An interesting feature that multiple sclerosis (MS) shares with
many other immune-mediated diseases is that susceptibility is
higher in females than males. However, the female to male ratio
(sex ratio) of MS appears to have been changing over time. A
recent meta-analysis of epidemiological studies has shown that
the worldwide sex ratio of MS has been substantially changing
over the past century.1 This change is particularly clear in
large population-based studies from Denmark and Canada.1,2 In
Canada, the sex ratio of MS patients born in the 1930s was lower
than 2: the ratio then increased to more than 3 females for each
male patient in the latest birth cohort analyzed (1976-1980).
This intriguing epidemiological phenomenon warrants particular
attention because the sex ratio of MS parallels MS incidence,
and the increasing frequency of MS among females is a key driver
of the increasing prevalence of this devastating disorder
worldwide. A change that occurs within a century is too short a
time for a genetic ca