Using non-normal SEM to resolve the ACDE model in the classical twin design

Koken Ozaki, Hideki Toyoda, Norikazu Iwama, Saori Kubo, Juko Ando

Research output: Contribution to journalArticlepeer-review

28 Citations (Scopus)


One of the biggest problems in classical twin studies is that it cannot estimate additive genetic (A), non-additive genetic (D), shared environmental (C), and non-shared environmental (E) effects, simultaneously, because the model, referred to as the ACDE model, has negative degrees of freedom when using Structural Equation Modeling (SEM). Therefore, instead of the ACDE model, the ACE model or the ADE model is actually used. However, using the ACE or ADE models almost always leads to biased estimates. In the present paper, the univariate ACDE model is developed using non-normal Structural Equation Modeling (nnSEM). In SEM, (1st- and) 2nd-order moments, namely, (means and) covariances are used as information. However, nnSEM uses higher-order moments as well as (1st- and) 2nd-order moments. nnSEM has a number of advantages over SEM. One of which is that nnSEM can specify models that cannot be specified using SEM because of the negative degrees of freedom. Simulation studies have shown that the proposed method can decrease the biases. There are other factors that have possible effects on phenotypes, such as higher-order epistasis. Since the proposed method cannot estimate these effects, further research on developing a more exhaustive model is needed.

Original languageEnglish
Pages (from-to)329-339
Number of pages11
JournalBehavior Genetics
Issue number2
Publication statusPublished - 2011 Mar
Externally publishedYes


  • Biases in estimators
  • Higher-order moments
  • Model identification
  • Non-normality
  • Univariate ACDE model
  • nnSEM

ASJC Scopus subject areas

  • Ecology, Evolution, Behavior and Systematics
  • Genetics
  • Genetics(clinical)


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