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  • Primer
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Behavioural genetics methods

Abstract

The question of why people show individual differences in their behaviours and capacities has intrigued researchers for centuries. Behaviour genetics offers us various methods to address this question. The answers are interesting for a range of research fields, varying from medicine to psychology, economics and neuroscience. Starting with twin and family studies in the late 1970s, the field of behaviour genetics has rapidly developed by applying molecular genetic techniques next to, and sometimes combined with, family data. The overarching conclusion at this point in time is that all measured human traits are to some extent heritable, and that many genetic variants, with each exerting a small effect, explain this heritability. Against this backdrop, we offer readers who might be less familiar with behaviour genetics a brief Primer on the topic. Sitting atop our list of goals is to be a resource for scholars interested in applying the widely useful techniques of the field to their particular specialty, regardless of what that might be.

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Fig. 1: Three common twin family models.
Fig. 2: Correlations in cognitive ability between twin pairs.
Fig. 3: Example raw and standardized variance path diagrams.
Fig. 4: A cross-lagged longitudinal MZ discordant twin design.
Fig. 5: The Wilson effect for cognitive ability.
Fig. 6: Estimates of within-family shrinkage for GWAS predictors across a range of phenotypes from a within-sibship study52.

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Acknowledgements

T.J.C.P. is supported by ZonMw grant 60-63600-98-834. Additionally, all of the authors thank the individuals who together provided incredibly constructive and useful feedback. All of these individuals contributed to the improvement of this Primer, and have their sincere gratitude for this.

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Contributions

Introduction (B.B.B. and T.J.C.P.); Experimentation (E.A.W.); Results (E.A.W.); Applications (E.A.W.); Reproducibility and data deposition (T.J.C.P.); Limitations and optimizations (B.B.B. and T.J.C.P.); Outlook (E.A.W. and B.B.B.); Overview of the Primer (T.J.C.P. and B.B.B.).

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Correspondence to Brian B. Boutwell.

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Nature Reviews Methods Primers thanks Thalia Eley, Elliott Rees, Lawrence Wilkinson and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Glossary

Akaike’s information criteria

A statistical method for evaluating how well a model fits the given data in which fit is penalized for the number of parameters estimated. In twin family studies, this term is often used to compare different possible models to determine which has the best fit.

Bayesian information criteria

A statistical method for evaluating model fit in which a penalty term is introduced for the number of parameters. This is a more explanatory tool that assesses the underlying data.

Cascade model

An extension of the extended twin family design that models covariances of twin pairs in addition to their siblings, spouses, parents and children. The Cascade model relaxes the assumptions of assortative mating and vertical transmission by allowing for the inclusion of latent phenotypes.

Classical twin design

The simplest twin design in which pairs of monozygotic and dizygotic twins are compared for similarity on some phenotype, and the observed covariances are used to calculate the relative magnitudes of genetic and environmental sources of variance.

Dizygotic

(DZ). Describes a pair of twins derived from two different sperm and two different ova who develop together in utero. DZ twins share approximately 50% of their DNA, similar to regular siblings.

Equal environment assumption

The assumption that monozygotic and dizygotic twin pairs experience the same environmental factors.

Extended twin family design

A type of twin family design that models observed covariances using the relatives of twins in addition to the focal twin covariances for some variable.

Gene–environment correlations

(rGE). A phenomenon in which genes may influence individual variations in exposure to certain types of environment.

Gene–environment interaction

An interplay between genes and environments in which different genomes can cause individuals to respond differently to the same environmental exposure.

Monozygotic

(MZ). Describes a pair of twins derived from a single sperm and ovum, who are therefore genetically identical.

Missing heritability

The observation that the sum total of genetic variants from a genome-wide association study cannot completely explain the heritabilities of complex traits derived from twin family studies.

Nuclear twin family design

A type of twin family design that models observed covariances of the parents of twins in addition to the twins themselves.

Placement effects

The non-random placement of children based on traits that are similar between biological and adoptive families.

Polygenic score

(PGS). A number generated from genome-wide association data that summarizes the estimated effect of a large number of summed genetic variants on a phenotype of interest.

Stealth model

An extension of the extended twin family design that models covariances of twin pairs in addition to their siblings, spouses, parents and children. The Stealth model relies on primary phenotypic assortment to model assortative mating, and on direct parent to offspring phenotypic transmission to model vertical transmission.

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Willoughby, E.A., Polderman, T.J.C. & Boutwell, B.B. Behavioural genetics methods. Nat Rev Methods Primers 3, 10 (2023). https://doi.org/10.1038/s43586-022-00191-x

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