Intelligence Bibliography

Long before they enter school, children are judged on their relative ability to master such cognitive tasks as solving puzzles, learning the alphabet, and counting. Many parents encourage their children to succeed on such tasks both in and out of school, demonstrating pleasure and disappointment based on their performances. Those who go on to do well on school tasks such as reading and science can expect to be rewarded with more opportunities than those who do not. It is no wonder, then, that children sense that their intellectual capabilities are bound to their value as individuals.

Our society has developed various means for the formal evaluation of intelligence. Since the development of the first “intelligence” test less than one hundred years ago, the “intelligence quotient,” or IQ, has become a household term meant to reflect the range and limits of a person’s cognitive potential. This widespread use is echoed by the increase of achievement testing nationwide. For example the company, Kaplan Test Prep and Admissions ran an advertisement recently that read, “higher score, brighter future [1],” which reflects the current cultural attitude about testing.

However, even experts have not been able to agree upon which abilities are most relevant to overall intelligence. The American Psychological Association’s Report on Intelligence published in 1995 [2] highlighted the lack of consensus among scientists regarding the term and the state of intelligence testing. Citing a study by Robert Sternberg and Douglas Detterman, [3] the task force reported, “when two dozen prominent theorists were recently asked to define intelligence, they gave two dozen somewhat different definitions.” While a small group of psychologists maintain that intelligence can be measured by IQ tests, others have sought to broaden our notion of what the term means.

Much of the current excitement about testing arose from a push to address gaps in achievement among various groups. Many researchers believe that disparities in performance result from economic and social factors. Others conclude that these gaps are the result of innate biological differences. Charles Murray and Richard Herrnstein, authors of the The Bell Curve [4], a controversial tome published in 1994, argue that achievement differences are not the result of unequal treatment, access to educational resources, or socio-economic status, but of genetic differences between individuals or groups. The most controversial claim made in the book lies in the analysis of how race contributes to these differences. The Bell Curve manipulates statistics to show that programs such as Affirmative Action [5] and Head Start [6] don’t work, while ignoring data from other studies that suggests otherwise.

The use of flawed science to help present a case for accepting the status quo is not new. Attempts to link intelligence to social hierarchies were made throughout the nineteenth and twentieth centuries. These have not been substantiated by science. Attempts to find a gene or set of markers implicated in the development or outcome of human intelligence have not succeeded, since no one understands how much of a role biology plays in human intelligence. The direction of current research is based on a model in which a single number on a linear scale defines the learning potential of an individual. Several psychologists who have questioned this approach have developed alternative models that attempt to expand the definition of intelligence to include a broader spectrum of abilities. Such models have investigated and described capacities such as creativity, social intelligence, and common sense as key to human adaptation. These include Howard Gardner’s “Theory of Multiple Intelligences [7],” Robert Sternberg’s  “Triarchic Mind [8],” and Daniel Goleman’s “Emotional Intelligence [9].” These preliminary approaches have received strong positive responses from specialists in the field. In fact, the primary way intelligence tends to be evaluated is based on a scale originally devised for a different purpose.


IQ: A Short History

The modern IQ test is a direct descendant of a measure designed by French psychologist Alfred Binet in 1905 [10]. As Steven Jay Gould points out in The Mismeasure of Man[11], though Binet originally designed his test to identify students who would need help in school, not as an overall measure of cognitive ability, his scale has come to be regarded as having laid the foundations for modern tests of intelligence. Lewis Terman, a researcher at Stanford University was the first to adapt Binet’s scale, revising and renaming it the “Stanford-Binet” test of intelligence in 1916 [12]. Terman believed that the dissemination of testing would bring to the attention of the government “tens of thousands of defectives” who should be kept from breeding. Thus the birth of modern notions of intelligence was closely tied to the eugenics movement of the same period. From the standpoint of eugenicists of the time, Terman’s test could be used for the purpose of identifying and eliminating from the gene pool, individuals with subnormal potential.

In an effort to underscore the scientific basis of these tests, researcher Charles Spearman developed a model that has come to be known as Spearman’s [13], or the general factor of intelligence. Spearman argued that grades or scores on seemingly unrelated tasks, such as arithmetic and reading ability, are highly correlated. These abilities, he argued, combine to form an overall abstract reasoning capacity, or “g,” that can be measured. Ever since, the notion of g has held sway over decades of research on human intelligence.

Beyond statistical correlations, there is no conclusive proof that IQ tests do more than detect who might encounter difficulty with school learning. In our society, school is a pathway to success later in life. Though Spearman and others claim that intelligence is related to success in school, this claim cannot be investigated until intelligence has been properly defined. Still, research into the biology of intelligence continues. Efforts range from genetic research to how inadequate nutrition during early development affects learning. The following is a survey of the current science.


Genetic Research on IQ

No set of genes or gene markers has been conclusively linked to the development of intelligence. Specific genes that have been studied are primarily those believed to be linked to the development of brain size. Yet no link between human brain size and intelligence has been established.

Other theories have been proposed to explain how differences in brain size and structure have evolved. One such theory detailed by British neuroscientist John R. Skoyles in his paper, “Human Evolution Expanded Brains to Increase Expertise Capacity, Not IQ [14],” argues that size differences are related to skill development. Skoyles maintains that the human brain increased in size over time due to a need for increased “expertise” or skill capacity associated with highly complex tasks that have varied from culture to culture. He argues that this explains differences in brain size among groups.

Contributing evidence comes from studies on children suffering from severe epilepsy who have had a brain hemisphere removed to prevent debilitating seizures. If removed early enough, half a brain seems to work nearly as well as a whole one. The remaining hemisphere often co-opts the functions previously handled by the missing portion [15]. Data from work with patients suffering from microcephaly, a congenital disorder which results in reduced brain size and function, reveals that though a majority of patients score considerably below average on IQ tests, a small portion test in the normal range [16]. These examples challenge the notion that a person’s brain size determines his or her cognitive ability.

Some genes that have been correlated to g are those associated with Cathepsin D (CTSD) and the cholinergic muscarinic 2 receptor (CHRM2) [17]. But it remains unclear what roles these particular genes play in the development of intelligence, if any. Genetic studies attempting to link genes to IQ have uncovered many “candidates” but little conclusive evidence. Research, at this point, has been restricted to analyzing levels of g and the presence or absence of these candidate genes.


Family Studies

Findings from twin, adoption, and family studies are the most commonly cited forms of evidence for a biological theory of intelligence. These studies compare individuals with very similar DNA (identical twins or related family members) with biologically unrelated children growing up in the same home or with children and their adoptive parents. This method attempts to distinguish traits a person is born with from those influenced by his or her environment. Molecular biologist, Robert Plomin has utilized such studies to estimate the heritability of intelligence at around .50 (50%) of the variance [18]. Other studies utilizing g as a cognitive measure have arrived at similar estimates. Longitudinal studies show that these effects increase with age. The heritabilitof g appears to rise to about .75 (75%) by late adolescence. One explanation for this shift is that family influences on cognition are deemed to diminish throughout development. Also possible, explains Plomin, is that additional gene expression delayed during childhood may be triggered as cognitive processes develop.

But do these studies provide evidence that intelligence is inherited? Causation has not been determined here. There are two significant problems associated with twin/adoption and family studies. First is the assumption that genetic effects can be separated from environmental effects. This position rests on the “equal environments assumption” (EEA), which posits that the environment of individuals in the same or different homes can be controlled for in such a way that genetic effects can be separated out. There have been serious critiques levied at EEA due to the way adoptive and non-adoptive environments are appraised as being different or alike [19]. Additionally, the idea that genetic and environmental effects are simply additive and work in isolation of one another is false.

Second, a majority of these studies do not account for how IQ outcomes are affected by class differences. Eric Turkheimer, et al. utilized the twin/adoption and family method to show that socioeconomic status modifies heritability of IQ in young children [20]. The study found that in families who subsisted on incomes at or below the poverty line, the heritabilty effects on IQ were close to zero, whereas in affluent families, these effects were quite high. They also found that parental education levels modified both the effects of heritability and environment, increasing the former and decreasing the latter as years of education increased. In cases where adequate nutrition, access to education, protection from exposure to environmental toxins, and similar issues have affected the development of individuals, heritability estimates have been shown to be expressed quite differently.

Another phenomenon that seems to refute current heritability estimates is the “Flynn effect [21],” which describes a steady worldwide rise in performance since testing began. A three-point rise in IQ per decade on average has been noted, even when tests have been re-standardized to account for these gains. The reasons for this rise are not known, but one explanation involves children’s need, and the need of people in general, to adapt to the increasing complexity of modern life. Obviously the rise cannot result from genetic mutation as the time frame is too narrow. Rather, the Flynn effect may demonstrate how flexible human cognitive development really is. As successive generations take in greater, and more complex, amounts of information from shifting sources such as television and radio, they learn to process the increase. The phenomenon calls into question the extent to which g is an inborn trait. Members of the American Psychological Association task force underscored in their 1995 report that: “…heritable traits can depend on learning and they may be subject to other environmental effects as well. The value of heritability can change if the distribution of environments (or genes) in the population is substantially altered [22].”


Environmental Effects on IQ

A wealth of research has identified multiple environmental factors that may contribute to variation in IQ scores. Many studies have focused on variables such as nutrition, exposure to toxic chemicals, family environment, and socio-economic status, and how these might affect test scores.

Quality and years of schooling have been shown to have an effect on IQ scores. For many children, particularly those from low-income backgrounds, school may be the primary transmitter of information. One study by Lee [23] followed a group of African-American students who were moved from a poor rural school in the south to an urban school with greater resources in Philadelphia. The groups’ average IQ score increased by one-half point each year that they attended the Philadelphia school. Ceci found a positive correlation between years of school attended and IQ scores [24]. His study found that when same-age children enter school a year apart, those with the additional year of school have higher mean scores. These results are borne out in our society. Children who attend poor schools in rural or urban areas tend to score lower on IQ tests than those that have access to a higher quality of education.  

A school-based program that has consistently demonstrated success in raising IQ scores is Head Start. The Department of Health and Human Services began implementing this program in 1964 in an effort to assist families with fewer economic resources by providing educational assistance to children from such families who were under the age of five years. Since its inception, Head Start has enrolled over 22 million children in its programs across the United States. Outcomes suggest that major cognitive gains, at least for the short term, result from this intervention. Follow up studies show that children who attended Head Start are less likely to need special education and more likely to finish high school than those who have not enrolled in the program [25].

Malnutrition during childhood often results in cognitive deficits. A Guatemalan study on undernourished preschoolers showed that those who were given a protein-rich dietary supplement over a ten-year period performed much better on IQ tests than those who did not [26]. There are obvious ethical problems involved in denying undernourished children an available nutritional supplement for the purposes of a study. Yet this data shows that, even though it is not known whether nutrition directly affects IQ, (since undernourished children tend to be less motivated and responsive to adults and therefore less active in their exploration), a child’s food intake clearly is somehow related to her or his ability to learn.

The effects of toxic substances, such as lead and alcohol, on IQ test scores are well known [27]. Children living in impoverished urban environments are at much greater risk for exposure to both. Building codes and zoning laws have reduced lead levels in recent years, but the effects of other toxic chemicals in these environments have not been well studied. Fetal Alcohol Syndrome, a condition resulting from the use of alcohol during pregnancy, leads to a range of deficits in cognitive functions such as attention and memory, as well as IQ [28].

Studies conducted on children who have been abused and neglected show that this often results in a higher than average rate of psychiatric and cognitive disorders [29]. Even in homes with less extreme conditions, the differences in the use of language and size of vocabulary have been shown to affect scores on verbal IQ tests. Parents’ years of schooling affect the IQ scores of children, as does early exposure to concepts such as counting [30]. Most of the evidence on environmental contributors to intelligence scores and intellectual development show a clear advantage for children who grow up in middle-class or affluent homes. This evidence should be weighted carefully in any discussion regarding between-group differences in IQ.


Between Group Differences in IQ

Some of the debate surrounding the biological basis of IQ focuses on differences in test scores between racial groups. Yet, these differences, such as those between African-Americans, Hispanics, and Whites, are shrinking. In the 1970’s, African-American IQ scores averaged a full standard deviation (15 points) below the average for Whites, with Hispanic scores falling somewhere in between the two averages. Since then, test scores for African-American and Hispanic groups have gradually risen, even outpacing the Flynn effect [31]. This has paralleled score increases on other achievement tests. One study tracked the scores of African-American five-year olds on the math section of the National Assessment of Educational Progress between 1978 and 1990. Results showed substantial gains over the twelve-year period [32]. Improvements like these have been attributed to programs focused on improving socio-economic status for minority groups as well those that have focused more on educational interventions. Unfortunately, despite the evidence, authors such as Herrnstein and Murray maintain that these kinds of efforts are wasted.



We do not yet know how to define human intelligence. Despite this, there is a strong push from some in the scientific community to quantify it. Attempts to describe human intelligence as an inborn trait rest on models like the IQ and Spearman’s g that have not been proven or agreed upon. Even when these models are used as a default, there is a lack of evidence for a genetic link.

Plomin has suggested that it would be useful to administer genetic tests to children to identify those with verbal learning disabilities and other cognitive disorders. He has further suggested that this type of testing occur before a child enters school in order to intervene appropriately prior to enrollment [33]. Based on the currently available data, these suggestions seem premature. Implementing such policies might well result in discrimination by parents, teachers, employers, and institutions, which, in turn, would be likely to affect performance on achievement tests.

Based on findings from current research, further study on brain plasticity and neural development, improvements in learning environments and teaching techniques, and policies which emphasize support for disadvantaged populations are likely to yield more positive outcomes in school achievement. Contrary to what the authors of The Bell Curve suggest, key social policies have demonstrated strong positive effects on IQ scores, particularly for disadvantaged groups.

Unfortunately however, a new educational climate is forming.  Increases in all kinds of academic testing have overtaken more balanced approaches to learning. Possibly the largest shift occurred in 2001 with the “No Child Left Behind [34]” Act implemented by the Bush Administration. Research has shown that self-esteem directly affects motivation to learn.  We will continue to see that performance on IQ and other standardized tests has an effect on the way students are treated and on their self esteem. Low scores affect the attitudes of teachers and other adults as well, so that this approach may ensure that students who struggle with testing will end up in classrooms and categories in which they won’t be expected to improve. In this way, performance on tests, rather than helping children to learn and improve, can become self-fulfilling prophecies of failure for both the children and the adults who are expected to teach them.



1 Retrieved on March 2, 2006, at:

2 Intelligence: Knowns and Unknowns. Board of Scientific Affairs, American PsychologicalAssociation. Released August 7, 1995.

3 Sternberg, Robert J. and Douglas K. Detterman. (Eds.). 1986. What is intelligence? Contemporary viewpoints on its nature and definition. Norwood, NJ: Ablex.

4 Herrnstein, Richard J. and Charles Murray. 1994. The bell curve: the reshaping of American life by difference in intelligence. New York: Free Press.

5 Affirmative Action information Retrieved on 02 Mar. 2006 at:

6 Head Start Program information retrieved from the United States Department of Health and Human Services’ Administration for Children and Families website on 02 March, 2006 at:

7 Gardner, Howard. 1983. Frames of mind: the theory of multiple intelligences. New York: Basic Books.

8 Sternberg, Robert J. 1986. The triarchic mind:  a new theory of human intelligence. New York: Viking.

9 Goleman, Daniel. 1998. Working with emotional intelligence. New York:  Bantam Books.

10 Binet, Alfred and Theodore Simon. 1912. A method of measuring the development of the intelligence in young children. Lincoln, IL: Courier Company.

11 Gould, Steven J. 1981/1996. The  Mismeasure of Man: the definitive refutation to the argument of ‘the bell curve.’  New York, NY: W.W. Norton & Company.

12 Terman, Lewis, et al. 1917. The Sanford revision extension of the Binet-Simon scale for measuring intelligence. Baltimore, MD: Warwick and York.

13 Spearman, C. 1904. General intelligence objectively determined and measured. American Journal of Psychology. 15: 201-293.

14 Skoyles, J. R. 1999. Human evolution expanded brains to increase expertise capacity, not IQ. Psycoloquy. 10:

15 Battro, Antonio. 2001. Half a brain is enough: the story of Nico. New York, NY: Cambridge University Press.

16 Dorman, C.  1991. Microcephaly and Intelligence. Developmental Medicine and ChildNeurology. 33(3): 267-69.

17 Plomin, R. 2003. Genetics, genes, genomics, and g (guest editorial). Molecular Psychiatry. 8: 1-5.

18 Plomin, R. 2004. Intelligence: genetics, genes, and genomics. Journal of Personality and Social Psychology. 86(1): 112-129.

19 Pam, A.; S. Kemker; C. Ross; and R. Golden. 1996. The equal environments assumption in MZ-DZ twin comparisons: an untenable premise of psychiatric genetics? Acta GeneticaeMedicae et gemellologiae: Twin Research. 45(3): 349-360.

20 Turkheimer, E.; A. Haley; M. Waldron; B. D’Onofrio; and I. Gottesman. 2003. Socioeconomic status modifies heritability of IQ in young children. Psychological Science. 14(6): 623-628.

21 Flynn, J.R. (1981). “The mean IQ of Americans: Massive gains 1932 to 1978.” Psychological Bulletin, Vol. 95, pp. 29-51.

22 “Intelligence: Knowns and Unknowns.” Board of Scientific Affairs, American Psychological Association. Released August 7, 1995.

23 Lee, E.S. (1951). “Negro intelligence and selective migration:  A Philadelphia test of the Klineberg hypothesis.” American Sociological Review. Vol. 16, pp. 227-232.

24 Ceci, S. J. 1991. How much does schooling influence general intelligence and its cognitive components? A reassessment of the evidence. Developmental Psychology. 27:  703-722.

25 Consortium for Longitudinal Studies. 1983. As the twig is bent…lasting effects of preschool programs. Hillsdale, NJ: Erlbaum.

26 Pollitt, E.; K. S. German; P. L. Engle. R. Martorell, and J. Rivera. 1993. Early supplementary feeding and cognition. Monographs of the Society for Research in ChildDevelopment. 58(235): 1-99.

27 McMichael, A.J.; P. A. Baghurst; N. R. Wigg; G. Vimpani; E. Robertson; and R. Roberts. 1988. Port Pirie cohort study: environmental exposure to lead and children’s abilities at the age of four years. New England Journal of Medicine. 319: 468-75.

28 Streissguth, A.; H. Barr; P. Sampson; B. Darby; and D. Martin. 1989. IQ at age 4 in relation to maternal alcohol use and smoking during pregnancy. Developmental Psychology. 25: 3-11.

29 Intelligence: Knowns and Unknowns. Board of Scientific Affairs, AmericanPsychological Association. Released August 7, 1995.

30 Rowe, D.; K. Jacobson; and E. Van den Oord. 1999. Genetic and environmental influences on vocabulary IQ: Parental education level as moderator. Child Development. 70: 1151-1162.

31 Flynn, J. R. 1981. The mean IQ of Americans: massive gains 1932 to 1978. Psychological Bulletin. 95: 29-51.

32 Grissmer, D.; S. Kirby, M. Berends; and S. Williamson. 1994. Student achievement and the changing American family. Santa Monica, CA: RAND Corporation.

33 Plomin R. (Ed), J. C. Defries; G. E.  McClearn; and P. McGuffin. 2000. Behavioral Genetics. American Psychological Association Publishers.

34 Retrieved on March 2, 2006 from:

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