For traits with a moderate heritability (h2 .40) the required sample sizes of the discovery samples are about twice as high as for a trait with a high heritability (h2 .80) (50). Simulations showed that even with high-heritability traits the sample size of for example TAG is still rather low. Besides sample size other factors, like proportion of SNPs having effect on the trait, are of importance (50). We have used 4 smoking dimensions from the TAG meta-analyses and the heritability of these phenotypes varied. The fact that a significant association with alcohol- and cannabis use was found for the polygenic risk scores of CPD and AOS but not for smoking initiation and smoking cessation might (partly) be explained by differences in heritability. In samples of the NTR the heritability was 75% for Nicotine Dependence, 51% for CPD, 60% (males) and 39% (females) for age at first cigarette and 36%–44% for smoking initiation (5,53,54). CPD might mirror a more ‘severe’ phenotypes that reflect addictive behavior (like nicotine dependence).
