Understanding Psychology (10th Ed)

 IQ AND HERITABILITY 
It is clear that intelligence in general shows a high degree of  heritability,  a measure 
of the degree to which a characteristic can be attributed to genetic, inherited factors. 
(e.g., Petrill, 2005; Miller & Penke, 2007; Plomin, 2009). As can be seen in Figure 1, 
the closer the genetic link between two related people, the greater the correspondence 
of IQ scores. Using data such as these, Herrnstein and Murray argued that differences 
between races in IQ scores were largely caused by genetically based differences in 
intelligence.
However, many psychologists reacted strongly to the arguments laid out in The 
Bell Curve, refuting several of the book’s basic arguments. One criticism is that even 
when attempts are made to hold socioeconomic conditions constant, wide variations 
culture-fair IQ test
A test that does 
not discriminate against the members 
of any minority group.
heritability
A measure of the degree 
to which a characteristic is related to 
genetic, inherited factors.
Study Alert
Remember that the diff er-
ences in IQ scores are much 
greater when comparing 
individuals than when 
comparing groups.
Exploring
D I V E R S I T Y
The Relative Infl uence of Genetics and 
Environment: Nature, Nurture, and IQ
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302 Chapter 
9 
Intelligence
remain among individual households. Furthermore, no one can convincingly assert 
that the living conditions of blacks and whites are identical even when their socio-
economic status is similar. In addition, as we discussed earlier, there is reason to 
believe that traditional IQ tests may discriminate against lower-SES urban blacks by 
asking for information pertaining to experience they are unlikely to have had (Amer-
ican Psychological Association Task Force on Intelligence, 1996; Hall, 2002; Horn, 
2002; Nisbett, 2007).
Moreover, blacks who are raised in economically enriched environments have 
IQ scores similar to whites in comparable environments. For example, in a study of 
black children who had been adopted at an early age by white middle-class families 
of above-average intelligence, the IQ scores of those children averaged 106—about 
15 points above the average IQ scores of unadopted black children in the study. Other 
research shows that the racial gap in IQ narrows considerably after a college educa-
tion, and cross-cultural data demonstrate that when racial gaps exist in other cul-
tures, the economically disadvantaged groups typically have lower scores. In short, 
the evidence that genetic factors play the major role in determining racial differences 
in IQ is not compelling (Scarr & Weinberg, 1976; Sternberg, Grigorenko, & Kidd, 2005; 
Fagan & Holland, 2007; Nisbett, 2009). 
Furthermore, drawing comparisons between different races on any dimension, 
including IQ scores, is an imprecise, potentially misleading, and often fruitless ven-
ture. By far, the greatest discrepancies in IQ scores occur when comparing individuals,
not when comparing mean IQ scores of different groups . There are blacks who score 
high on IQ tests and whites who score low, just are there are whites who score high 
and blacks who score low. For the concept of intelligence to aid in the betterment of 
society, we must examine how individuals perform and not the groups to which they 
belong (Angoff, 1988; Fagan & Holland, 2002, 2007).