Group Differences in IQ Scores

281
Evaluating Intelligence Tests
FIGURE
7.12
A verages
A Representation of Ethnic Group
Differences in IQ Scores
The average IQ score of Asian Americans is
about four to six points higher than the
average score of European Americans, who
average twelve to fifteen points higher
than African Americans and Hispanic
Americans. Notice, however, that the variation within each of these groups is much
greater than the variation among them.
African Americans and Hispanic Americans
European Americans
Asian Americans
attributed to genetic influences and about half can be attributed to environmental
influences. (As discussed in the appendix, variability is the degree to which scores
spread out around an average score.)
Group Differences in IQ Scores
Much of the controversy over the roles played by genes and the environment in intelligence has been sparked by efforts to explain differences in the average IQ scores earned
by particular groups of people. For example, the average scores of Asian Americans are
typically the highest among various ethnic groups, followed, in order, by European
Americans, Hispanic Americans, and African Americans (e.g., Fagan, 2000; Herrnstein
& Murray, 1994; Lynn, 1996; Taylor & Richards, 1991). Further, the average IQ scores
of people from high-income areas in the United States and elsewhere are consistently
higher than those of people from low-income communities with the same ethnic
makeup (Jordan, Huttenlocher, & Levine, 1992; McLoyd, 1998; Rowe, Jacobson, & Van
den Oord, 1999).
To understand these differences and where they come from, we must remember two
things. First, group scores are just that; they do not describe individuals. So even though
the average IQ score of Asian Americans is higher than the average IQ score of European Americans, there will still be many European Americans who score well above the
Asian American average and large numbers of Asian Americans who score below the
European American average (see Figure 7.12). Second, inherited characteristics are not
necessarily fixed. As already mentioned, living in a favorable environment can improve
a child’s intellectual performance somewhat (Humphreys, 1984). There is also evidence
that living in an impoverished environment can impair the development of cognitive
skills (Turkheimer et al., 2003).
Why is there a relationship between IQ scores and
family income? Four factors seem to account for the correlation. First, parents’ jobs and
status depend on characteristics related to their own intelligence. And this intelligence
is partly determined by a genetic component that, in turn, contributes to their children’s cognitive ability level. Second, parents’ income affects their children’s environment in ways that can increase or decrease the children’s IQ scores (Bacharach &
Baumeister, 1998). Third, motivational differences may play a role. Parents in upperand middle-income families tend to provide more financial and psychological support
for their children’s motivation to succeed and excel in academic endeavors (Atkinson
& Raynor, 1974; Erikson et al., 2005; Nelson-LeGall & Resnick, 1998). As a result, children from middle- and upper-income families may exert more effort in testing situations
Socioeconomic Differences
LINKAGES
How does motivation affect
IQ scores? (a link to Motivation
and Emotion)
282
in review
Chapter 7 Thought, Language, and Intelligence
INFLUENCES ON IQ SCORES
Source of
Effect
Description
Examples of
Evidence for Effect
Genetics
Genes appear to play a
significant role in IQ test
performance.
The IQ scores of siblings who
share no common environment
are positively correlated. There is
a greater correlation between
scores of identical twins than
between those of nonidentical
twins.
Environment
Environmental conditions
interact with genetic
inheritance. Nutrition, medical
care, sensory and intellectual
stimulation, interpersonal
relations, and influences on
motivation are all significant
features of the environment.
IQ scores have risen among
children who are adopted into
homes that offer a stimulating,
enriching environment.
Correlations between IQs of
twins reared together are higher
than for those reared apart.
?
1. Intelligence is influenced by both _____ and _____.
2. Children living in poverty tend to have _____ IQ scores than those in middleincome families.
3. IQ scores of children whose parents encourage learning tend to be _____ than
those of children whose parents do not.
and therefore obtain higher scores (Bradley-Johnson, Graham, & Johnson, 1986; Zigler
& Seitz, 1982). Fourth, because colleges, universities, and businesses usually select
people with higher scores on various cognitive ability tests, those with higher IQs—
who tend to do better on such tests—may have greater opportunities to earn more
money (Sackett et al., 2001).
Ethnic Differences Some have argued that the average differences in IQ among
various ethnic groups in the United States and other developed countries are due
mostly to heredity (Rowe, 2005; Rushton & Jensen, 2005). However, the existence of
hereditary differences among individuals within groups does not indicate whether differences between groups result from similar genetic causes (Lewontin, 1976). Notice
again in Figure 7.12 that variation within ethnic groups is much greater than variation
among the mean scores of those groups (Zuckerman, 1990).
We must also take into account the large differences among the environments in
which the average African American, Hispanic American, and European American child
grows up. To take only the most blatant evidence, the latest U.S. Census Bureau figures
show 24.4 percent of African American families and 22.5 percent of Hispanic American families living below the poverty level, compared with 11.8 percent of Asian American families and 8.2 percent of European American families (U.S. Census Bureau,
2004). Compared with European Americans, African American children are more likely
to have parents with poor educational backgrounds, as well as inferior nutrition, health
care, and schools (Evans, 2004; Wilson, 1997). All of these conditions are likely to pull
down scores on intelligence tests (Brooks-Gunn, Klebanov, & Duncan, 1996). Cultural
factors may also contribute to differences among the average scores of various ethnic
groups. For example, those differing averages may partly reflect differences in the degree
to which parents in each group tend to encourage their children’s academic achievement (Steinberg, Dornbusch, & Brown, 1992).