These curves are based upon the “current” proband-wise monozygotic-twin concordance-rates for men and women in Canada and upon the change in the sex-ratio observed in Canada between the two time-periods . However, in deriving this set of curves, two further assumptions have been made. First it has been assumed that the currently observed values for Zw2 and Zm2 are accurate . Second, it is assumed that C (defined in ) is equal to one half (i.e., that the prevalence of MS has doubled between the two time intervals). Of course, the actual level of environmental exposure (whatever this represents) is unknown. Nevertheless, the environmental exposure has been scaled such that the difference in the actual exposure level between these two time-periods (whatever this is), multiplied by the unknown hazard rate, is assigned the arbitrary value of one exposure unit (see ). In these curves, the probability of a “sufficient” exposure to the entire set of environmental events (PE) is assumed to have changed (in some manner) as the actual level of exposure (x) of the population has increased between the two time intervals of study. This change in (PE), however, could be due to a change in only one, in some, or in all of the relevant environmental factors. NB: Although the two time-periods of (1941–1945) and (1976–1980), which were used for parameter estimation, are shown along the x-axis, this axis represents an increasing (but unknown) environmental exposure. It is not a time-axis.