Brief on Genetic Determinism and Intelligence 

I.Overview of Intelligence and Genetics

II. Intelligence Testing

III. Information Related to Spearman's

IV. Finding Genes Correlated with Intelligence

V. Brain Size, Genetics, and Intelligence

VI. Brain Maps, Genetics and Intelligence

VII. Genetic and Environmental Determinants

A. Birth Effects 
B. Socioeconomic Status
C. Shared Environmental Influences 
D. Varying Contributions of Genes and the Environment with Aging 
E. Working Memory and Proessing Speed
F. Between Group Differences in Intelligence

VIII. Alternative Theories of Human Intelligence

I. Overview of Intelligence and Genetics

Deary I. J.  2001. Human intelligence differences: a recent history. Trends Cognitive Science, 5(1): 127-30.

  • This article provides a summary regarding what is known about human intelligence.  It gives a brief summary of the following:  the known and unknowns about human intelligence differences, the taxonomy of psychometric intelligence differences, the stability and aging of psychometric intelligence differences, the environmental and genetic influences on psychometric intelligence differences, the predictive validity of psychometric intelligence differences, and understanding intelligence differences.

Gould, Steven J. 1996. The mismeasure of man: the definitive refutation to the argument of The bell curve. New York, NY: W.W. Norton and Company.

  • This book argues against between-group differences in intelligence and IQ presented by the authors of The Bell Curve.

Grissmer, David W., Sheila Kirby, Mark Berends, and Stephanie Williamson. 1994. Student achievement and the changing American family. Santa Monica, CA: RAND Corporation.

Lubinski, D.  2004. Introduction to the special section on cognitive abilities:  100 years after Spearman’s (1904)  ‘General intelligence,’ objectively determined and measured.Journal of Personality and Social Psychology. 86(1):  96-112.

  • This study discusses cognitive abilities, their hierarchical organization, Spearman’s g as well as the various ways that cognitive abilities are critical in models of human behavior and important life outcomes.

Newson, A., and R. Williamson. 1999. Should we undertake genetic research on intelligence? Bioethics. 13(3):  327-42.

  • This paper is based on the ethical concerns about performing research on the genetic basis of intelligence.  It goes into the areas of the role of genetic information, the value placed on intelligence, and the allocation of resources.

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

  • This editorial discusses the importance of studying the relationship between intelligence and genetics.  The concept of g and its effect on cognition and social outcomes is stressed.

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

  • This review article discusses current genetic findings related to intelligence as well as the need for future research.  Two findings discussed are the increasing heritability of intelligence with age and how genes affect diverse cognitive capabilities. Future research should involve the findings of the Human Genome Project and functional genomics in order to locate brain pathways between genes and intelligence.

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

Neisser, U; G. Boodoo; T. J. Bouchard; W. A. Boykin; N. Brody; S. J. Ceci; D. F. Halpern; J. C. Loehlin; R. Perloff; R. J. Sternberg; and S. Urbina. 1996.  Intelligence: knowns and unknowns.  American Psychologist. 51(2): 77-101.


II.  Intelligence Testing

Begley, S.  Good genes may count, but many factors make up a high IQ. 2003. Wall Street Journal. 241(120): B1.

  • This is an overview of various factors that are involved in determining intelligence and IQ.  It introduces the possibility that 50 % of the variation in memory, verbal and spatial abilities, and abstract reasoning can be accounted for by genetic differences.  Furthermore, the article talks about the relationship between heretibility and the environment as well as Robert Plomin’s attempts to find genes correlated with high IQs.

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

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


III.  Information Related to Spearman’s g

Duncan, J;  J. Rudiger; J., Kolodny; D. Bor; H. Herzog; A. Ahmed; F. Newell; and H. Emslie. 2001. A neural basis for general intelligence. Science. 289(5478):  457-60.

  • This is a very technical paper that provides data to suggest that Spearman’s g reflects the function of a specific neural system including a specific region of the lateral frontal cortex.

Spinath F. M; A. Ronald; N. Harlaar; T. Price; and R.  Plomin. 2003. Phenotypic g early in life:  On the etiology of general cognitive ability in a large population of twin children aged 2-4 years. Journal of Child Psychol Psychiatry.  31(2):  195-211.

  • This study showed through principal component factor analysis that 50-55% of the total variance in cognitive ability was accounted for by g.  It showed that g is evident early in life; the genetic influence is less in early childhood (20-30%) than in middle childhood (40%) than after adolescence (50%).  It also showed that a shared environment is more important in early childhood than after adolescence.


IV.  Finding Genes Correlated with Intelligence

Bearden, C. E.; T. G. Van Erp; J. R. Monterosso; T. J. Simon; D. C. Glahn; P. A. Saleh; N. M. Hill; D. M. McDonald-McGinn; E. Zackai; B. S. Emanuel; and T. D. Cannon. 2004. Regional brain abnormalities in 22q11.2 deletion syndrome:  association with cognitive abilities and behavioral symptoms. Neurocase. 10(3):  198-206.

  • This study observed the neurocognitive and behavioral functions in children with 22q11.2 deletions to examine regional brain abnormalities.  The study performed on 13 cases and 9 controls showed that children with the deletion showed a 4.3% (not significant) decrease in total brain volume as compared to controls.  Furthermore cases had reduction in white matter, increased cerebrospinal fluid.  In addition, the authors found that in cases, but not controls, bilateral temporal gray and white matter volumes were predictors of cognitive performance (significant).

Correia, H. R.; S. C. Balseiro, and M. L. de Areia. 2005. Are genes of human intelligence related to the metabolism of thyroid and steroid hormones? Endocrine changes may explain human evolution and higher intelligence. Med Hypotheses. 65(6):  1016-1023.

  • The authors propose that steroid hormones influence the genes for human intelligence.  Evolutionary changes are the basis for the argument.  Furthermore, the authors propose that neuroactive hormone therapy might provide significant improvement in cognitive deficits and neurodegenerative diseases in all stages of life.

Futamura, T.; A. Kakita; M. Tohmi; H. Sotoyama; H. Takahashi; and  H.  Nawa. 2003. Neonatal perturbation of neurotrophic signaling results in abnormal sensorimotor gating and social interaction in adults:  implication for epidermal growth factor in cognitive development. Molecular Psychiatry.  8(1):  19-29.

  • This study was performed on rats and suggested that epidermal growth factor receptor (EGFR) stimulation abnormalities in the neonatal stage can result in cognitive and behavioral dysfunctions in adulthood.  This research is important because schizophrenic patients have both EGF and EGFR abnormalities.

Malaspina, D.; A. Reichenberg; M. Weiser; S. Fennig; M. Davidson; S. Harlap; R. Wolitzky; J. Rabinowitz; E. Susser; and  H. Y.  Knobler. 2005. Paternal age and intelligence:  implications for age-related genomic changes in male germ cells. Psychiatric Genetics 15(2):  117-25.

  • The authors propose that genetic changes in germ cells of older men possibly underlie the association between advancing paternal age and schizophrenia.  This study showed that overall paternal age accounted for 2% of variance in IQ scores of 44175 Israeli children. 

Morley, K. I., and G. W. Montgomery. 2001. The genetics of cognitive processes:  candidate genes in humans and animals. Behavioral Genetics. 31(6):  511-31.

  • This article is based on an extensive literature search that was performed to identify candidate genes contributing to cognition.  The literature search included papers from 1986-2001.  From the search, 150 candidate genes were identified and 76 genes implicated in human cognitive processes were found.  Table 4 of the paper shows a list of the strongest candidate genes for cognition along with their associated phenotypes.

Payton, A.; F. Holland; P. Diggle; P. Rabbitt; M. Horan; Y. Davidson; L. Gibbons; J. Worthington; W. E. R. Ollier; and N. Pendleton. 2003. Cathepsin-D exon 2 polymorphism associated with general intelligence in a healthy older population. Molecular Psychiatry. 8(1):  14-18.

  • It has been previously shown that there are 4 quantitative trait loci (QTL) that influence general intelligence.  These include two in the IGFR and the Msx1 homeobox.  This study presents the findings that an additional QTL has been identified in exon 2 of the Cathepsin 2 gene.

Plomin, R.; L. Hill;  I. W. Craig; P. McGuffin; S.  Purcell; P.  Sham; D.  Lubinski; L. A. Thompson; P. J. Fisher; D. Turic; and M. J. Owen. 2001.  A genome-wide scan of 1842 DNA markers for allelic associations with general cognitive ability:  a five-stage design using DNA pooling and extreme selected groups. Behavior Genetics. 31(6):  497-509.

  • The goal of this study was to identify quantitative trait loci that were connected to high g as compared to average g.  The design was to use simple sequence repeats (SSRs) in people with a range of IQ scores.  The results of the study showed that power decreased significantly near the SSRs.  It suggests an alternative way to perform the study using SNPs. 

Posthuma, D.; M. Luciano; E. J. Geus; M. J. Wright; P. E. Slagboom; G. Montgomery; D.  I.  Boomsma; and N. G. Martin. 2005.  A genome wide scan for intelligence identifies quantitative trait loci on 2q and 6p.  American Journal of Human Genetics. 77(2):  318-26.

  • This study presents a genome wide scan involving 634 sibling pairs to identify chromosomal regions that explain variation in IQ.  Quantitative trait loci (QTL) 2q and 6p were identified.  It has been previously shown that the 2q region is associated with autism while the 6p region is associated with reading disability and dyslexia.

Seydel, C.  2003. Brainy genes boost IQ. Science Now.  214(1):  2.  

  • This brief article comments on the findings of two genes associated with IQ.  Tony Payton looked at genes associated with the mental decline in Alzheimer’s disease.  He identified mutations in the cathepsin D gene (CTSD).  David Comings looked at mutations in the CHRM gene and found individuals possessing the mutation to have lower IQ scores. 


V. Brain Size, Genetics, and Intelligence

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

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

Kouprina, N.; A. Pavlicek; G. H. Mochida; G. Solomon; W. Gersch; Y. Yoon; R. Collura; M. Ruvolo; J. C. Barrett; G. Woods; C. A. Walsh; J. Jurka; and V. Larionov. 2004. Accelerated evolution of the ASPM gene controlling brain size begins prior to human brain expansion. PloS Biology. 2(5):  126-43.

  • This study looked at sequences of the ASPM gene in chimpanzees, orangutan, and rhesus macaque.  The results show that large portions of the gene are conserved between species.  However, there were segments that were varied and were consistent with positive selection thereby promoting evolutionary change.  The researchers suggest that this positive selection in the ASPM gene correlates to differences in cerebral cortex size.

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

Tan U.; M. Tan; P. Polat; Y. Ceylan; S. Suma; and A. Okur.  1999. Magnetic resonance imaging brain size/IQ relations in Turkish university students. Intelligence. 27(1):  83-92.

  • This study focused on IQ and brain size in 103 right and left-handed men and women.  There was a correlation of 40 between total area and IQ.  Men showed anterior cerebral area to be correlated with IQ while women had total and posterior cerebral areas correlated with IQ.  Handedness did not yield significant relations to IQ in this study.

Wade, N. September 8, 2005. Researchers say human brain is still evolving. The New York TimesNew York Edition.


VI. Brain Maps, Genetics, and Intelligence

Thompson, P.; T. Cannon; K. Narr; T. Van Erp; V. P. Poutanen; M. Huttunen; J. Lonnqvist; C. G. Standertskjold-Nordenstam; J. Kaprio; M. Khaledy; R. Dail; C. Zoumalan; and A. Toga. 2001. Genetic influences on brain structure. Nature Neuroscience. 4(12):  1253-58.

  • This report on 3-D brain maps shows how genes may determine differences in cognitive and linguistic skills.  Additionally, it proposes a potential genetic source for diseases that affect the human cortex.

Toga, A., and P. M. Thompson. 2005. Genetics of brain structure and intelligence. Annual Reviews Neuroscience. 28(1):  1-23.

  • This review showcases studies that show complex cortical patterns associated with cognitive ability and the heritability of g.  The authors hypothesize that these genetic links are under the control of the brain, which is in turn under genetic control.  The authors believe that genetics predisposition is the major determinant of these factors.


VII. Genetic and Environmental Determinants

A.  Birth Effects

Jacobs, N.; S. Van Gestel; C. Derom; E. Thiery; P. Vernon; R. Derom; and R. Vlietinck.  2001. Heritability estimates of intelligence in twins:  effect of chorion type. Behavior Genetics. 31(2):  209-17.

  • This study examines differences in IQ based on the timing of zygotic splitting in dichorionic and monochorionic twin pairs.  The results show that the monochorionic twins resembled each other more than dichorionic monozygotic twins.

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.

Luciano, M.; M. J. Wright; and N. G. Martin.  2004. Exploring the etiology of the association between birth weight and IQ in an adolescent twin sample. Twin Research  7(1):  62-71.

  • This twin study explores the etiology of the association between low birth weight and intellectual development. The authors found that genetic variance in birth weight overlapped with that of verbal IQ but not full IQ.  The results also showed the importance of shared environmental effects on birth weight (~60%).


B.  Socioeconomic Status

Petrill, S. A.; A. Pike; T.  Price; and R. Plomin. 2004. Chaos in the home and socioeconomic status are associated with cognitive development in early childhood:  environmental mediators identified in a genetic design.  Intelligence. 32(5):  445-61.

  • This study shows that socioeconomic status and chaos mediate a modestly significant portion of the shared environment in influencing verbal and nonverbal skills in children aged 3 and 4.

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.

Turkheimer, E.; A. Haley; M. Waldron; B. D’Onfrio; and I. Gottesman. 2003.  Socioeconomic status modifies heritability of IQ in young children.  Psychological Science. 14(6):  623-8.

  • The authors suggest that in economically impoverished families, 60% of the variation in IQ is a result of shared environment and that the contribution of genes is essentially 0.  Yet, the opposite results are true for affluent families.  Weschler Intelligence Scale scores were obtained for 7-year-old twins.  Many of the twins were raised in families that were at or below the poverty level.  Modeling was done and it showed the proportions of IQ variance attributable to genes and the environment vary nonlinearly with socioeconomic status. 

Walker, S. O.;  S. A. Petrill, and R.  Plomin. 2005. A genetically sensitive investigation of the effects of the school environment and socio-economic status on academic achievement in seven-year-olds. Educational Psychology. 25(1):  55-74.

  • The authors of this study of same-sex mono and dizygotic twins offer that, exclusive of genetic influence, shared environment accounts for 12% of the variance in academic achievement while non-shared environment accounts for 19% of the variance in academic achievement.


C.  Shared Environmental Influences

Jacobs, N.; F. Rijskijk; C. Derom; M. Danckaerts; E. Thiery; R. Derom; R. Vlietinck; and J. Van Os. 2002. Child psychopathology and lower cognitive ability:  a general population twin study of the causes of association.  Molecular Psychiatry. 7(1):  368-74.

  • This paper examines the association between cognitive ability and child psychopathology.  The three possible causes of the association are described as additive genetic factors, common environmental factors, and individual-specific environmental factors.  A twin study was done looking at these associations.  The results suggest that most of the environmental factors that do influence cognitive ability do not influence child psychopathology.

McMichael, A. J.; P. A.. Baghurst; N. R. Wigg; G. Vimpani; E. Robertson; and R. Roberts; 1998. 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.

Nakao, K.; J. Takaishi; K. Tatsuta; H. Katayama; M. Iwase; K. Yorifuji; and M. Takeda. 2000. The influences of family environment on personality traits.  Psychiatry Clin Neurosci.54(1):  91-5.

  • This study was performed in an attempt to clarify the influences of family environment on the development of personality traits.  Interviews were conducted on 150 school children, and the traits examined were extraversion, maturity, and intellect.  Analysis showed 8% of variance in extraversion, 14% in maturity, and 10% in intellect was due to family environment.

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.

Rietveld, M. J.; G. C. Van Baal; C. V. Dolan; and D. I.  Boomsa. 2000. Genetic Factor Analyses of Specific Cognitive Abilities in 5-Year-Old Dutch Children. Behavioral Genetics. 30(1):  29-40.

  • This study analyzed the genetic and environmental factor structures of intellectual abilities in 209 twin pairs.  Six subtests of a Dutch intelligence test (RAKIT) looked at both verbal and nonverbal factors.  A shared environment had a single common factor structure.  Heritability estimates ranged from 15-65%.

Sundet, J. M.; K. Tambs, J. R. Harris; P. Magnus; and T. M. Torjussen. 2005. Resolving the genetic and environmental sources of the correlation between height and intelligence:  a study of nearly 2600 Norwegian male twin pairs. Twin Res Hum Genet. 8(4): 307-11.

  • The goal of this study was to divide the correlation between intelligence test scores and standing height into environmental and genetic components.  It showed a major part of the correlation to be related to share environments.  In addition, the study showed statistically significant effects of genes in unshared environments.


D.  Varying Contributions of Genes and the Environment with Aging

Bishop, E. G.;  S. Cherny;  R. Corley; R. Plomin; J. DeFries; and J. K. Hewitt. 2003. Development genetic analysis of general cognitive ability from 1 to 12 years in a sample of adoptees, biologic siblings, and twins. Intelligence. 31(1):  31-50.

  • This study was a longitudinal genetic analysis performed to examine continuity and change in the etiology of cognitive ability from infancy to adolescence.  The study was performed on twins and adopted and non-adopted siblings.  The data suggest that heritability increases and shared environmental influence decreases.

McGue, M. and K. Christensen.  2001. The heritability of cognitive functioning in very old adults:  evidence from Danish twins aged 75 years and older. Psychol Aging. 16(2):  272-80.

  • This same-sex Danish twin study on individuals 75 and older was conducted to examine heritable influences on cognitive function.  Findings suggest that heritability accounted for 26-54% of the variance in 4 cognitive tests that were performed.  The rest of the variance was attributed to other environmental factors.

Petrill, S. A.; P. Lipton; J. K. Hewitt; R. Plomin; S. S. Cherny; R. Corley; and J. C.  DeFries. 2004. Genetic and environmental contributions to general cognitive ability through the first 16 years of life. Developmental Psychology. 40(5):  805-13.

  • This study was conducted to determine the genetic and environmental influences on the development of general cognitive ability through the first 16 years of life.  The study used sibling data from the Colorado Adoption Project.  Modeling showed that genetic factors mediated phenotypic stability throughout the entire period.  Instability between ages appeared to be a result of non-shared environmental influences.


E.  Working Memory and Processing Speed

Ando, J.; Y. Ono; and M. J. Wright. 2001. Genetic structure of spatial and working memory. Behavior Genetics. 31(6):  615-24.

  • Working memory includes storage memory and executive functions that are essential in all forms of cognition.  This study was looking at the genetic structure of storage and executive functions by conducting a twin study using spatial and verbal tasks.  Study results support the relationship between WM and cognitive ability.  In addition, study results support differences in storage and executive functions to be heritable and comparable to cognitive ability.

Finkel, D. and N. L.  Pederson. 2000.  Contribution of age, genes, and environment to the relationship between perceptual speed and cognitive ability. Psychol Aging. 15(1):  56-64.

  • The goal of this study was to examine the genetic influences on cognitive ability in adulthood.  The study specifically was looking at perceptual speed and cognitive aging.  The results showed that 90% of age-related variance in cognitive factor was shared with perceptual speed while 70% of the genetic variance in cognitive factor was shared with perceptual speed.

Posthuma, D.; E. J. Mulder; D. I. Boomsma; and E. J.  de Geus. 2002. Genetic analysis of IQ, processing speed and stimulus-response incongruency effects. Biol Psychol. 61(1-2):  157-82.

  • This twin and sibling study shows that the ability to perform well under conditions of stimulus-response incongruency is a viable phenotype of cognitive ability.  The heritability of processing speed was 33-48%, accuracy was 41%, and stimulus-response incongruency effects were 3-32%.

F.  Between Group Differences in Intelligence

Fraser, Steven. ed. 1995. The bell curve wars: race, intelligence, and the future of America. New York, NY: Basic Books.

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

Sternberg, R.; E. Grigorenko; and K. Kidd. 2005.  Intelligence, race, and genetics. American Psychologist. 60(1):  46-60.

  • This article argues that since the definition of intelligence is unclear, it is impossible to definitely conclude its relation to other constructs, including intelligence.  They further state that there is no gene shown to be linked to intelligence and therefore a genetic link of race to intelligence cannot be made.


VIII. Alternative Theories of Human Intelligence

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

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

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

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

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