Methods are described for assessing an individual's likelihood of developing or exhibiting a multifactorial trait, and for predicting the effectiveness of a drug treatment regimen in an individual. The methods include determining a plurality of genotypes for the individual at a plurality of biallelic polymorphic loci, using the genotypes to compute a score for the individual, and comparing the score to at least one threshold value. Genetic tests are also described for assessing an individual's likelihood of developing or exhibiting a multifactorial trait.