Altogether 81 DZ twin pairs (32 same-sex and 49 opposite-sex) and 54 MZ twin pairs with cotinine (>10 ng/ml in both co-twins) and 3-hydroxycotine measures available in the FinnTwin12 and FinnTwin16 samples were used to estimate the heritability of NMR. Intraclass correlations were calculated and quantitative genetic modelling was used to estimate alternative variance components models in Mx [39]. We evaluated the presence of additive genetic (A), genetic effects due to dominance (D), shared environmental effects (C), and environmental effects not shared by twins (E).
