Genome-wide association tests have been hugely successful at finding genes and even specific mutations that contribute to traits ranging from human height to schizophrenia. At its most basic, the idea is that a group of individuals with a shared phenotype should also share some genetic variants in common that are causally related to the trait in question. Unfortunately, there are other reasons that individuals who share a trait, such as cardiovascular disease or epilepsy, might share genetic variants in common. For example, a gene might seem to be associated with epilepsy within a given population, but it may be that a subgroup of the affected individuals shares a common ancestry that they aren’t aware of, and the associated gene may simply reflect that fact.  Read more