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Shared-Environmental Contributions to High Cognitive Ability

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Abstract

Using a combined sample of adolescent twins, biological siblings, and adoptive siblings, we estimated and compared the differential shared-environmentality for high cognitive ability and the shared-environmental variance for the full range of ability during adolescence. Estimates obtained via multiple methods were in the neighborhood of 0.20, and suggest a modest effect of the shared environment on both high and full-range ability. We then examined the association of ability with three measures of the family environment in a subsample of adoptive siblings: parental occupational status, parental education, and disruptive life events. Only parental education showed significant (albeit modest) association with ability in both the biological and adoptive samples. We discuss these results in terms of the need for cognitive-development research to combine genetically sensitive designs and modern statistical methods with broad, thorough environmental measurement.

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Notes

  1. Let us assume a high heritability coefficient for adult IQ (0.8). Let us also assume that spouses select mates for psychometric IQ per se, and that the phenotypic spousal correlation perfectly induces the spousal genetic correlation. The latter two assumptions are generally not true, but they simplify the model, and enable a “worst-case” estimate of the biometric bias induced by assortative mating. Under these assumptions, the assortative mating coefficient will be \( \sqrt {0.8} \times 0.34 \times \sqrt {0.8} \), or 0.27. The expected additive-genetic correlation for DZ twins will therefore be 0.5 × (1 − 0.27), or 0.64. If we attempt to compute the Falconer estimate of narrow-sense heritability in twin data (a 2 = (r MZ − r DZ), which typically assumes no assortative mating) using r DZ = 0.64a 2 + c 2, we discover that the Falconer estimate only yields 0.72a 2, an underestimate of over 25%. The “missing” variance will be consumed by the Falconer shared-environmentality estimate (c 2 = 2r DZ − r MZ).

  2. On the suggestion of an anonymous referee, we conducted post-hoc analyses to investigate whether parental psychopathology predicted offspring IQ. We examined composite indicators of two of the primary phenotypes of interest in SIBS, substance abuse and behavioral disinhibition. As an indicator of substance abuse, we used scores on the first principal component extracted from measures of nicotine dependence (e.g., symptom count, per-day frequency of use), heavy alcohol consumption (e.g., frequency of drinking, largest number of drinks in one day), alcohol abuse and dependence (e.g., withdrawal and tolerance, interference with social and occupational functioning), illicit drug abuse and dependence (e.g., symptom count, number of different drugs used), and frequency of illicit drug use. As an indicator of behavioral disinhibition, we used scores on the first principal component extracted from measures such as lifetime symptom counts for conduct disorder, relevant life-events (e.g., trouble with the law, early age of first intercourse), a reverse-scored socialization scale, and personality scales measuring aggression and low constraint. For each family, we summed original parents’ scores on the substance abuse measure and on the behavioral disinhibition measure. We then estimated the regression of offspring IQ onto each of these, using mixed-effect linear models that controlled for sibling-pair clustering, sex, and ethnicity. The coefficient for the behavioral disinhibition variable was significant neither among biological (b = −1.18, SE = 1.28) nor adopted (b = −1.67, SE = 1.45) participants. Despite suggestive results for the substance abuse variable among biological participants (b = −2.09, SE = 1.05), its coefficient was significant in neither that subsample nor among adoptees (b = −0.63, SE = 1.29).

  3. However, one interesting aspect of our life-events results is the marked decline in ability for biological offspring at the upper life-events extreme (scores greater than eleven, which occurred for only two families). This suggests that a sufficiently disruptive environment can detrimentally affect cognitive ability, or alternately, that instability-producing behavior in parents and IQ decrements in their natural children may stem from common genetic factors. There were no adoptees with family life-events scores of this magnitude, so the effect of extreme environmental instability, independent of heredity, cannot be estimated. In general, adoptive samples do not represent the negative extremes of family environments, so results from adoption studies cannot rule out the possibility that exceptionally unfavorable environments can produce exceptionally unfavorable effects.

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Acknowledgments

This work was supported by US Public Health Service grants # DA005147, AA009367, DA13240, AA11886, and MH66140, and by grant # 13575 from the John Templeton Foundation (“The Genetics of High Cognitive Abilities”). The authors give their special thanks to Brian M. Hicks for his assistance in the analysis of the life-events and parental-psychopathology data.

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Authors and Affiliations

  1. Department of Psychology, University of Minnesota, Minneapolis, MN, USA

    Robert M. Kirkpatrick, Matt McGue & William G. Iacono

Authors
  1. Robert M. Kirkpatrick
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  2. Matt McGue
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  3. William G. Iacono
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Corresponding author

Correspondence to Robert M. Kirkpatrick.

Additional information

Edited by Robert Plomin.

Appendix: Life events item content

Appendix: Life events item content

Items from mother’s life events interview:

  1. 1.

    Divorce.

  2. 2.

    Former spouse remarrying.

  3. 3.

    Death of spouse.

  4. 4.

    Separation from spouse due to conflict.

  5. 5.

    Separation from spouse due to necessity.

  6. 6.

    Arguing a lot with spouse.

  7. 7.

    Treatment for emotional problem.

  8. 8.

    Treatment for alcohol/drugs.

  9. 9.

    Suicide attempt.

  10. 10.

    Fired/laid off.

  11. 11.

    Financial problems.

  12. 12.

    Bankruptcy.

  13. 13.

    Public assistance (welfare, etc.).

  14. 14.

    Public assistance cut off.

  15. 15.

    Arrested or jailed.

  16. 16.

    Moving 50 + miles.

  17. 17.

    Serious/chronic illness.

  18. 18.

    Hospitalization.

Items from father’s life events interview:

  1. 19.

    Treatment for emotional problems.

  2. 20.

    Treatment for alcohol/drugs.

  3. 21.

    Suicide attempt.

  4. 22.

    Fired/laid off.

  5. 23.

    Arrested or jailed.

  6. 24.

    Serious/chronic illness.

  7. 25.

    Hospitalization.

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Kirkpatrick, R.M., McGue, M. & Iacono, W.G. Shared-Environmental Contributions to High Cognitive Ability. Behav Genet 39, 406–416 (2009). https://doi.org/10.1007/s10519-009-9265-0

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