my own view is that something used in modern industry may be implicated. For instance, the Scandinavian countries on the list, and Saudi, and Sardinia, all have large mining and metal industries. Not saying this is the answer, but surely someone, somewhere , has done an analysis of the type of pollutants produced by different industries, for how long these have been produced by, or used in, the industry, and the numbers of the population exposed. Then maybe look at incidence in certain areas, even the jobs done by parents of type ones - there may also be an interplay with diet (most of the countries at the top of the list are overall wealthy). It is complex, as of course there is often a genetic element also but it surely would not be impossible to do the necessary research? maybe someone already is aware of at least part of the answer but any change would be just too difficult?