Third, the SNP score was not significantly related to alcohol use at ages 14, 17, and 20, indicating that these SNPs, even though they increase the likelihood of smoking at ages 20 and 24, play no detectable role in alcohol use. The sensitivity of this analysis is limited by the relatively small genetic effect on nicotine use (r2 < 1%) as well as the small sample size, and requires confirmation in larger samples. While the present results do not rule out the existence of a positive effect of environmental nicotine exposure on alcohol use, such a hypothesized effect is too small to be detected in this sample. There was a significant SNP score effect on alcohol use at age 24, but in the opposite direction as that observed for cigarettes. This result was unexpected and while possibly spurious, nevertheless fails to support the notion that the nicotine SNP score increase risk for alcohol use. One might conjecture that it represents a trend for early adults to specialize in their drug use habits and use available resources (e.g., money, time) for
