title: Causal Inference and Observational Research: The Utility of Twins.
abstract: Valid causal inference is central to progress in theoretical and applied psychology. Although the randomized experiment is widely considered the gold standard for determining whether a given exposure increases the likelihood of some specified outcome, experiments are not always feasible and in some cases can result in biased estimates of causal effects. Alternatively, standard observational approaches are limited by the possibility of confounding, reverse causation, and the nonrandom distribution of exposure (i.e., selection). We describe the counterfactual model of causation and apply it to the challenges of causal inference in observational research, with a particular focus on aging. We argue that the study of twin pairs discordant on exposure, and in particular discordant monozygotic twins, provides a useful analog to the idealized counterfactual design. A review of discordant-twin studies in aging reveals that they are consistent with, but do not unambiguously establish, a causal effect of lifestyle factors on important late-life outcomes. Nonetheless, the existing studies are few in number and have clear limitations that have not always been considered in interpreting their results. It is concluded that twin researchers could make greater use of the discordant-twin design as one approach to strengthen causal inferences in observational research.
authors: Department of Psychology, University of Minnesota, Minneapolis Institute of Public Health, University of Southern Denmark, Odense, Denmark mcgue001@umn.edu.
authors: Matt
authors: M
authors: McGue
authors: Institute of Public Health, University of Southern Denmark, Odense, Denmark Research Center for Prevention and Health, Glostrup Hospital, Glostrup, Denmark;
authors: Merete
authors: M
authors: Osler
authors: The Danish Twin Registry and The Danish Aging Research Center Institute of Public Health, University of Southern Denmark, Odense, Denmark.
authors: Kaare
authors: K
authors: Christensen
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