Starting about 15 years ago, geneticists began to collect DNA from thousands of people who shared traits, to look for clues to each trait’s cause in commonalities between their genomes, a kind of analysis called a genome-wide association study (GWAS). What they found, first, was that you need an enormous number of people to get statistically significant results — one recent GWAS seeking correlations between genetics and insomnia, for instance, included more than a million people. Second, in study after study, even the most significant genetic connections turned out to have surprisingly small effects. The conclusion, sometimes called the polygenic hypothesis, was that multiple loci, or positions in the genome, were likely to be involved in every trait, with each contributing just a small part. (A single large gene can contain several loci, each representing a distinct part of the DNA where mutations make a detectable difference.) - www.quantamagazine.org