The concept of GWAS was formed in the 1990’s and supported by theoretical work such as that of Risch and Merikangas (15), however at that time genotyping the genome at an adequate density was not possible. Association studies were performed for one, ten or sometimes 100 SNPs, covering targeted genes or regions. Claims of “lack of association” were obviously exaggerated and to everyone’s frustration positive findings rarely replicated and almost never consistently. Genome-wide scans became possible around the turn of the millennium, initially at low marker densities and often with sample pooling to reduce cost. Today, before the end of the decade, studies testing 1 million SNPs across the genome on many thousands of subjects are published weekly. There have been many success stories that have confirmed the value of the GWAS approach. In just the last three years GWAS have led to hundreds of associations of common DNA variants with over 80 diseases and traits (17) including schizophrenia, bipolar disorder, major depression, autism, attention deficit disorder, neuroticism, alcohol dependence, smoking behavior and other psychiatric or related disorders and traits
