.

Figure 4-1 shows the classic aging pattern of a steeper decline on the Performance Scale than on the Verbal Scale. Figure 4-2, which is also based on the 1981 WAIS-R standardization data, shows the rate of decline of standard scale scores on three Verbal subtests (Information, Vocabulary, Arithmetic) and three Performance subtests (Picture Completion, Picture Arrangement, Digit Symbol). The age-related decline in subtest-scaled scores after ages 25-34 is clearly greater for all three of these Performance subtests than for any of the three Verbal subtests. Similar patterns of decline in mental abilities have been found on the Primary Mental Abilities Test (Schaie, 1983), the Stanford-Binet IV (Thorndike, et al., 1986), and other individual and group intelligence tests.

The difficulties experienced in interpreting the results of cross-sectional investigations such as those described here are discussed in Chapter 1. These types of studies compare the scores of different age cohorts, people of differ-

X Information Vocabulary Arithmetic

A Picture Completion ♦picture Arrangement ■ Digit Symbol

X Information Vocabulary Arithmetic

A Picture Completion ♦picture Arrangement ■ Digit Symbol

16-17 16-19 20-24 25-34 35-44 45-54 55-64 65-69 70-74

16-17 16-19 20-24 25-34 35-44 45-54 55-64 65-69 70-74

Age Interval (Years)

Figure 4-2 Mean scaled scores on six WAIS-R subtests by age. (Based on data in the Manual for the Wechsler Adult Intelligence Scale-Revised. Copyright © 1981 by The Psychological Corporation. Reproduced by permission. All rights reserved.)

ent ages who have been brought up in different social and cultural conditions. An important variable that cannot be controlled when comparing different age cohorts is education, which is significantly related to scores on intelligence tests. Because comparisons of the intelligence test scores of different age cohorts confound the effects of education and age, it is impossible to conclude from the results of a cross-sectional study how much of the observed decline is due to age per se and how much is due to differences in education or other cohort-related variables. Because intelligence test scores are positively correlated with both educational level and socioeconomic status, it is possible that the lower test scores obtained by older adults are caused by the fact that they had less formal education and grew up during less intellectually stimulating times. In addition to educational experiences, people of different ages may vary in motivation, susceptibility to fatigue, response times, test anxiety, social isolation, depression, attentiveness, sensory acuity, nutrition, physical health, and other factors that can effect test scores (Reese & Rodeheaver, 1985).

It is more difficult to conduct longitudinal studies than cross-sectional studies of changes in ability test scores with age. Assuming that the selected sample is representative of the population to which the results will be generalized, the problem of differential mortality or selective attrition, in which individuals of lower intelligence are less likely to be available for repeated testings, still remains. In addition, the practice effect of having taken a particular test on one or more previous occasions can inflate scores in longitudinal studies. Another methodological shortcoming of longitudinal studies is the regression effect, in which individuals who score very high or very low on an initial administration of a test tend to obtain scores closer to the mean on a subsequent administration of the same test. Such methodological and individual factors can interfere with an accurate estimate of the effects of chronological age per se on intelligence test scores. In any case, the results of several longitudinal investigations indicate that intelligence test scores tend to remain fairly stable or to decrease only slightly after early adulthood (Bayley & Oden, 1955; Campbell, 1965; Nisbet, 1957; Owens, 1953,1966). Most of these studies, however, were conducted on college graduates, so the results are not necessarily representative of the general population. Other longitudinal studies conducted with people of average intelligence (Charles & James, 1964; Eisdorfer, 1963; Tuddenham, Blumenkrantz, & Wilkin, 1968) and noninstitu-tionalized mentally retarded adults (Baller, Charles, & Miller, 1967; Bell & Zubek, 1960) have, however, yielded similar findings. The findings have been interpreted as indicating that intelligence continues to increase somewhat during early adulthood and reaches a plateau in the late twenties. After that, a slow decline is observed in individuals of below-average intelligence, particularly if they fail to use their abilities. On the other hand, people of above-average intelligence may show no decline at all or may even improve until their early fifties.

In general, the results of longitudinal studies reveal a slower decline in cognitive abilities in later adulthood than those obtained from cross-sectional studies. Warner Schaie and his coworkers (Schaie, 1979, 1983, 1990, 1994) have found, however, that whether and how much cognitive abilities decline in later life depends on the specific ability and the individual. They maintain that, except perhaps for individuals with cardiovascular disorders or other serious illnesses, declines in cognitive abilities during later life are by no means inevitable.

The conclusion that the magnitude of age-related declines in abilities varies with the specific ability and the individual is supported by the results of both cross-sectional and longitudinal data. In the Seattle Longitudinal Studies, Schaie (1990, 1994) examined the relationships of age to scores on the SRA Mental Abilities Tests of Verbal Meaning, Spatial Orientation, Inductive Reasoning, Number, and Word Fluency. Figure 4-3 shows that the percentage of individuals who declined in these abilities increased with age. During middle adulthood, the rate of decline was greatest for spatial orientation and inductive reasoning and less for word fluency, verbal meaning, and number. However, scores on verbal meaning, which is a somewhat speeded test, showed the greatest drop in old age. Though the number of individuals

Verbal Meaning]

Spatial Orientatior i

InductiveReasoning i

Number

Word Fluency

Verbal Meaning]

Spatial Orientatior i

InductiveReasoning i

Number

Word Fluency

10 20 30

Percentage Declining in Specific Ability

Figure 4-3 Percentage of individuals showing decline in specific intellectual abilities during four age periods. (Based on data from Schaie, 1990.) Cambridge, UK: Cambridge University Press.

10 20 30

Percentage Declining in Specific Ability

Figure 4-3 Percentage of individuals showing decline in specific intellectual abilities during four age periods. (Based on data from Schaie, 1990.) Cambridge, UK: Cambridge University Press.

who showed a decline in one or more of these five abilities increased with age, only a small percentage declined in all five.

A lively debate concerning age decrements in cognitive abilities has been waged by Warner Schaie and John Horn over the years. Horn maintains that intelligence may begin to decline as early as the twenties and thirties and that it becomes particularly noticeable during middle age. Following his mentor, R. B. Cattell, Horn makes a distinction between fluid and crystallized ability. Crystallized ability is Cattell's term for cognitive ability (knowledge, skills) acquired through experience and education; it is specific to certain fields, such as school learning, and applied-in tasks where habits have become fixed. In contrast, fluid ability is inherent, genetically determined mental ability, as manifested in problem-solving or novel responses. The items in Figure 4-4, which are samples from the Culture Fair Intelligence Test, are illustrative of the kinds of tasks involving fluid intelligence. Horn states that the decline in abilities during middle age is seen particularly in scores on measures of fluid intelligence that deal with the flexibility of thinking and problem solving (Horn, 1985; Horn & Donaldson, 1976,1980; Horn &Hofer, 1992). A number of other investigations (e.g., Christensen et al., 1994) have also found greater age-related declines in fluid ability than in crystallized ability.

While recognizing that cognitive abilities decline with age, Schaie (1983) believes that the decrement is much less than Horn asserts and is caused in

MATRICES

CONDITIONS

Cattell Culture Free Test

Figure 4-4 Sample items from the Culture Fair Intelligence Test. (Copyright © 1949, 1960, by the Institute for Personality and Ability Testing, Inc. All rights reserved. Reproduced by permission.)

CONDITIONS

Figure 4-4 Sample items from the Culture Fair Intelligence Test. (Copyright © 1949, 1960, by the Institute for Personality and Ability Testing, Inc. All rights reserved. Reproduced by permission.)

large measure by differences in the experiences of younger and older adults during their formative years. The wide range of individual variations in intelligence, in addition to its multidimensionality and modifiability, led Schaie to conclude that it is a very plastic variable. Environments in which there are varied opportunities for intellectual stimulation and a flexible lifestyle can, according to Schaie, contribute to the maintenance of an optimal level of intellectual functioning in adulthood. Rather than simply accepting the conclusion that a decline in intelligence during later adulthood is inevitable, Schaie feels that psychologists should explore ways of improving memory, abstract reasoning, concentration, and other aspects of intellectual functioning in older adults. In this regard, Schaie and his coworkers (Baltes & Willis, 1982; Schaie & Willis, 1986; Willis, 1990) have had some success in training older adults to improve their performance on intelligence tests. In many instances, however, it is not clear whether the training procedures truly arrest and reverse declines in cognitive abilities or whether the subjects simply learn new test-taking skills. Furthermore, both older and younger adults show improvements in test scores as a result of the training.

Improvements in cognitive skills can be obtained not only by instruction in specific skills but also by anxiety-reduction training (Yeasavage & Rose, 1984). In addition, older adults can continue to perform effectively by limiting their involvement in areas where deficits are more obvious and by emphasizing activities in which ability losses are less noticeable. They can also compensate for losses in abilities by employing reminders, being more careful, and finding other ways of accomplishing certain tasks. Even when there are significant losses in cognitive abilities, most people have enough reserve capacity to continue functioning at an acceptable level. Adaptation to the environment and daily life seldom requires all of one's capacities, and older adults usually have untapped abilities that can be called upon when needed. Furthermore, older adults often possess highly specialized knowledge and abilities that are not tapped by conventional tests of intelligence. These "practical abilities" enable many older adults to be more competent than younger adults in dealing with everyday problems that are not always covered in school.

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