In the quoted passage below, I came across a sentence I don't understand:
In 1955 Maurice Duverger published The Political Role of Women, the first behavioralist, multinational comparison of women’s electoral participation ever to use election data and survey data together. His study analyzed women’s patterns of voting, political candidacy, and political activism in four European countries during the first half of the twentieth century. Duverger’s research findings were that women voted somewhat less frequently than men (the difference narrowing the longer women had the vote) and were slightly more conservative.
Does the italicized sentence mean the difference of men and woman voting rates narrows the time that women have for voting?
Why shouldn't the sentence be "the difference narrower, the longer women had the vote"?
Thank you so much.