Aging and Intelligence

27-7 How does aging affect crystallized and fluid intelligence?

Does intelligence increase, decrease, or remain constant as we age? The answer depends on the type of intellectual performance we measure:

How do we know? Developmental psychologists use longitudinal studies (restudying the same group at different times across their life span) and cross-sectional studies (comparing members of different age groups at the same time) to study the way intelligence and other traits change with age. (See Appendix A for more information.) With age we lose and we win. We lose recall memory and processing speed, but we gain vocabulary and knowledge (FIGURE 27.4 below). Fluid intelligence may decline, but older adults’ social reasoning skills increase, as shown by an ability to take multiple perspectives, to appreciate knowledge limits, and to offer helpful wisdom in times of social conflict (Grossman et al., 2010). Decisions also become less distorted by negative emotions such as anxiety, depression, and anger (Blanchard-Fields, 2007; Carstensen & Mikels, 2005).

Age-related cognitive differences help explain why older adults are less likely to embrace new technologies (Charness & Boot, 2009; Lenhart, 2015). These cognitive differences also help explain why mathematicians and scientists produce much of their most creative work during their late twenties or early thirties, when fluid intelligence is at its peak (Jones et al., 2014). In contrast, authors, historians, and philosophers tend to produce their best work in their forties, fifties, and beyond—after accumulating more knowledge (Simonton, 1988, 1990). Poets, for example, who depend on fluid intelligence, reach their peak output earlier than prose authors, who need the deeper knowledge reservoir that accumulates with age. This finding holds in every major literary tradition, for both living and dead languages.

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27-8 How stable are intelligence test scores over the life span?

What about the stability of early-life intelligence scores? For most children, casual observation and intelligence tests before age 3 only modestly predict their future aptitudes (Humphreys & Davey, 1988; Tasbihsazan et al., 2003). Even Albert Einstein was once thought “slow”—as he was in learning to talk (Quasha, 1980).

By age 4, however, children’s performance on intelligence tests begins to predict their adolescent and adult scores. The consistency of scores over time increases with the age of the child. By age 11, the stability becomes impressive, as Ian Deary and his colleagues (2004, 2009, 2013) discovered. Their amazing longitudinal studies have been enabled by their country, Scotland, which did something that no nation has done before or since. On June 1, 1932, essentially every child in the country born in 1921—87,498 children around age 11—took an intelligence test. The aim was to identify working-class children who would benefit from further education. Sixty-five years later to the day, Patricia Whalley, the wife of Deary’s co-worker, Lawrence Whalley, discovered the test results on dusty storeroom shelves at the Scottish Council for Research in Education, not far from Deary’s Edinburgh University office. “This will change our lives,” Deary replied when Whalley told him the news.

And so it has, with dozens of studies of the stability and the predictive capacity of these early test results. For example, when the intelligence test administered to 11-year-old Scots in 1932 was readministered to 542 survivors as turn-of-the-millennium 80-year-olds, the correlation between the two sets of scores—after nearly 70 years of varied life experiences—was striking (FIGURE 27.5). Ditto when 106 survivors were retested at age 90 (Deary et al., 2013). Another study that followed Scots born in 1936 from ages 11 to 70 confirmed the remarkable stability of intelligence, independent of life circumstance (Johnson et al., 2010).

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Children and adults who are more intelligent also tend to live healthier and longer lives. Why might this be the case? Deary (2008) proposes four possible explanations:

27-9 What are the traits of those at the low and high intelligence extremes?

One way to glimpse the validity and significance of any test is to compare people who score at the two extremes of the normal curve. The two groups should differ noticeably, and with intelligence testing, they do.

THE LOW EXTREME At one extreme of the intelligence test normal curve are those with unusually low scores. The American Association on Intellectual and Developmental Disabilities guidelines list two criteria for a diagnosis of intellectual disability (formerly referred to as mental retardation):

Intellectual disability is a developmental condition that is apparent before age 18, sometimes with a known physical cause. Down syndrome, for example, is a disorder of varying intellectual and physical severity caused by an extra copy of chromosome 21.

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People diagnosed with a mild intellectual disability—those just below the 70 score—might be better able to live independently today than many decades ago, when they were institutionalized. The tests have been periodically restandardized. As that happened, individuals who scored near 70 on earlier tests have suddenly lost about 6 test-score points. Two people with the same ability level could thus be classified differently, depending on when they were tested (Kanaya et al., 2003; Reynolds et al., 2010). As the intellectual-disability boundary has shifted, more people have become eligible for special education and for Social Security payments. And in the United States (one of only a few industrialized countries with the death penalty), fewer people are now eligible for execution: The U.S. Supreme Court ruled in 2002 that the execution of people with an intellectual disability is “cruel and unusual punishment.” For people near that cutoff score of 70, intelligence testing can be a high-stakes competition. And so it was for Teresa Lewis, a “dependent personality” with limited intellect, who was executed by the state of Virginia in 2010. Lewis, whose reported test score was 72, allegedly agreed to a plot in which two men killed her husband and stepson in exchange for a split of a life insurance payout (Eckholm, 2010). If only she had scored 69.

THE HIGH EXTREME In one famous project begun in 1921, Lewis Terman studied more than 1500 California schoolchildren with IQ scores over 135. Terman’s high-scoring children (the “Termites”) were healthy, well adjusted, and unusually successful academically (Friedman & Martin, 2012; Koenen et al., 2009; Lubinski, 2009a). When restudied over the next seven decades, most had attained high levels of education (Austin et al., 2002; Holahan & Sears, 1995). Many were doctors, lawyers, professors, scientists, and writers, though no Nobel Prize winners. (The two future physics Nobel laureates Terman tested failed to score above his gifted-sample cutoff [Hulbert, 2005].)

Recent studies have followed the lives of precocious youths who had aced the math SAT at age 13—by scoring in the top quarter of 1 percent of their age group. By their fifties, these 1650 math whizzes had secured 681 patents (Lubinski et al., 2014). Compared with the math aces, 13-year-olds scoring high on verbal aptitude were, by age 38, more likely to have become humanities professors or written a novel (Kell et al., 2013). About 1 percent of Americans earn doctorates. But among those scoring in the top 1 in 10,000 on the SAT at age 12 or 13, 63 percent had done so.

One of psychology’s whiz kids was Jean Piaget, who by age 15 was publishing scientific articles on mollusks and who went on to become the twentieth century’s most famous developmental psychologist (Hunt, 1993). Children with extraordinary academic gifts are sometimes more isolated, shy, and in their own worlds (Winner, 2000). But most thrive.