Group Differences in Intelligence Test Scores

Instructor: Daniel Murdock

Daniel has taught Public Health at the graduate level and has a Ph.D. in Behavioral Sciences & Health Education.

Why do some groups of people have higher average scores on IQ tests than others? In this lesson, we'll examine three possible explanations for group differences in IQ scores: genetic causes, social and environmental causes, and test bias.

Group Differences in IQ

First developed over a century ago, intelligence quotient (IQ) tests are still widely used today to measure an individual's mental agility and ability. Despite the widespread use of IQ tests, the relevance and legitimacy of these tests is hotly debated among educators and researchers. One of the most controversial issues is the observation that IQ scores vary between racial and ethnic groups and sexes.

Alfred Binet invented the first practical IQ test in 1904.
Alfred Binet

Although current research suggests there is little difference between the average IQ scores of men and women, some studies have found differences in the capacity of males and females in performing certain mental tasks. For example, researchers have found that females tend to perform better on verbal abilities, while males tend to perform better on visuospatial abilities.

The connection between race and intelligence is a particularly controversial issue in IQ testing. Recent research has broadly shown that there are significant variations in average IQ scores across different racial and ethnic groups.

This research raises two important questions:

  1. If these findings reflect real differences in average group intelligence, what are the possible causes of those differences?
  2. Are intelligence tests inappropriately biased toward certain groups?

In this lesson, we'll examine three possible explanations for group differences in IQ scores: genetic causes, social and environmental causes, and test bias.

Heredity as an Explanation

Researchers generally agree that our genes and our environment have an interactive influence on our intelligence. However, there are differing views regarding how much each factor contributes to intelligence and how the factors interact. Most researchers believe there is a reaction range to IQ, in which heredity places an upper and lower limit on the IQ that can be attained by a given person.

Evidence for the influence of heredity on intelligence comes from three types of studies:

  • Family studies show that intelligence is highly correlated among family members.
  • Twin studies show that intelligence is more highly correlated among identical twins than fraternal twins.
  • Adoption studies show that the IQ scores of adopted children somewhat resemble those of their biological parents.

Environment as an Explanation

Even if our individual IQ is largely due to heredity, group differences may not be. Many scholars believe that group differences in IQ are best understood as environmental in origin.

Adoption studies provide evidence that our upbringing plays an important role in our mental ability. Researchers have found that adopted children show some resemblance to their adoptive parents in IQ.

Family studies also show that siblings who are reared together are more similar in IQ than siblings reared apart. And when children are removed from deprived environments and placed in homes that are more conducive for learning, they show increases in IQ.

Some environmental factors that affect IQ are unequally distributed across different population groups, which many believe is the primary cause of group differences in IQ. Key examples include:

  • Socioeconomic environment: Racial and ethnic minority children are more likely to experience socioeconomic disadvantages, which are correlated with lower IQ scores.
  • Health and nutrition: Racial and ethnic minority children are statistically more likely to be exposed to poorer nutrition, prenatal health care, and postnatal health care. Poor health and nutrition can negatively affect cognitive development and functioning.
  • Education: Racial and ethnic minority children are more likely to be exposed to poorer quality schools than white children, which limits opportunities for cognitive development. Gender biases in the education system may also help explain why men and women perform differently on certain mental abilities in IQ tests.

Bias in Intelligence Testing

Another possible explanation for group differences in IQ is that they are merely a statistical artifact, or a result of the particular research technique employed rather than an accurate representation of the world as it truly exists.

Several studies have suggested that IQ tests may be biased against certain groups, particularly racial and ethnic minorities. Some scholars have dismissed these claims, however, by pointing out that IQ tests have been shown to be equally valid predictors of future academic achievement for black and white Americans. Still, this defense may not completely rule out the possibility of bias.

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