Intelligence

B

Environmental Influences

If genetic influences account for between 40 and 80 percent of the variation in intelligence, then environmental influences account for between 20 and 60 percent of the total variation. Environmental factors comprise all the stimuli a person encounters from conception to death, including food, cultural information, education, and social experiences. Although it is known that environmental factors can be potent forces in shaping intelligence, it is not understood exactly how they contribute to intelligence. In fact, scientists have identified few specific environmental variables that have direct, unambiguous effects on intelligence. Many environmental variables have small effects and differ in their effect on each person, making them difficult to identify.

Schooling is an important factor that affects intelligence. Children who do not attend school or who attend intermittently score more poorly on IQ tests than those who attend regularly, and children who move from low-quality schools to high-quality schools tend to show improvements in IQ. Besides transmitting information to students directly, schools teach problem solving, abstract thinking, and how to sustain attention—all skills required on IQ tests.

Many researchers have investigated whether early intervention programs can prevent the lowered intelligence that may result from poverty or other disadvantaged environments. In the United States, Head Start is a federally funded preschool program for children from families whose income is below the poverty level. Head Start and similar programs in other countries attempt to provide children with activities that might enhance cognitive development, including reading books, learning the alphabet and the numbers, learning the names of colors, drawing, and other activities. These programs often have large initial effects on IQ scores. Children who participate gain as much as 15 IQ points compared to control groups of similar children not in the program. Unfortunately, these gains seem to last only as long as the intervention lasts. When children from these programs enter school, their IQ declines to the level of control groups over a period of several years. This has come to be known as the “fade-out” effect.

Even though early intervention preschool programs do not seem to produce lasting IQ gains, some studies suggest they may have other positive long-term effects. For example, the Consortium for Longitudinal Studies reported that participants are less likely to repeat grades, less likely be placed in remedial classes, and more likely to finish high school than comparable nonparticipants—even though both groups show about the same levels of academic achievement. Preschoolers in early intervention programs may also benefit from improved health and nutrition, and their mothers may sometimes benefit from additional education that the programs provide. Because a substantial portion of the variation in intelligence is due to environmental factors, early intervention programs should be able to produce significant and lasting IQ gains once the specific environmental variables that influence IQ have been identified. Researchers continue to search for the interventions that will increase IQ and, ultimately, academic achievement.

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Two environmental variables known to affect intelligence are family size and birth order. Children from smaller families and children who are earlier-born in their families tend to have higher intelligence test scores. These effects, however, are very small and amount to only a few IQ points. They are detectable only when researchers study very large numbers of families.

Although there has been substantial debate about the effects of other environmental variables, certain substances in the prenatal environment may influence later intelligence. For example, some pregnant women who consume large amounts of alcohol give birth to children with fetal alcohol syndrome, a condition marked by physical abnormalities, mental retardation, and behavioral problems. Even exposure to moderate amounts of alcohol may have some negative influence on the development of intelligence, and to date no safe amount of alcohol has been established for pregnant women. Scientists have also discovered that certain substances encountered during infancy or childhood may have negative affects on intelligence. For example, children with high blood levels of lead, as a result of breathing lead-contaminated air or eating scraps of lead-based paint, tend to have lower IQ scores. Prolonged malnutrition during childhood also seems to influence IQ negatively. In each of these cases, a correlation exists between environmental factors and measured intelligence, but one cannot conclude that these factors directly influence intelligence. Other environmental variables in this category include parenting styles and the physical environment of the home.

Although the nature-nurture debate has raged for some time, research points to a conclusion that appeals to common sense: Intelligence is about half due to nature (heredity) and about half due to nurture (environment). The exact mechanisms by which genetic and environmental factors operate remain unknown. Identifying the specific biological and environmental variables that affect intelligence is one of the most important challenges facing researchers in this field.

C

Sex Differences

Are women smarter or are men smarter? Psychologists have studied sex differences in intelligence since the beginning of intelligence testing. The question is a very complicated one, though. One problem is that test makers sometimes eliminate questions that show differences between males and females to eliminate bias from the test. Intelligence tests, therefore, may not show gender differences even if they exist. Even when gender differences have been explicitly studied, they are hard to detect because they tend to be small.

There appear to be no substantial differences between men and women in average IQ. But the distribution of IQ scores is slightly different for men than for women. Men tend to be more heavily represented at the extremes of the IQ distribution. Men are affected by mental retardation more frequently than are women, and they also outnumber women at very high levels of measured intelligence. Women’s scores are more closely clustered around the mean.

Although there are no differences in overall IQ test performance between men and women, there do seem to be differences in some more specialized abilities. Men, on average, perform better on tests of spatial ability than do women. Spatial ability is the ability to visualize spatial relationships and to mentally manipulate objects. The reason for this difference is unknown. Some psychologists speculate that spatial ability evolved more in men because men were historically hunters and required spatial ability to track prey and find their way back from hunting forays. Others believe that the differences result from parents’ different expectations of boys’ and girls’ abilities.

Many studies have examined whether gender differences exist in mathematical ability, but the results have been inconsistent. In 1990 American researchers statistically combined the results of more than 100 studies on gender differences in mathematics using a technique known as meta-analysis. They found no significant differences in the average scores of males and females on math tests. Research also indicates that the average girl’s grades in mathematics courses equal or exceed those of the average boy. Other studies have found that boys and girls perform equally well on math achievement tests during elementary school, but that girls begin to fall behind boys in later years. For example, male high school seniors average about 45 points higher on the math portion of the SAT than do females.

A 1995 study examined the performance of more than 100,000 American adolescents on various mental tests. The study found that on average, females performed slightly better than males on tests of reading comprehension, writing, perceptual speed, and certain memory tasks. Males tended to perform slightly better than girls on tests of mathematics, science, and social studies. In almost all cases, the average sex differences were small.

Are differences in abilities between men and women biologically based or are they due to cultural influences? There is some evidence on both sides. On the biological side, researchers have studied androgenized females, individuals who are genetically female but were exposed to high levels of testosterone, a male hormone, during their gestation. As these individuals grow up, they are culturally identified as female, but they tend to play with “boys’ toys,” like blocks and trucks, and have higher levels of spatial ability than females who were not exposed to high levels of testosterone. Further evidence for a biological basis for spatial gender differences comes from comparisons of the brains of men and women. Even when corrected for body size, males tend to have slightly larger brains than females. Some scientists speculate that this extra brain volume in males may be devoted to spatial ability.

On the cultural side, many social scientists argue that differences in abilities between men and woman arise from society’s different expectations of them and from their different experiences. Girls do not participate as extensively as boys do in cultural activities thought to increase spatial and mathematical ability. As children, girls are expected to play with dolls and other toys that develop verbal and social skills while boys play with blocks, video games, and other toys that encourage spatial visualization. Later, during adolescence, girls take fewer math and science courses than boys, perhaps because of stereotypes of math and science as masculine subjects and because of less encouragement from teachers, peers, and parents. Many social scientists believe cultural influences account for the relatively low representation of women in the fields of mathematics, engineering, and the physical sciences.

It is important to remember that sex differences, where they exist, represent average differences between men and women as groups, not individuals. Knowing whether an individual is female or male reveals little about that person’s intellectual abilities.

D

Racial and Ethnic Differences

Numerous studies have found differences in measured IQ between different self-identified racial and ethnic groups. For example, many studies have shown that there is about a 15-point IQ difference between African Americans and whites, in favor of whites. The mean scores of IQ scores of the various Hispanic American subgroups fall roughly midway between those for blacks and whites. Although these differences are substantial, there are much larger differences between people within each group than between the means of the groups. This large variability within groups means that a person’s racial or ethnic identification cannot be used to infer his or her intelligence.

The debate about racial and ethnic differences in IQ scores is not about if the differences exist but what causes them. In 1969 Arthur Jensen, a psychology professor at the University of California at Berkeley, ignited the modern debate over racial differences. Jensen published a controversial article in which he argued that black-white differences in IQ scores might be due to genetic factors. Further, he argued that if IQ had a substantial genetic component, remedial education programs to improve IQ should not be expected to raise IQ as they were currently being applied. In 1994 American psychologist Richard Herrnstein and American social analyst Charles Murray renewed the debate with the publication of The Bell Curve (1994). Although only a small portion of the book was devoted to race differences, that portion of the book received the most attention in the popular press. Among other arguments, Herrnstein and Murray suggested it was possible that at least some of the racial differences in average IQ were due to genetic factors. Their arguments provoked heated debates in academic communities and among the general public.

As discussed earlier, research supports the idea that differences in measured intelligence between individuals are partly due to genetic factors. However, psychologists agree that this conclusion does not imply that genetic factors contribute to differences between groups. No one knows exactly what causes racial and ethnic differences in IQ scores. Some scientists maintain that these differences are in part genetically based. Supporters of this view believe that racial and ethnic groups score differently on intelligence tests partly because of genetic differences between the groups. Others think the cause is entirely environmental. In this view, certain racial and ethnic groups do poorer on IQ tests because of cultural and social factors that put them at a disadvantage, such as poverty, less access to good education, and prejudicial attitudes that interfere with learning. Representing another perspective, many anthropologists reject the concept of biological race, arguing that races are socially constructed categories with little scientific basis (see Race). Because of disagreements about the origins of group differences in average IQ, conclusions about these differences must be evaluated cautiously.

Some research indicates that the black-white differential in IQ scores might be narrowing. Several studies have found that the difference in average IQ scores between African Americans and whites has shrunk to 10 points or less, although research has not established this trend clearly. The National Assessment of Educational Progress, a national longitudinal study of academic achievement, also shows that the performance of African Americans on math and science achievement tests improved between 1970 and 1996 when compared to whites.

Educators and researchers have focused much attention on explaining why some ethnic groups perform more poorly than others on measures of intelligence and academic achievement. Another topic of research is why some ethnic groups, particularly Asian Americans, perform so well academically. Compared to other groups, Asian American students get better grades, score higher on math achievement and aptitude tests, and are more likely to graduate from high school and college. The exact reasons for their high academic performance are unknown. One explanation points to Asian cultural values and family practices that place central importance on academic achievement and link success in school with later occupational success. Critics counter that this explanation does not explain why Asian Americans excel in specific kinds of abilities.

The academic and occupational successes of Asian Americans have caused many people to presume Asian Americans have higher-than-average IQs. However, most studies show no difference between the average IQ of Asian Americans and that of the general population. Some studies of Asians in Asia have found a 3 to 7 point IQ difference between Asians and whites, in favor of Asians, but other studies have found no significant differences.

See also Psychological Testing.

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