The role of the children of twins design in elucidating causal relations between parent characteristics and child outcomes.
BACKGROUND
Determination of causal connections between parental measures and child outcomes using typical samples is limited by the inability to account for all confounds, both environmental and genetic. This paper discusses the strength of the Children of Twins (COT) design to highlight the role of specific environments.


METHODS
A new analytical model is presented which helps differentiate and quantify the environmental and genetic processes underlying associations between family-level risk factors and child adjustment. In order to illustrate the COT design, the relation between smoking during pregnancy and child birth weight (BW) is examined in a sample of female twins and their children from Norway and the United States.


RESULTS
The results illustrate that smoking during pregnancy is influenced by genetic factors. However, the Children of Twins model supports the claim that smoking during pregnancy has a direct environmental influence on BW and that genetic and shared environmental confounds cannot account for the association.


CONCLUSIONS
An assessment of the strengths and limitations of the Children of Twins design and a comparison with other research strategies suggest that the design plays a unique role in the study of developmental psychology and psychopathology. Finally, the authors describe how methodological advances and future applications of the design will provide additional insight into the causal processes underlying children's adjustment to environmental stimuli.
D'Onofrio
Brian M
BM
Department of Psychology, University of Virginia, Charlottesville 22904-4400, USA. bmd8q@virginia.edu
Turkheimer
Eric N
EN
Eaves
Lindon J
LJ
Corey
Linda A
LA
Berg
Kare
K
Solaas
Marit H
MH
Emery
Robert E
RE
D000328:Adult; D000428:Alcohol Drinking; D001724:Birth Weight / Q000187:drug effects* / Q000235:genetics*; D015992:Body Mass Index; D015984:Causality; D015340:Epidemiologic Research Design; D005260:Female; D006801:Humans; D008297:Male; D008425:Maternal Behavior / Q000502:physiology*; D008957:Models, Genetic; D008960:Models, Psychological; D009664:Norway / Q000453:epidemiology; D010290:Parents / Q000523:psychology; D011247:Pregnancy; D012044:Regression Analysis; D012907:Smoking / Q000009:adverse effects* / Q000235:genetics; D012931:Social Environment; D014429:Twins, Dizygotic / Q000235:genetics / Q000523:psychology; D014430:Twins, Monozygotic / Q000235:genetics / Q000523:psychology; D014768:Virginia / Q000453:epidemiology