Skip to main content Accessibility help

The Wilson Effect: The Increase in Heritability of IQ With Age

  • Thomas J. Bouchard (a1)


Ronald Wilson presented the first clear and compelling evidence that the heritability of IQ increases with age. We propose to call the phenomenon ‘The Wilson Effect’ and we document the effect diagrammatically with key twin and adoption studies, including twins reared apart, that have been carried out at various ages and in a large number of different settings. The results show that the heritability of IQ reaches an asymptote at about 0.80 at 18–20 years of age and continuing at that level well into adulthood. In the aggregate, the studies also confirm that shared environmental influence decreases across age, approximating about 0.10 at 18–20 years of age and continuing at that level into adulthood. These conclusions apply to the Westernized industrial democracies in which most of the studies have been carried out.

  • View HTML
    • Send article to Kindle

      To send this article to your Kindle, first ensure is added to your Approved Personal Document E-mail List under your Personal Document Settings on the Manage Your Content and Devices page of your Amazon account. Then enter the ‘name’ part of your Kindle email address below. Find out more about sending to your Kindle. Find out more about sending to your Kindle.

      Note you can select to send to either the or variations. ‘’ emails are free but can only be sent to your device when it is connected to wi-fi. ‘’ emails can be delivered even when you are not connected to wi-fi, but note that service fees apply.

      Find out more about the Kindle Personal Document Service.

      The Wilson Effect: The Increase in Heritability of IQ With Age
      Available formats

      Send article to Dropbox

      To send this article to your Dropbox account, please select one or more formats and confirm that you agree to abide by our usage policies. If this is the first time you use this feature, you will be asked to authorise Cambridge Core to connect with your <service> account. Find out more about sending content to Dropbox.

      The Wilson Effect: The Increase in Heritability of IQ With Age
      Available formats

      Send article to Google Drive

      To send this article to your Google Drive account, please select one or more formats and confirm that you agree to abide by our usage policies. If this is the first time you use this feature, you will be asked to authorise Cambridge Core to connect with your <service> account. Find out more about sending content to Google Drive.

      The Wilson Effect: The Increase in Heritability of IQ With Age
      Available formats


Corresponding author

address for correspondence: Thomas J. Bouchard Jr., 280 Storm Peak Court, Steamboat Springs, CO 80487, USA. E-mail:


Hide All
Benyamin, B., Wilson, V., Whalley, L. J., Visscher, P. M., & Deary, I. J. (2005). Large, consistent estimates of the heritability of cognitive ability in two entire populations of 11-year-old twins from Scottish Mental Surveys of 1932 and 1947. Behavior Genetics, 35, 525534.
Bergen, S. E., Gardner, C. O., & Kendler, K. S. (2007). Age-related changes in heritability of behavioral phenotypes over adolescence and young adulthood: A meta-analysis. Twin Research and Human Genetics, 10, 423433.
Bickley, P. G., Keith, T. Z., & Wolfe, L. E. (1995). The three-stratum theory of cognitive abilities: Test of the structure of intelligence across the life span. Intelligence, 20, 309328.
Boomsma, D. I., Busjahn, A., & Peltonen, L. (2002). Classical twin studies and beyond. Nature Reviews: Genetics, 3, 872882.
Bouchard, T. J. Jr. (1993). The genetic architecture of human intelligence. In Vernon, P. A. (Ed.), Biological approaches to the study of human intelligence (pp. 3393). Norwood, NJ: Ablex.
Bouchard, T. J. Jr. (1995). Longitudinal studies of personality and Intelligence: A behavior genetic and evolutionary psychology perspective. In Saklofske, D. H. & Zeidner, M. (Eds.), International handbook of personality and intelligence (pp. 81106). New York: Plenum.
Bouchard, T. J. Jr. (1996). Behavior genetic studies of intelligence, yesterday and today: The long journey from plausibility to proof — The Galton Lecture. Journal of Biosocial Science, 28, 527555.
Bouchard, T. J. Jr. (1997). Experience producing drive theory: How genes drive experience and shape personality. Acta Paediatrica, 86(Suppl. 422), 6064.
Bouchard, T. J. Jr. (2009). Genetic influence on human intelligence (Spearman's g): How much? Annals of Human Biology, 36, 527544.
Bouchard, T. J. Jr., Lykken, D. T., Tellegen, A., & McGue, M. (1996). Genes, drives, environment and experience: EPD theory — Revised. In Benbow, C. P. & Lubinski, D. (Eds.), Intellectual talent: Psychometrics and social issues (pp. 543). Baltimore, MD: John Hopkins University Press.
Bouchard, T. J Jr., & McGue, M. (1981). Familial studies of intelligence: A review. Science, 212, 10551059.
Bouchard, T. J. Jr., & McGue, M. (2003). Genetic and environmental influences on human psychological differences. Journal of Neurobiology, 54, 445.
Bouchard, T. J. Jr., & Segal, N. L. (1985). Environment and IQ. In Wolman, B. J. (Ed.), Handbook of intelligence: Theories, measurements, and applications (pp. 391464). New York: Wiley.
Bratko, D., Butkovic, A., & Chamorro-Premuzic, T. (2010). The genetics of general knowledge: A twin study from Croatia. Personality and Individual Differences, 48, 403407.
Burks, B. S. (1928). The relative influence of nature and nurture upon mental development: A comparative study of foster parent-offspring child resemblance and true parent-true child resemblance. Yearbook of the National Society for the Study of Education, 27, 219316.
Burks, B. S. (1938). On the relative contributions of nature and nurture to average group differences in intelligence. Proceedings of the National Academy of Sciences, 24, 276282.
Calvin, M. C., Deary, I. J., Webbink, D., Smith, P. B., Fernandes, C., Lee, S. H., … Visscher, P. M. (2012). Multivariate genetic analyses of cognition and academic achievement from two population samples of 174,000 and 166,000 school children. Behavior Genetics, 42, 699710.
Davis, O. S. P., Haworth, C. M. A., & Plomin, R. (2009). Dramatic increase in heritability of cognitive development from early to middle childhood: An 8-year longitudinal study of 8,700 pairs of twins. Psychological Science, 20, 13011308.
De Bellis, M. D., Keshavan, M. S., Beers, S. R., Hall, J., Frustaci, K., Masalehdan, A., … Boring, A. M. (2001). Sex differences in brain maturation during childhood and adolescence. Cerebral Cortex, 11, 552557.
Deary, I. J. (2012). Intelligence. Annual Review of Psychology, 63, 453482.
Devlin, B., Daniels, M., & Roeder, K. (1997). The heritability of IQ. Nature, 388, 468471.
Erlenmeyer-Kimling, L., & Jarvick, L. F. (1963). Genetics and intelligence: A review. Science, 142, 14771479.
Finkel, D., Pedersen, N. L., McGue, M., & McClearn, G. E. (1995). Heritability of cognitive abilities in adult twins: Comparison of Minnesota and Swedish Data. Behavior Genetics, 25, 421431.
Giedd, J. N., Blumenthal, J., Jeffries, N. O., Castellanos, F. X., Liu, H., Zijdenbos, A., … Rapoport, J. L. (1999). Brain development during childhood and adolescence: A longitudinal MRI study. Nature Neuroscience, 2, 861863.
Gould, S. J. (1981). The mismeasure of man. New York: W. W. Norton.
Gould, S. J. (1996). The mismeasure of man (2nd ed.). New York: W. W. Norton.
Haworth, C. M. A., Wright, M. J., Luciano, M., Martin, N. G., de Geus, E. J. C., van Beijsterveldt, C. E. M., … Plomin, R. (2010). The heritability of general cognitive ability increases linearly from childhood to young adulthood. Molecular Psychiatry, 15, 11121120.
Hayes, K. J. (1962). Genes, drives, and intellect. Psychological Reports, 10, 299342.
Horn, J. M., & Loehlin, J. C. (2010). Heredity and environment in 300 adoptive families: The Texas Adoption Project. New Brunswick, NJ: AldineTransaction.
Johnson, W. (2010a). Extending and testing Tom Bouchard's experience producing drive theory. Personality and Individual Differences, 49, 296301.
Johnson, W. (2010b). Understanding the genetics of intelligence: Can height help? Can corn oil? Current Directions in Psychological Science, 19, 177182.
Johnson, W., McGue, M., & Deary, I. J. (in press). Normative cognitive aging. In Finkel, D. & Reunolds, C. A. (Eds.), Behavior genetics of cognition. New York: Springer.
Johnson, W., McGue, M., & Iacono, W. G. (2005). Disruptive behavior and school grades: Genetic and environmental relations in 11-year-olds. Journal of Educational Psychology, 97, 391405.
Johnson, W., Nijenhuis, J. T., & Bouchard, T. J. Jr. (2008). Still just 1 g: Consistent results from five test batteries. Intelligence, 36, 8195.
Kamin, L. J. (1974). The science and politics of IQ. Potomac, MD: Erlbaum.
Karama, S., Colom, R., Johnson, W., Deary, I. J., Haier, R., Waber, D. P., … Evans, A. C. (2011). Cortical thickness correlates of specific cognitive performance accounted for by the general factor of intelligence in healthy children age 6 to 18. NeuroImage, 55, 14431453.
Leahy, A. M. (1935). Nature-nurture and intelligence. Genetic Psychology Monographs, 17, 241305.
Lee, T., Henry, J. D., Trollor, J. N., & Sachdev, P. S. (2010). Genetic influences on cognitive functions in the elderly: A selective review of twin studies. Brain Research Reviews, 64, 113.
Lenroot, R. K., Schmitt, J. E., Ordaz, S. J., Wallace, G. L., Neale, M. C., Lerch, J. P., … Giedd, J. N. (2009). Differences in genetic and environmental influences on the human cerebral cortex associated with development during childhood and adolescence. Human Brain Mapping, 30, 163174.
Lessov-Schlaggar, C. N., Hardin, J., DeCarli, C., Krasnow, R. E., Reed, T., Wolf, P. A., … Carmelli, D. (2012). Longitudinal genetic analysis of brain volumes in normal elderly male twins. Neurobiology of Aging, 33, 636644.
Lewontin, R. C., Rose, S., & Kamin, L. J. (1984). Not in our genes: Biology ideology, and human nature. New York: Pantheon.
Malykh, S. B., Iskoldsky, N. V., & Gindina, E. D. (2005). Genetic analysis of IQ in young adulthood: A Russian twin study. Personality and Individual Differences, 38, 14751485.
Martin, N. G., Eaves, L. J., Heath, A. C., Jardine, R., Feingold, L. M., & Eysenck, H. J. (1986). Transmission of social attitudes. Proceedings of the National Academy of Sciences USA, 83, 43644368.
Matheny, A. (1990). Developmental behavior genetics: Contributions from the Louisville Twin Study. In Hahn, M. E., Hewitt, J. K., Henderson, N. D. & Benno, R. H. (Eds.), Developmental behavior genetics: Neural, biometrical, and evolutionary approaches. Oxford: Oxford University Press.
McClearn, G. E., Johanson, B., Berg, S., Pedersen, N. L., Ahern, F., Petrill, S. A., & Plomin, R. (1997). Substantial genetic influence on cognitive abilities in twins 80 or more years old. Science, 276, 15601563.
Moffitt, T. E., Caspi, A., Harkness, A. R., & Silva, P. A. (1993). The natural history of change in intellectual performance: Who changes? How much? Is it meaningful? Journal of Child Psychology and Psychiatry, 34, 455506.
Nisbet, R. E., Aronson, J., Blair, C., Dickens, W., Flynn, J., Halpern, D. F., & Turkheimer, E. (2012). Intelligence: New findings and theoretical developments. American Psychologist, 62, 130159.
Petrill, S. A., & Deater-Deckard, K. (2004). The heritability of general cognitive ability: A within-family adoption design. Intelligence, 32, 403409.
Plomin, R. (1987). Obituary — Ronald S. Wilson (1933–1986). Behavior Genetics, 17, 211217.
Plomin, R., & DeFries, J. C. (1980). Genetics and intelligence: Recent data. Intelligence, 4, 1524.
Plomin, R., Fulker, D. W., Corley, R., & DeFries, J. C. (1997). Nature, nurture and cognitive development from 1 to 16 years: A parent-offspring adoption study. Psychological Science, 8, 442447.
Plug, E., & Vijverberg, W. (2003). Schooling, family background, and adoption: Is it nature or is it nurture? Journal of Political Economy, 111, 611641.
Rabbitt, P., & Lunn, M. (2008). Death, dropout, and longitudinal measurements of cognitive change in old age. Journals of Gerontology Series B — Psychological Sciences and Social Sciences, 63, 271278.
Raz, N., Gunning-Dixon, F., Head, D., Rodrique, K. M., Williamson, A., & Acker, J. D. (2004). Aging, sexual dimorphism, and hemispheric asymmetry of the cerebral cortex: Replicability of regional differences in volume. Neurobiology of Aging, 25, 377396.
Reynolds, C. A., Finkel, D., McArdle, J. J., Gatz, M., Berg, S., & Pedersen, N. L. (2005). Quantitative genetic analysis of latent growth curve models of cognitive abilities in adulthood. Developmental Psychology, 41, 316.
Scarr, S. (1996). How people make their own environments: Implications for parents and policy makers. Psychology, Public Policy, and Law, 2, 204228.
Scarr, S. (1997). Behavior genetic and socialization theories of intelligence: Truce and reconciliation. In Sternberg, R. J. & Grigorenko, E. L. (Eds.), Intelligence: Heredity and environment (pp. 341). New York: Cambridge University Press.
Scarr, S., & McCartney, K. (1983). How people make their own environments: A theory of genotype — environment effects. Child Development, 54, 424435.
Scarr, S., & Weinberg, R. A. (1978). The influence of ‘family background’ on intellectual attainment. American Sociological Review, 43, 674692.
Schofield, W. (1986). Psychotherapy: The purchase of friendship. New Brunswick, NJ: Transaction Books.
Scott, J. P. (1990). Foreword. In Hahn, M. E., Hewitt, J. K., Henderson, N. D. & Benno, R. H. (Eds.), Developmental behavior genetics: Neural, biometrical, and evolutionary approaches. New York: Oxford.
Scott, J. P., & Fuller, J. L. (1965). Genetics and the social behavior of the dog. Chicago: University of Chicago Press.
Segal, N. L., & Johnson, W. (2009). Twin studies of general mental ability. In Kim, Y.-K. (Ed.), Handbook of behavior genetics (pp. 81100). New York: Springer.
Segal, N. L., McGuire, S. A., & Stohs, J. H. (2012). What virtual twins reveal about general intelligence and other behaviors. Personality and Individual Differences, 53, 405410.
Sundet, J. M., Tambs, K., Harris, J. R., Magnus, P., & Torjussen, T. M. (2005). Resolving the genetic and environmental sources of the correlation between height and intelligence: A study of nearly 2600 Norwegian male twin pairs. Twin Research and Human Genetics, 8, 307311.
Sundet, J. M., Tambs, K., Magnus, P., & Berg, K. (1988). On the question of secular trends in the heritability of intelligence test scores: A study of Norwegian twins. Intelligence, 12, 4759.
Teasdale, T. W., & Owen, D. R. (1984). Heritability and familial environment in intelligence and educational level—a sibling study. Nature, 309, 620622.
Vinkhuyzen, A. A. E., van der Sluis, S., Maes, H. H., & Posthuma, D. (2012). Reconsidering the heritability of intelligence in adulthood: Taking assortative mating and cultural transmission into account. Behavior Genetics, 42, 187198.
Visscher, P. M., Hill, W. G., & Wray, N. R. (2008). Heritability in the genomic era — Concepts and misconceptions. Nature Reviews Genetics, 9, 255266.
Wilson, R. S. (Ed.). (1977). Mental development in twins. Amsterdam: Elsevier.
Wilson, R. S. (1978). Syncronies in mental development: An epigenetic perspective. Science, 202, 939948.
Wilson, R. S. (1983). The Louisville twin study: Developmental synchronies in behavior. Child Development, 54, 298316.



Altmetric attention score

Full text views

Total number of HTML views: 0
Total number of PDF views: 0 *
Loading metrics...

Abstract views

Total abstract views: 0 *
Loading metrics...

* Views captured on Cambridge Core between <date>. This data will be updated every 24 hours.

Usage data cannot currently be displayed