Fig. 1From: Causal effect of smoking on DNA methylation in peripheral blood: a twin and family studySome possible directed acyclic graphs for the cross-twin cross-trait correlation. a The cross-twin cross-trait correlation is due to familial confounding. b The cross-twin cross-trait correlation is due to the causal effect of X on Y. c The cross-twin cross-trait correlation is due to the causal effect of Y on XBack to article page