Behavior Genetics

, Volume 19, Issue 4, pp 593–604

Twins reared together: Minimizing shared environmental effects

  • D. A. Grayson


The assumption that genetic variance is primarily (if not all) additive is usually made in biometric-genetic analyses of data collected on twins raised together. It is known amongst those familiar with twin methods that this assumption may lead to overestimates of heritability and underestimates of shared environmental variance (E2), although this limitation is not always made clear to genetically naive readers of such applications. The concept of “emergenic” genetic mechanisms (a potentially extreme epistatic or nonadditive mechanism) discussed by Lykken (1982) raises the possibility that genetic variance may be substantially nonadditive in some applications. The aims of the present paper are to investigate quantitatively the potential size of such nonadditivity and such misestimations and to provoke discussion on the empirical plausibility (or otherwise) of epistatic effects. For if substantially present, the results of conventional twin analyses are substantially biased.

Key Words

twins nonadditivity emergenesis model misspecification shared environment 


Unable to display preview. Download preview PDF.

Unable to display preview. Download preview PDF.


  1. Eaves, L. J. (1977). Inferring the causes of human variation.J. Roy. Stat. Soc. (Ser. A) 140:324–355.Google Scholar
  2. Eaves, L. J., Last, K., Martin, N. G., and Jinks, J. L. (1977). A progressive approach to non-additivity and genotype-environmental covariance in the analysis of human differences.Br. J. Math. Stat. Psychol. 30:1–42.Google Scholar
  3. Eaves, L. J., Last, K. A., Young, P. A., and Martin, N. G. (1978). Model-fitting approaches to the analysis of human behaviour.Heredity 41:249–320.PubMedGoogle Scholar
  4. Feldman, M. W., and Lewontin, R. C. (1975). The heritability hang-up.Science,190:1163–1168.PubMedGoogle Scholar
  5. Goldberger, A. S. (1977). Twin methods: A skeptical view. In Taubman, P. (ed.),Kinometrics: Determinants of Socioeconomic Success Between and Within Families, North-Holland, Amsterdam, Chap. 6.Google Scholar
  6. Jardine, R., Martin, N. G., and Henderson, A. S. (1984). Genetic covariation between neuroticism and the symptoms of anxiety and depression.Genet. Epidemiol. 1:89–107.PubMedGoogle Scholar
  7. Jinks, J. L., and Fulker, D. W. (1970). Comparison of the biometrical genetical, Mova, and classicall approaches to the analysis of human behaviour.Psychol. Bull. 75:311–349.Google Scholar
  8. Lykken, D. J. (1982). Research with twins: The concept of emergenesis.Psychophysiology 19:361–373.PubMedGoogle Scholar
  9. Martin, N. G., and Eaves, L. J. (1977). The genetical analysis of covariance structure.Heredity 38:79–95.PubMedGoogle Scholar
  10. Plomin, R., and Daniels, D. (1987). Why are children in the same family so different from one another?Behav. Brain Sci. 10:1–60.Google Scholar
  11. Wilson, S. (1982). Heritability.J. Appl. Prob. 19A:71–85.Google Scholar

Copyright information

© Plenum Publishing Corporation 1989

Authors and Affiliations

  • D. A. Grayson
    • 1
  1. 1.Psychology DepartmentUniversity of SydneyAustraliá

Personalised recommendations