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. Author manuscript; available in PMC: 2011 Dec 1.
Published in final edited form as: Arch Gen Psychiatry. 2010 Dec;67(12):1309–1315. doi: 10.1001/archgenpsychiatry.2010.159

Figure 1. Interpretation of Co-Twin Control Results.

Figure 1

Should marriage be environmentally or causally linked to reductions in antisocial behavior, we would expect to observe this association at the individual-level, within DZ twin pairs discordant for exposure, and within MZ twin pairs discordant for exposure (scenario A). By contrast, the failure to observe an association within discordant MZ twin pairs would imply that the association of exposure with outcome is solely attributable to selection processes. In particular, if exposure was associated with outcome at the individual-level and in discordant DZ twins (scenario B), we would infer that the selection process was genetic in origin. If the exposure was associated with outcome only at the individual-level (scenario C), we would infer that the selection process was genetic and shared environmental in origin.