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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.

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Corresponding author

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


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