Behavior genetic studies reveal that personality traits, like psychiatric disorders, have a genetic basis. Twin, adoption, and family studies indicate that personality factors are heritable, with about 50% of the variance of the underlying components accounted for by additive and non-additive genetic factors (1-3). However, identifying the genetic variants associated with personality traits is challenging. A large number of studies have tested several candidate genes, especially for the Neuroticism factor, but these studies have produced largely inconclusive results. Similarly, genetic linkage studies, usually based on 400-500 microsatellite markers, have suggested loci for Neuroticism (4-7), but only a few genomic regions (e.g., 12q) have been reported in multiple studies. Difficulties in identifying specific loci suggest that, as has been observed for a number of quantitative traits, genetic influences on these complex traits are likely attributable to many genes, each with a small effect size. To detect such small genetic effects, there is a growing interest in high throughput genotyping technologies that examine large numbers (e.g., 500,000) of single nucleotide polymorphisms (SNPs) densely mapped across the entire genome. Using these more incisive
