An additional cause of false positive findings is hidden population structure. Case and control samples may differ in the distribution of their ancestry, either owing to control sampling effects, as discussed above, or to confounding when different ancestries carry higher disease risk and are, as a result, over-represented in cases. Even after exclusion of individuals with evidence of recent non-European ancestry, the British population is heterogeneous, having been shaped by several waves of immigration from southern and northern Europe. Whether the differences between these incoming populations are sufficiently large to distort the findings of population-based case-control studies is an open question.
