We found a high degree of within-substance overlap in heritable influences on use and dependence. In contrast to the more moderate genetic correlation of 0.63 reported by our group for AD and average alcohol intake (Whitfield et al., 2004), we found that 90.9% of genetic contributions to heaviness of alcohol consumption (during heaviest period of use) were shared with AD symptoms (see also Grant et al., in press). For cannabis phenotypes the genetic correlation was 0.979. The nearly perfect correlation between genetic contributions to use and genetic contributions to dependence for both alcohol and cannabis suggests that an indicator of heaviness of use may act as a proxy for dependence symptomatology (and vice versa). The implications for linkage and association studies of alcohol and cannabis use and dependence are far-reaching. Data collected on one of the two phenotypes can be highly informative for investigations focusing on the other phenotype; it may even be possible to integrate findings from studies examining heavy use with those examining dependence. Furthermore, the substantial genetic overlap in dependence symptomatology across the two substances suggests that
