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Quantifying and addressing parameter indeterminacy in the classical twin design.

Matthew C Keller1, William L Coventry

  • 1Center for Society and Genetics, University of California-Los Angeles, 1339 Murphy Hall, Los Angeles, CA 90095-1405, USA. matthew.c.keller@gmail.com

Twin Research and Human Genetics : the Official Journal of the International Society for Twin Studies
|July 2, 2005
PubMed
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The classical twin design (CTD) has inherent limitations in separating genetic and environmental influences due to parameter indeterminacy. This study quantifies biases and proposes strategies like extended twin-family designs to improve behavioral genetic research.

Area of Science:

  • Behavioral Genetics
  • Quantitative Genetics
  • Twin Studies

Background:

  • The classical twin design (CTD) is widely used to estimate genetic and environmental contributions to phenotypic variation.
  • CTD is known to suffer from parameter indeterminacy, where different genetic and environmental models can explain the same twin pair covariation.
  • Common assumptions in CTD, such as negligible higher-order epistasis or absence of dominant genetic effects/common environment, lead to biased parameter estimates.

Purpose of the Study:

  • To quantify the biases in parameter estimation inherent in the classical twin design.
  • To discuss and evaluate alternative strategies for addressing parameter indeterminacy in twin studies.
  • To emphasize the importance of explicitly presenting parameter indeterminacy for accurate interpretation of findings.

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Main Methods:

  • Quantification of parameter estimation biases within the classical twin design framework.
  • Exploration of extended twin-family designs incorporating additional relatives to reduce indeterminacy.
  • Graphical representation of parameter indeterminacy to visualize equally likely parameter value spaces.

Main Results:

  • The classical twin design, under common simplifying assumptions, consistently produces biased estimates for genetic and environmental effects.
  • Extended twin-family designs can reduce, but not entirely eliminate, parameter indeterminacy.
  • Explicitly presenting the range of possible parameter values aids in understanding the limitations of twin study findings.

Conclusions:

  • The classical twin design's assumptions lead to unavoidable biases, necessitating careful interpretation of results.
  • Alternative designs and explicit representation of indeterminacy are crucial for advancing behavioral genetics.
  • Mindfulness of methodological assumptions is vital for researchers utilizing twin designs.