Ireland is relatively isolated at the North Western extreme of Europe, which may be advantageous in investigating a complex genetic disorder such as schizophrenia. The current Irish population, though not a genetic isolate, is more homogeneous than the US population29–31. Fewer ancestral haplotypes increase LD and the power to detect association. Studies of Y chromosome and mtDNA point to a common genetic legacy in Ireland that probably extends back to the repopulation of the island, after the last Ice Age approximately 15,000 years ago, from population centres in the Iberian peninsula and south western France32–34. The genetic structure of the population has been minimally influenced by more recent human migrations over the last three millennia34. In the experience of the blood bank staff, non-Irish donors are very rare, in agreement with the history of minimal in-migration to Ireland, and would have been excluded on the basis of questions about their grandparents.
