Disorders such as autism  schizophrenia and epilepsy each affect about 1% of the population and are therefore defined as “common disorders” But are they really? I mean  they are clearly really that common  but are they really “disorders”? Are they natural categories that reflect some shared underlying etiology or are they simply groupings based on sets of shared symptoms? Genetics is providing an answer to that question and demonstrating that so-called “common disorders” are really collections of rare disorders with similar symptoms