the article referenced by davidr looks the most authoritative - google throws up quite a number of more dubious pages - that article suggests that there may be several causes of similar symptoms but one strong pattern is the genetic nature of one found strongly linked with Scandinavia - re Viking settlement of Man my understanding is that most (tho not all) of the Norse settlers on Man were 2nd or later generations from the Western Isles and also from the Dublin Norse after their defeat.