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Genetic Influences on Intelligence Testing Results

written by: •edited by: Stephanie Mojica•updated: 3/14/2011

As genetic knowledge increases, so do the questions that can be asked. One of the most often heard ones is: 'What are the genetic influences on intelligence testing results?' This article discusses this question.

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    What is Intelligence?

    Intelligence is a reasonably obscure concept. What does it mean when someone is called intelligent? We all have a pretty good idea what to expect of that person, and yet it seems that everybody has a slightly different definition of intelligence. To make matters even more convoluted, several types of intelligence can be discerned as follows: analytic intelligence, practical intelligence and social intelligence.

    Nevertheless, most intelligence tests seem to focus on a few specific types of intelligence: analytical intelligence, verbal intelligence and spatial intelligence. Intelligence tests of this kind result in a tangible number, which makes it a lot easier to talk about the concept of intelligence (even though the number doesn’t say everything.)

    In the rest of this article, the term ‘intelligence’ will be used to refer to the intelligence that is measured during these tests, while discussing the genetic influences on intelligence testing.

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    Intelligence and Genetics

    Intelligence is one of those fields where the nature-nurture debate still rages on. Is one born intelligent, or can one be taught to be intelligent, or both? Today, most geneticists take a moderate position in these matters, recognizing that many behavioral characteristics, including intelligence, are influenced by a complex interaction between genes and environment, and that it is quite difficult to separate the effects of genes and those of the environment.

    Nevertheless, many modern studies indicate that intelligence has a moderate to high heritability (averagely between 0.4 and 0.8), so there certainly seem to be genetic influences on intelligence testing results. Based on these results, some people have argued that intelligence is innate and that enhanced education will have no influence whatsoever on the level of intelligence.

    This, however, is based on a misconception. When a characteristic has a high heritability, this does not mean that changing the environment has no influence on the characteristic. Heritability provides information only about how much of the variation in a certain characteristic is genetically determined. There is no universal heritability for a trait, heritability is given for a certain population at a certain time. So, environmental factors can potentially affect traits with a high heritability.

    Another misconception about heritability is that the difference in average intelligence between certain groups of people is based on genetic differences between these groups. But heritability provides no information about the causes of difference between groups. The only thing it indicates is the degree to which phenotypic variance within a group is genetically based. The relatively high heritability of intelligence does not mean that the difference in intelligence between groups is genetic.

    All this is complemented by the fact that it is very difficult to separate genetic and environmental effects in human beings. For example, it is possible that intelligent parents will also provide a more mentally stimulating environment for their children. In this case, it would be very difficult to separate the genetic effects from the environmental ones.

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    • Gardner, H. (1999). Intelligence Reframed: Multiple Intelligences for the 21st Century. Basic Books.
    • Pierce, B.A. (2002). Genetics: A Conceptual Approach. W.H. Freeman.
    • Plomin, R. (1999). Genetics and general cognitive ability. Nature. 402, pp. C25 – C29.