ReadingReading: Racial Controversy.

Module 7.4 - LO8: Discuss the role of nature and nurture in intelligence.

In the early 1900s, psychologists believed intelligence was primarily inherited. This idea reappeared in a relatively recent book, The Bell Curve, by psychologist Richard Herrnstein and political scientist Charles Murray (1994). But what brought these authors the greatest publicity was their statement that racial differences in IQ scores were caused primarily by genetic or inherited factors. This and other statements from Herrnstein and Murray’s book set off such a heated and often misguided public debate that the American Psychological Association (APA) formed a special task force of prominent researchers. The goal of the APA task force was to summarize what is known about intelligence (Neisser et al., 1996). The issues of race, genetics, and intelligence are still hotly debated (Rushton & Jensen, 2005). We now focus on the complex question of racial differences in IQ scores.

Difference between IQ Scores


To help you understand the controversy surrounding racial differences in IQ scores, please look at the figure in the upper right. Notice that there are two distributions of IQ scores: The orange bell-shaped curve shows the distribution of IQ scores for African Americans (blacks), and the blue bell-shaped curve shows the distribution of scores for Caucasians (whites). Although there is much overlap in IQ scores (indicated by overlapping of orange and blue areas), researchers generally agree that the average or mean IQ score for African Americans is about 15 points lower than the average IQ score for Caucasians (Bouchard, 1995). This 15-point average difference in IQ scores means that although there are many African Americans with high IQ scores, they are proportionally fewer in number compared to Caucasians.

Two Explanations

There are at least two possible explanations for this 15-point difference in average IQ scores. One explanation is that the differences are due to inherited or genetic factors: African Americans are genetically inferior to whites. Another explanation is that the difference is due to a number of environmental factors: African Americans have fewer social, economic, and educational opportunities than whites do.

Although the authors of The Bell Curve emphasized the role of genetic factors, you’ll see that the APA task force and many other psychologists disagreed.

Cause of IQ Differences

Group Differences

In a careful review of The Bell Curve, one of the leading researchers in the area of intelligence concluded the book offered no convincing evidence that genetic factors were primarily responsible for the 15-point IQ difference between African Americans and Caucasians (R. J. Sternberg, 1995). This conclusion is based largely on the distinction between whether genetic factors can influence the development of intelligence in an individual and whether they can influence the development of intelligence among races. The APA task force said there is good evidence that genetic factors play a significant role in the development of an individual’s intelligence. However, there is no convincing evidence that genetic factors play a primary role in the differences in intelligence among races. A tremendous amount of research data challenges Herrnstein and Murray’s statement that IQ differences among races are caused primarily by genetic factors (Neisser et al., 1996; R. J. Sternberg et al., 2005).

Although no one knows exactly what causes the difference in IQ scores shown in the above graph, many psychologists suggest a number of environmental factors, such as differences in socioeconomic classes, educational opportunities, family structures, and career possibilities (Loehlin, 2000). Recent research that shows the difference in IQs between African Americans and whites is narrowing by 4 to 7 points suggests that environmental factors can significantly influence IQ (Dickens & Flynn, 2006). Thus, one of The Bell Curve’s major conclusions—that racial differences in IQ scores are based primarily on genetic factors—is not supported by the evidence (Neisser et al., 1996). Two prominent researchers concluded that The Bell Curve’s argument for racial inferiority appeared to be based on scientific evidence, but closer examination shows that it was not (S. J. Gould, 1996; R. J. Sternberg, 1995).

Differences in Skin Color

Another problem with The Bell Curve is its assumption that skin color is a meaningful way to identify races. For example, based on skin color, to which race would you assign the individuals in the four photos on the left? Researchers report that skin color is not reliable in identifying racial makeup because recent studies on DNA (genetic instructions) indicate that people around the world are much more alike than different (Shriver, 2005). In fact, no matter the color of one’s skin, genetic instructions in people around the world vary by only about 3% to 5% (M. C. King & Motulsky, 2002). Thus, differences in skin color are only skin deep, and skin color is not a reliable measure to assign people to different races when comparing IQ scores (Venter, 2000).

Credits: bell curve, Hernstein and Murray, 1994; fair (#1); medium (#2); light (#3); Hispanic (#4) © Anthony Barboza
LO8: Discuss the role of nature and nurture in intelligence