It should be noted that not all the evidence for relations between the s and l alleles of the 5-HTTLPR and smoking are consistent. For example, a recent meta-analysis found that smokers carrying the s allele more likely to relapse than those homozygous for the l allele (Munafo et al., 2004). Differences between the samples may contribute to this discrepancy, (ie., our population was not in treatment nor attempting to quit smoking), However, if indeed the s allele confers less 5-HTT and increased 5-HT, the biological plausibility of those carrying the s allele being more dependent seems less clear in light of the strong inverse association between depressive symptoms and quitting. One would expect those with higher levels of 5-HT to have reduced affective impairment during quitting and be more likely to succeed. In the absence of direct or indirect assessment of 5-HT, however, all these arguments must be considered speculative, albeit hypothesis generating.
