The role of genetics in determining a person's intelligence is a controversial subject. Few would deny that genes play some role, but many are uncomfortable with the idea that genes determine intelligence. For if intelligence is a genetic trait the implication is that some people are born to be smart, others are not, and education and upbringing cannot change it. The results of scores of studies and contentious debate in the scientific community have produced little consensus about the relationship between genetics and intelligence. At least part of the problem stems from the fact that "intelligence" is defined differently by different people.
Although this issue has been argued since the 1870s—when Francis Galton proposed his controversial and arguably racist notions about the heritability of intelligence—the debate was reignited during the 1990s when Richard Herrnstein (1930–94) and Charles Murray (1943–) published The Bell Curve: Intelligence and Class Structure in American Life (New York: Touchstone Books, 1996). Herrnstein and Murray expressed their beliefs that between 40% and 80% of intelligence is determined by genetics and that it is intelligence levels, not environmental circumstances, poverty, or lack of education, that are at the root of many of our social problems. Critics argued that Herrnstein and Murray not only manipulated and misinterpreted data to support their contention that intelligence levels differ among ethnic groups, but also reintroduced outdated and harmful racial stereotypes. Many observers did agree with Herrnstein and Murray's premises that intellect is spread unevenly among individuals and population subgroups, that innate intelligence is distributed through the entire population on a "bell curve," with most people near to the average and fewer at the high and low ends, and even that the distribution varies by race and ethnicity. But few have been willing to accept the idea that intelligence is entirely genetically encoded, permanently fixed, and unresponsive to environmental influences.
One traditional measure of intelligence is a standardized intelligence quotient (IQ) test, which measures an individual's ability to reason and solve problems. Nearly all studies about the link between intelligence and genetics rely on results obtained from IQ tests, which generally provide an overall score along with measures of verbal ability and performance ability. Although there are several versions of IQ tests available, test takers generally perform comparably on all of them, presumably indicating that they all measure comparable aspects of cognitive ability. Critics of IQ tests as measures of intelligence contend that they do not measure all abilities in the complex realm of intelligence. They observe that the tests measure one small segment of the diverse abilities that comprise intelligence, evaluate only analytic abilities, fail to assess creative or practical abilities, and measure only a small sample of the skills that define the domain of intelligent human behavior.
Despite concern about whether IQ tests fully measure intelligence, nearly all scientific study of the contributions of genetics and environment to intelligence have focused on measuring IQ and examining individuals who differ in their familial relationships. For example, if genetics plays the predominant role in IQ, then identical twins should have IQs that compare more closely than the IQs of fraternal twins, and siblings' IQs should correlate more highly than those of cousins. In "Genetics of Childhood Disorders, II: Genetics and Intelligence" (Journal of the American Academy of Child and Adolescent Psychiatry, vol. 38, no. 5, May 1999), Alan Kaufman observed that scientists who argue in favor of genetic determination of IQ cite these data to support their assertion that:
- Identical (monozygotic) twins' IQs are more similar than those of fraternal (dizygotic) twins.
- IQs of siblings correlate more highly than IQs of half-siblings, which, in turn, correlate higher than IQs of cousins.
- IQ correlations between a biological parent and child living together are higher than those between an adoptive parent and child living together.
Kaufman asserted that the following results support scientists who believe that environment more strongly determines IQ:
- IQs of fraternal twins correlate more highly than IQs of siblings of different ages despite the same degree of genetic similarity.
- Unrelated siblings reared together, such as biological and adopted children, have IQs that are more similar than biological siblings reared apart.
- Correlations between IQs of an adoptive parent and a child living together are similar to correlations of a biological parent and a child living apart.
- Siblings reared together have IQs that are more similar than siblings reared apart, and the same finding holds for parents and children, when they live together or apart.
The preponderance of evidence from twin, family, and adoption studies supports increasing heritability of intelligence over time, ranging from 20% in infancy to 60% in adulthood, along with environmental factors estimated to contribute about 30%. Kaufman suggested that heredity is important in determining a person's IQ, but environment is also crucial. Based on twin studies, the heritability percentage for IQ is approximately fifty, which is comparable with the heritability value for body
weight. Kaufman believed that the genetic contribution to weight and intelligence are comparable. He observes that many overweight people have a genetic predisposition for a large frame and a metabolism that promotes weight gain, while naturally thin people have the opposite genetic predisposition. Nonetheless, for most people environmental factors such as diet and exercise have a substantial impact on weight. Similarly, genetics and environment interact to determine IQ, and people with genetic, familial relationships such as parents and siblings frequently share common environments.
In "Virtual Twins: New Findings on Within-Family Environmental Influences on Intelligence" (Journal of Educational Psychology, vol. 92, no. 3, September 2000), psychologist Nancy Segal examined "virtual twins"—genetically unrelated siblings (typically adopted) of the same age who were reared together from early infancy—to assess environmental influences on intelligence. The results of this study reveal that the IQ correlation of virtual twins fell considerably below correlations reported for identical twins, fraternal twins, and full siblings. Segal interpreted these results as a demonstration of the modest effects of environment on intellectual development and as supporting a predominantly genetic role in determining intelligence.
Robert Plomin detailed similar findings from adoption studies in "Genetics of Childhood Disorders, III: Genetics and Intelligence" (Journal of the American Academy of Child and Adolescent Psychiatry, vol. 38, no. 6, June 1999). He observed that adoption studies have a substantial estimated heritability, finding that identical twins reared apart are almost as similar for measures of intelligence as identical twins reared together. Plomin also looked at 245 children adopted in the first month of life from the Colorado Adoption Project to see if there is more of a parent-offspring or an adoptive parent–adopted children correlation of IQ scores. He found significant correlations between biological mothers and their adopted-away children and almost no parent-offspring correlations for adoptive parents and their adopted children, suggesting that family environment shared by parents and offspring does not contribute as strongly as genetic influences to parent-offspring resemblance for selected measures of intelligence.