The authors would like to correct the following mistakes to the abstract:
1. ‘genetic factors (0.49 vs. 0.22 for initiation . . .’ should be ‘genetic factors (0.50 vs. 0.22 for initiation. . .’.
2. ‘heritabilities ranging from 0.39 to 0.52 in the measures studied’ should be ‘heritabilities ranging from 0.44 to 0.54 in the measures studied’.