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6.7.2 Nature or nurture: What determines intelligence?

The question of what is responsible for a person’s intelligence, nature or nurture, is only important because it is associated with sociopolitical issues where intelligence has been identified with phenotypical race as representative of the affect of invariable “nature”. Consistent differences have been found between phenotypical races on general measures of intelligence. The direction of the differences shows that African Americans score consistently lower than Americans of European background (Jensen, 1968, 1984). The research created a storm in American psychology, not least because it was published just as Black people were finding their way forward after centuries of oppression. Jensen argued that about 80 percent of intelligence is biologically determined and fixed and that therefore compensatory programs for the underprivileged are useless and a waste of resources.

Strong support for the nature theory is found in the studies of twins reared apart compared to fraternal twins reared together (Bouchard & McGue, 1981). If intelligence has a large nature component and is biological inherited then twins reared apart should produce very similar intelligence scores, whereas if the environment is primarily responsible then the fraternal twins raised in the same environment should yield more similarity. In fact the results showed that identical twins reared apart, had more similar scores (.82 correlation) than fraternal twins reared in the same homes. Although most researchers agree that there is a biological component in intelligence (Plomin, 1990), Jensen’s application of these findings to phenotypical race is rejected by most researchers today. It would be more fruitful to examine the relationship between brain size and intelligence (Jensen & Johnson, 1993).

6.7.3 Sources of bias in intelligence testing.

The fact that Afro-Americans scored one standard deviation below samples of European-Americans led Jensen to conclude that significant intellectual differences existed between the two groups. A serious controversial contention occurred in psychology and the broader society when Jensen and others (Herrnstein & Murray, 1994) interpreted these differences as racial variances in intellectual capacity. However, other research has challenged Jensen’s ”nature” conclusions. For example Humphreys (1985) analyzed results from a very large data bank and found that “race” only correlated .17 with intelligence, but .86 with socioeconomic status. The large correlation for socioeconomic status was attributed to the very adverse conditions associated with the lives of Black people who struggle economically that in turn affect opportunity and intelligence scores.

All the readers of this chapter will now also be aware of the large role played by the cultural context in cognition. Can we really know what component of intelligence can be attributed to nature or nurture? Hebb (1949) made a distinction between the genetic capacity associated with the brain that he called intelligence A, and intelligence B which developed in interaction with the cultural context. Vernon (1969) provided an additional distinction that he called intelligence C that was measured by a person’s performance on an intelligence test. These distinctions provide us with two sources of bias in the conceptualization of the construct. First, especially in the cross-cultural context, the intelligence test employed may not adequately sample B, thereby yielding a performance that does not reflect a person’s intelligence adequately. Since not all sources of test variance are controlled for in intelligence testing the lack of equivalence of test scores may be the result of cultural bias. Many variables can contribute to bias including language differences, inadequate items used to measure content, and variable motivation to respond well on the test. The results of intelligence testing then may be nothing more than an assessment of intelligence C performance on the intelligence test, not measuring either A or B. Jensen, Herrnstein and Murray who advocate in favor of racial differences in intelligence seem to confuse intelligence C (the actual measurement) as representing intelligence A (the hardwired capacity).

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