The effects of heredity, sensorimotor abilities, health and disease, nutrition, substance abuse, and other biological factors on intellectual functioning may be even more important than specific experiences and education. For example, Lindenberger and Baltes (1994) found that visual and auditory acuity, which decline with age, were among the best predictors of scores on measures of knowledge, reasoning, memory, word fluency, and response speed in older adults. Differences in auditory acuity, in particular, accounted for a large portion of the age-related variability in scores on cognitive tests.
Related to differences in sensory abilities is response speed. The fact that older adults tend to respond more slowly than their younger contemporaries has prompted the hypothesis that age differences in the speed with which cognitive processes take place contribute to differences in intellectual functioning between young and old adults. Evidence supporting this hypothesis comes from various sources (e.g., Lindenberger, Mayr, & Kliegl, 1993; Miller, 1994; Nettelbeck & Rabbitt, 1992). In a summary of research on reaction time and information processing, Miller (1994) concluded that more intelligent people have faster and less variable reaction times, that reaction time becomes longer with age, and that it increases more rapidly with increased task complexity in older than in younger adults. Furthermore, Miller concludes that people with large brains (relative to their body sizes) are more intelligent, that more intelligent brains use less energy, and that age differences in intelligence reflect differences in myelination of neural axons.
Many supporters of a neurological explanation of age-related declines in intelligence view it as the result of small changes in the brain produced by high blood pressure, alcoholism, and other pathological conditions (Rinn, 1988). It is certainly true that intellectual functioning is affected by health status and that people with higher intellectual abilities are healthier and live longer than those with lower abilities. Self-reports of physical and mental health confirm the results of medical diagnoses in this regard (Perlmutter &
Nyquist, 1990). It is arguable whether good health produces better cognitive abilities, or vice versa, but it is clear that poor health can lead to a loss of energy, depression, and less motivation to express oneself intellectually and socially (Perlmutter et al., 1987).
As discussed in Chapter 3, organic brain disorders can have a pronounced effect on behavior and abilities. This is particularly evident in Alzheimer's disease, a disorder that afflicts approximately 20% of individuals in the 75- to 84-year age range and about 47% of those over 85 (Evans et al., 1989). An even greater percentage of older Americans suffer from hypertension, another disorder that is associated with reduced intellectual functioning (Hertzog, Schaie, & Gribbin, 1978; Sands &Meredith, 1992) and which can lead to cardiovascular disease and stroke. By interfering with the oxygen flow to the brain, a major stroke can affect not only intellectual abilities but also speaking, walking, and other skills. The brain's blood supply can also be temporarily reduced by emphysema, acute infection, poor nutrition, injuries, and surgery. The loss of neuronal tissue, changes in metabolic rate, and a decline in blood circulation also have depressing effects on cognitive functioning. Although aging is accompanied by the death of many brain neurons, the remaining neurons continue to grow even in people's seventies and eighties (Coleman, 1986).
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