How can it be that a mental illness is found in one or some, localities but not others?

Because the brain works off electrical impulses I suspect it is easily impacted by external sources of electricity such as magnetic deposits of metals in the ground, electricity in power lines, magnetic forces of any sort, as well as unstable elements in ground and solar radiation. Another factor may well be time. The more time we have to sit and dwell upon ourselves the more problems we would be likely to discover. Of less likely impact could be training. We live in a culture where everyone is encouraged to cry their life away about any real or imagined wrong ever done us in our childhood.