The correlations between the 4 polygenic risk scores for smoking were moderate to low to non-significant. The highest correlation was found between the polygenic scores for CPD and former smoking and it was negative, suggesting that being an ex-smoker is associated with a high number of cigarettes per day. This can be explained by the fact that former smokers reported on the maximum number of cigarettes smoked per day while smokers report on their current number of cigarettes per day. The moderate correlations between the 4 polygenic risk scores might be the result of a lot of error variance resulting from random, non-generalizable, non-linear and/or interactive genetic effects. Previous twin studies have suggested some overlap between smoking-related variables, varying from only a small proportion of shared genetic variance (45,46) between age at first cigarette and smoking variables, to a higher genetic overlap between smoking persistence and initiation (47). A study of the Netherlands Twin Register showed two separate dimensions for smoking initiation and nicotine dependence, but those dimensions were not independent (5).
