Contrast the results of cross-sectional and longitudinal research on intelligence.
Describe the typical pattern of performance on tests of fluid and crystallized intelligence across the years of adulthood.
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1. As adults get older, does their intelligence increase or decrease? The earliest studies (conducted between 1920 and 1960) found that intelligence peaks in young adulthood, and then drops substantially during middle and later adulthood. This graph shows a typical pattern of results on one type of intelligence test.
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| Time of birth | Time of Measurement |
| Cohort | 1950 |
| 1880 | Age 70 |
| 1890 | Age 60 |
| 1900 | Age 50 |
| 1910 | Age 40 |
| 1920 | Age 30 |
| 1930 | Age 20 |
2. However, these early studies used the cross-sectional research design, testing different age groups of people (called cohorts) in the same year. For example, the 70-year-old individuals tested in 1950 had been born in 1880, before compulsory public education existed in the United States. We now realize that the cross-sectional approach fails to consider generational differences in education and other life experiences, so it exaggerates the negative impact of aging.
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3. Later research using longitudinal studies, which retested the same group of people (same cohort) over many years, produced different results. Intelligence held steady across the adult years, or even increased slightly!
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4. However, we now realize that the results of some longitudinal studies are overly optimistic. Within the distribution of scores at the first time of testing, the individuals with the lowest scores tend to drop out at later testing times. This could artificially inflate the average score at later testing times.
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5. In addition, researchers now believe that even if overall intelligence doesn’t change much across adulthood, it has two subcomponents that do change as we age—in different ways. Fluid intelligence (quick, flexible, abstract thinking) seems to decline in later life, while crystallized intelligence (accumulated knowledge and skills) tends to increase in healthy middle-aged and older adults.
Practice 1: Describing Cross-Sectional and Longitudinal Studies
Select each button to show a representative example of that type of study.
As people move from young adulthood, to middle age, and then to late adulthood, does their intelligence change? Researchers now realize that the answer to this question depends on how you study the issue.
One method of studying developmental change uses cross-sectional studies, which test people of different ages at the same point in time. The other method uses longitudinal studies, which test the same group of people at different points in time.
| Year of Measurement | |||
|---|---|---|---|
| Year of Birth | 1990 | 2000 | 2010 |
| 1950 | Age 40 | Age 50 | Age 60 |
| 1960 | Age 30 | Age 40 | Age 50 |
| 1970 | Age 20 | Age 30 | Age 40 |
Practice 2: Comparing Cross-Sectional and Longitudinal Studies of Intelligence
Select each button to compare the results from the two types of studies.
When used to measure changes across the decades of adulthood, cross-sectional and longitudinal studies tend to produce different results.
Practice 3: Comparing Fluid and Crystallized Intelligence
Select each of the buttons to compare the results of tasks measuring crystallized and fluid intelligence.
The specific tasks used to measure fluid intelligence and crystallized intelligence also show different patterns of performance across the adult years.
Quiz 1
Drag each term for the method of studying developmental changes to the gray area in front of the appropriate research scenario. When all the terms have been placed, select the CHECK ANSWER button.
Quiz 2
Drag each term for each type of intelligence to the gray area in front of the appropriate description. When all the terms have been placed, select the CHECK ANSWER button.