IV. The Influence of Heredity and Environment
Today, researchers generally agree that heredity and environment have an interactive influence on intelligence. Many researchers believe that there is a reaction range to IQ, which refers to the limits placed on IQ by heredity. Heredity places an upper and lower limit on the IQ that can be attained by a given person. The environment determines where within these limits the person’s IQ will lie. Despite the prevailing view that both heredity and environment influence intelligence, researchers still have different opinions about how much each contributes and how they interact. Hereditary Influences Evidence for hereditary influences on intelligence comes from the following observations:
Heritability of Intelligence Heritability is a mathematical estimate that indicates how much of a trait’s variation in a population can be attributed to genes. Estimates of the heritability of intelligence vary, depending on the methods used. Most researchers believe that heritability of intelligence is between 60 percent and 80 percent. Heritability estimates apply only to groups on which the estimates are based. So far, heritability estimates have been based mostly on studies using white, middle-class subjects. Even if heritability of IQ is high, heredity does not necessarily account for differences between groups. Three important factors limit heritability estimates:
Biological Influences Brain Anatomy Several studies report a positive correlation (+.33) between brain size (adjusted for body size) and intelligence score. Moreover, as adults age, brain size and nonverbal intelligence test scores fall in concert. Some studies suggest that highly educated people die with more synapses. The direction of the relationship between brain size and intelligence remains unclear. Larger brain size may enable greater intelligence, but it is also possible that greater intelligence leads to experiences that exercise the brain and build more connections, thus increase its size. Or, some third factor may be at work. Some evidence suggests that highly intelligent people differ in their neural plasticity. Neural Processing Speed People who score high on intelligence tests tend to retrieve information from memory more quickly. Research also suggests that the correlation between intelligence score and the speed of taking in perceptual information tends to be about +.3 to +.5. Those who perceive quickly are especially likely to score higher on tests based on perceptual rather than verbal problem solving. The brain waves of highly intelligent people register a simple stimulus, such as a flash of light, more quickly and with greater complexity. The evoked brain response also tends to be slightly faster when people with high intelligence rather than low intelligence scores perform a simple task, such as pushing a button when an X appears on the screen. As yet, psychologists have no firm idea of why fast reactions on simple tasks should predict intelligence test performance. Gender Differences in Intelligence Although gender similarities far outnumber gender differences, we find the differences in abilities more interesting. Research indicates that, compared with males, females are better spellers; are more verbally fluent; are better at remembering and locating objects; are more sensitive to touch, taste, and color; and are better emotion detectors. Males’ mental ability scores vary more than females’, and thus boys outnumber girls at both the low extreme and the high extreme. Boys outperform girls in spatial ability tests and at math problem solving, but they underperform them in math computation. According to different perspectives, these differences may be explained as evolutionarily adaptive for each gender or as the result of social expectations and divergent opportunities. Ethnic Similarities and Differences in Intelligence Test Scores American Blacks average about 10 points lower than White Americans on intelligence tests. European New Zealanders outscore native Maori New Zealanders, Israeli Jews outscore Israeli Arabs, and most Japanese outscore the stigmatized Japanese minority. Research suggests that environmental differences are largely responsible for these group differences. Consider: (1) genetics research indicates that the races are remarkably alike under the skin; (2) race is not a neatly defined biological category; (3) Asian students outperform North American students on math achievement and aptitude tests; (4) intelligence test performance of today’s better-fed, better educated, and more test-prepared population exceeds that of the 1930s population by the same margin that the score of the average White today exceeds that of the average Black; (5) White and Black infants tend to score equally well on tests measuring preferences for looking at novel stimuli—a predictor of future intelligence; and (6) in different eras, different ethnic groups have experienced periods of remarkable achievement. Environmental Influences Evidence for environmental influences on intelligence comes from the following observations:
Possible Causes of the Flynn Effect The precise cause for the Flynn effect is unclear. Researchers speculate that it may be due to environmental factors such as decreased prevalence of severe malnutrition among children, enhancing of skills through television and video games, improved schools, smaller family sizes, higher level of parental education, or improvements in parenting. Cultural and Ethnic Differences Studies have shown a discrepancy in average IQ scores between whites and minority groups in the United States. Black, Native American, and Hispanic people score lower, on average, than white people on standardized IQ tests. Controversy exists about whether this difference is due to heredity or environment. Hereditary Explanations A few well-known proponents support hereditary explanations for cultural and ethnic differences in IQ:
Environmental Explanations Many researchers believe that environmental factors primarily cause cultural and ethnic differences. They argue that because of a history of discrimination, minority groups comprise a disproportionately large part of the lower social classes, and therefore cultural and ethnic differences in intelligence are really differences among social classes. People in lower social classes have a relatively deprived environment. Children may have:
However, not all minority groups score lower than whites on IQ tests. Asian Americans achieve a slightly higher IQ score, on average, than whites, and they also show better school performance. Researchers suggest that this difference is due to Asian American cultural values that encourage educational achievement. Are Intelligence Tests Biased? Intelligence tests are “biased” in the sense that they are sensitive to performance differences caused by cultural experience. However, tests are not biased in that they predict as accurately for one group as they do for another. For example, the predictive validity is roughly the same for men and women, for Blacks and Whites, and for rich and poor. Stereotype threat is a self-confirming concern that one will be evaluated based on a negative stereotype. The phenomenon sometimes appears in intelligence testing among African-Americans and among women of all colors. The Dynamic of Intelligence The Stability of Intelligence Over the Lifespan The stability of intelligence test scores increases with age. By age 4, children’s performance on intelligence tests begins to predict their adolescent and adult scores. After about age 7, intelligence scores, though certainly not fixed, stabilize. Extremes of Intelligence At one extreme of the normal distribution are people whose intelligence scores fall below 70. To be labeled as having an intellectual disability (formerly referred to as mental retardation), a child must have both a low test score and difficulty adapting to the normal demands of living independently. Intellectual disability sometimes results from known physical causes, such as Down syndrome, a disorder of varying severity that is attributed to an extra chromosome in the person’s genetic makeup. Most mentally challenged adults can, with support, live in mainstream society. At the other extreme are the “gifted.” Contrary to the popular myth that they are frequently maladjusted, research suggests that high-scoring children are healthy, well adjusted, and academically successful. Controversy surrounds “gifted child” programs in which the “gifted” are segregated and given academic enrichment not available to the masses. Critics note that tracking by aptitude sometimes creates a self-fulfilling prophecy: Those implicitly labeled “ungifted” can be influenced to become so. Denying lower-ability students opportunities for enriched education can widen the achievement gap between ability groups and increase their social isolation from one another |
Vocabulary to Know
Reaction Range Heritability Flynn Effect Downs Syndrome Mental Retardation Stereotype Threat Quizlet |
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