Polygenic disorders are due to the additive effect of multiple genes interacting with the environment. Because of the small effect size of each gene and considerable genetic heterogeneity, when single genes are examined, the outcome of association and linkage analyses are variable from study to study. Techniques are needed that take these unique characteristics of polygenic disorders into consideration. The present invention discloses that the formation of a polygenic score, consisting of the additive effect of multiple candidate genes, and its assessment using receiver operating characteristic (ROC) plots, provides such a technique. Six genes previously shown to be associated with Alzheimer's disease were examined, APOE, ACE, ACP1, ESR1, PNMT and SLC6A4. The total fraction of the variance, the area under the ROC plots, and the range of risks were similar for both groups indicating that despite genetic heterogeneity and the small effect size of most genes, consistent risk analyses could be obtained by examining the additive effect of these multiple genes. The present invention also discloses diagnostic tests for determining a subject's risk of developing Alzheimer's Disease or specifically Late Onset Alzheimer's Disease.