It tends to be more common in women than men although there are geographical variations in this. There are also other complex geographical variations and it tends to show an increased incidence with increasing latitude from the equator in both hemispheres. There are, however, exceptions to this and there appear to be marked variations and some areas irrespective of latitude. There also appears to be some relationship to the age of geographical migration in that those migrating from a high incidence area to a low incidence area after adolescence is lower than when the migration occurs in childhood. There does, however, appear to be an environmental factor, possibly infective, which has not yet been defined.
Genetic factors are clearly important.
The disease is low in some races such as oriental races and studies on twins show a higher concordance rate amongst monozygotic twins than in dizygotic twins and siblings.