Since the initial findings by our laboratories showing a strong association of the A1 allele of the D2 dopamine receptor gene and alcoholism, a number of reports have either failed to support the association, while a Meta Analysis confirms the association of this allelic variant in alcoholism (p<10−7). Moreover, the overwhelming evidence supports an association of variants of the DRD2 gene with risk in developing compulsive disease (CD). Specifically, variants of the DRD2 gene (A1, B1, Int6-Ex7, D1) have been correlated with increased risk of severe alcoholism, crack/cocaine dependence, polysubstance abuse, carbohydrate bingeing, obesity, ADHD, Tourette' and severe gambling. We utilized Bayes' Theorem as a mathematical method to assist in evaluating the predictive value of the A1 allele of the DRD2 gene in compulsive disease. According to Bayes' rule, the predictive value positive (PV+) of a screening test is the probability that a person has the disease given that the test is positive. Whereas, the predictive value negative (PV−) of a screening test is the probability that a person does not have the disease given that the test is negative. We found that when we combined data including severe alcoholism, severe cocaine dependence, polysubstance abuse, severe overeating, ADHD and restricted our control data to only well characterized accessed controls screened for alcohol, drug and in some cases tobacco abuse, the PV+ was 45.5% risk for compulsive disease. Moreover, interpretation of a negative result from the A1 allelic genotype is very predictive, since we found that PV− = 81%. Finally, an analysis of pooled data related to CD resulted in an odds ratio of 3.62 (95% confidence limits [2.59-5.07]) with a P value of 0.00001 indicating a similarly strong positive correlation of the DRD2 gene variant and this disease (Yates 2 = 64.9, df = 1, p< 1×10−5).