Evidence for Genetic Links to Intelligence
Much of the research into the connection between genes and intelligence has focused on attempting to determine the relative roles of biological inheritance and social influence in developing intelligence. Such attempts have usually involved a combination of four methods: associations of parental intelligence with the intelligence of offspring, associations of the intelligence of siblings (brothers and sisters), comparisons of dizygotic (fraternal) twins
and monozygotic (identical) twins, and adoption studies.
To the extent that mental qualities are inherited, one should expect blood relatives to share these qualities with each other more than with nonrelatives. In an article published in 1981 in the journal Science, T. J. Bouchard, Jr., and Matt McGue examined studies that looked at statistical relationships of intellectual abilities among family members. These studies did reveal strong associations between mental capacities of parents and children and strong associations among the mental capacities of siblings. Further, if genes are involved in establishing mental abilities, one should expect that the more genes related people share, the more similar they will be in intelligence. Studies have indicated that fraternal twins are only slightly more similar to each other than are nontwin siblings. Identical twins, developing from a single egg with identical genetic material, have even more in common. Bouchard
and McGue found that there was an overlap of about 74 percent in the intellectual abilities of identical twins and an overlap of about 36 percent in the intellectual abilities of fraternal twins.
Family members may be similar because they live in similar circumstances, and identical twins may be similar because they receive nearly identical treatment. However, studies of adopted children show that the intellectual abilities of these children were more closely related to those of their biological parents than to those of their adoptive parents. Studies of identical twins who were adopted and raised apart from each other indicate that these twins have about 62 percent of their intellectual abilities in common.
Twin studies, in particular, have helped to establish that heredity is involved in a number of intellectual traits. Memory, number ability, perceptual skills, psychomotor skills, fluency in language use, and proficiency in spelling are only a few of the traits in which people from common genetic backgrounds tend to be similar to each other. However, psychometricians have not reached agreement on the extent to which mental abilities are products of genes rather than of environmental factors such as upbringing and opportunity. Some researchers estimate that only 40 percent of intellectual ability is genetic; others set the estimate as high as 80 percent.
It is important to keep in mind that even if most differences in mental abilities among human beings were caused by genetics, members of families would still show varied abilities. If, for example, there is a gene for high mathematical ability (gene A) and a gene for low mathematical ability (gene a), it is quite possible that a woman who has inherited each gene (Aa) from her parents will marry a man who has inherited each gene (Aa) from his parents. In this case, there is a 1 in 4 probability that they will have a child who is mathematically gifted (AA) and a 1 in 4 probability that they will have a child who is mathematically slow (aa). This example, although grossly simplified, gives an idea of the effect of variation in the genes inherited.
The Problem of Defining and Measuring Intelligence
Debates over genetic links to intelligence are complicated by the problem of precisely defining and accurately measuring intelligence. It may be that abilities to build houses, draw, play music, or understand complex mathematical procedures are inherited as well as learned. Which of these abilities, however, constitute intelligence? Because of this debate, some people, such as Harvard psychologist Howard Gardner, have argued that there is no single quality of intelligence but rather multiple forms of intelligence.
If there is no single ability that can be labeled “intelligence,” this means that one cannot measure intelligence or determine the extent to which general intellectual ability may be genetic in character. An intelligence quotient (IQ), the measure of intelligence most commonly used to study genetic links to intellectual ability, is based on the view that there is a great deal of overlap among various mental traits. Although a given individual may be skilled at music or writing and poor at mathematics, on average, people who are proficient in one area tend to be talented in other areas. Proponents of IQ measures argue that this overlap exists because there is a single, underlying, general intelligence that affects how people score on tests of various kinds of mental abilities. The opponents of IQ measures counter that even if one can speak of intelligence rather than “intelligences,” it is too complex to be reduced to one number.
Impact and Applications
The passing of mental abilities from parents to children by genetic inheritance is a politically controversial issue because genetic theories of intelligence may be used to justify existing social inequalities. Social and economic inequalities among racial groups, for example, have been explained as differences among groups in inherited intelligence levels. During the nineteenth century, defenders of slavery claimed that black slaves were by nature less intelligent than the white people who held them in slavery. After World War I, the Princeton University psychologist C. C. Brigham concluded from results of army IQ tests that southern European immigrants had lower levels of inherited intelligence than native-born Americans and that blacks had even more limited intelligence. White supremacists and segregationists used Brigham’s results to justify limiting the access of blacks to higher education and other opportunities for advancement. In 1969, Berkeley psychologist Arthur R. Jensen touched off a storm of debate when he published an article that suggested that differences between black and white children in educational success were caused in part by genetic variations in mental ability.
Wealth and poverty, even within racial and ethnic groups, have been explained as consequences of inherited intelligence. Harvard psychologist Richard Herrnstein and social critic Charles Murray have argued that American society has become a competitive, information-based society in which intellectual ability is the primary basis of upward mobility. They have maintained, furthermore, that much of intellectual ability is genetic in character and that people tend to marry and reproduce within their own social classes. Therefore, in their view, social classes also tend to be intellectual classes: a cognitive elite at the top of the American social system and a genetically limited lower class at the bottom.
Scientific truth cannot be established by accusing theories of being inconvenient for social policies of equal opportunity. Nevertheless, it is not clear that genetic differences in intelligence are necessarily connected to social status. Even those who believe that inherited intelligence affects social position generally recognize that social status is affected by many other factors such as parental wealth, educational opportunity, and cultural attitudes.
It seems evident that there are genetic links to mental ability. At the same time, however, the extent to which genes shape intellectual capacities, whether these capacities should be combined into one dimension called intelligence, and the validity of measures of intelligence remain matters of debate. The scientific debate, moreover, is difficult to separate from social and political debates.
Key terms
dizygotic organism
:
an organism developed from two separate ova; fraternal twins are dizygotic
intelligence quotient (IQ)
:
the most common measure of intelligence; it is based on the view that there is a single capacity for complex mental work and that this capacity can be measured by testing
monozygotic organism
:
developed from a single ovum (egg); identical twins are monozygotic because they originate in the womb from a single fertilized ovum that splits in two
psychometrician
:
one who measures intellectual abilities or other psychological traits
Bibliography
Bock, Gregory R., Jamie A. Goode, and Kate Webb, eds. The Nature of Intelligence. New York: John Wiley and Sons, 2001. Presents the debate between evolutionary psychologists, who argue against general intelligence and for an intelligence that develops and evolves based on particular, extraspecies domains, and behavior geneticists, who believe general intelligence is fundamental and who focus their work on intraspecies differences. Illustrations, bibliography, and index.
Devlin, Bernie, et al. Intelligence, Genes, and Success: Scientists Respond to “The Bell Curve.” New York: Springer, 1997. Presents a scientific and statistical reinterpretation of The Bell Curve’s claims about the heritability of intelligence and about IQ and social success. Bibliography, index.
Fish, Jefferson M., ed. Race and Intelligence: Separating Science from Myth. Mahwah, N.J.: Lawrence Erlbaum, 2002. An interdisciplinary collection disputing race as a biological category, arguing that there is no general or single intelligence and that cognitive ability is shaped through education. Bibliography, index.
Fraser, Steven, ed. The “Bell Curve” Wars: Race, Intelligence, and the Future of America. New York: Basic Books, 1995. Scholars from a variety of disciplines and backgrounds provide a brief, critical response to the book by Herrnstein and Murray. Bibliography.
Gardner, Howard. Frames of Mind: The Theory of Multiple Intelligences. 10th anniversary ed. New York: Basic Books, 1993. Argues that there is no single mental ability to be inherited. New introduction, bibliography, index.
Gould, Stephen Jay. The Mismeasure of Man. New York: Norton, 1996. An influential criticism of IQ as a measure of intelligence and of the idea that intellectual abilities are inherited. Bibliography, index.
Herrnstein, Richard J., and Charles Murray. The Bell Curve: Intelligence and Class Structure in America. New York: Free Press, 1994. The authors maintain that IQ is a valid measure of intelligence, that intelligence is largely a product of genetic background, and that differences in intelligence among social classes play a major part in shaping American society. Illustrations, bibliography, and index.
Heschl, Adolf. The Intelligent Genome: On the Origin of the Human Mind by Mutation and Selection. Drawings by Herbert Loserl. New York: Springer, 2002. Chapters include “Learning: Appearances Are Deceptive,” “The ’Wonder’ of Language,” “How to Explain Consciousness,” and “The Cultural Struggle of Genes.”
Lynch, Gary, and Richard Granger. Big Brain: The Origins and Future of Human Intelligence. Art by Cheryl Cotman. New York: Palgrave Macmillan, 2008. Chronicles the evolution of the human brain. Describes the functions of memory, cognition, and intelligence and explains how the brain can potentially be enhanced.
Murdoch, Stephen. IQ: A Smart History of a Failed Idea. Hoboken, N.J.: J. Wiley and Sons, 2007. Chronicles the history of intelligence testing from the late nineteenth through the twentieth centuries. Argues that IQ testing is neither a reliable nor a helpful tool in predicting human behavior or an individual’s future success or failure.
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