The search for the genetic roots of psychiatric illnesses and behavioral disorders such as schizophrenia, autism and ADHD has a long history, but until recently, it was one marked by frustration and skepticism. In the past few years, new techniques have begun to reveal strong evidence for the role of specific genes in some cases of these conditions but in a way few people expected.

For a long time, it was widely assumed that Single-nucleotide polymorphisms (SNPs, or “snips” as everyone calls them) were responsible for psychiatric disorders, in what’s called the “common-variant model” of disease. The idea was that any given risk variant might be quite common, but it would only increase your risk of suffering a disease by a small amount. Those who carried a large number of risk variants would develop the disease. Those with a moderate number might get mild symptoms, and so on.

Yet this just didn’t work out. About ten years ago, it became feasible to scan huge numbers of SNPs quickly and cheaply. These “genome-wide association studies” (GWAS) tested hundreds of thousands of variants. Moving quickly to exploit the new technology, psychiatrists conducted GWAS after GWAS comparing people with diseases to those without — but very little came out. There are a few common SNPs which seem to be associated with some disorders, like autism and schizophrenia, but only a handful, and they have very small effects.