But a study just published in the European Journal of Human Genetics says there is no evidence to support the earlier theory. The latest research investigated more than 200 people in the islands where small populations with little immigration makes it easier to study families.
The lead author of the report Dr Jim Wilson, himself from Orkney, said: “In the 1970s it was suggested that the close family relationships and cousin marriage in small islands might have been a contributing factor, but our first published paper has shown both that cousin marriage is rare and that it does not contribute to the risk of MS.”
Dr Wilson said the 1970s study gave a “false positive” and the new research used a larger sample of people and more advanced DNA methods to come to a different conclusion.
It is estimated there are 10,500 MS sufferers in Scotland – one in 500 – and Orkney and Shetland have the highest rates.
Dr Wilson said the causes of MS are not well understood, although it may be related to daylight as the number of cases rise further from the Equator. Previous studies revealed only 3½ per cent of Scots have optimal levels of Vitamin D in their blood.