MS occurs in about one in 1,000 persons in European and North American Caucasian populations. It is rare in Asia and Africa. Two-thirds of MS patients are female. First degree relatives of affected persons have about a 20-40 fold increased risk for developing MS. For identical twins, where one twin is affected by MS, the second twin is also affected about 30% of the time. Half siblings of affected persons have a slightly lower risk of MS than full siblings.