There are four major limitations of this study: First, it is observational rather than experimental. We can't be sure that the differences between samples DAO and DASD have to do with the abuse of stimulants rather than some unmeasured cohort differences between the samples. Experimental studies of addiction (where subjects are randomly assigned to alcohol only or alcohol and stimulants) are only possible with animals, which have their own inherent limitations, especially in studies of abstinence (where the human condition necessarily includes issues of choice, motivation, and compliance). Nonetheless, the demographic similarity between alcoholics DAO and DASD suggests that the differences between groups are likely the result of stimulant abuse. Second, the study is cross-sectional rather than longitudinal, which limits our ability to definitively attribute differences between STAA and LTAA to duration of abstinence. This limitation was discussed above. Third, STAA were abstinent a minimum of six weeks. The study is therefore silent with regard to subcortical volume effects of alcohol and stimulant abuse and dependence in very early abstinence. This might be particularly relevant to our lack of finding
