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During lectures and public appearances, I'm sometimes asked why Libertarians and other third-party movements make ample noise yet never win a sufficient number of elected offices in the United States to at least rattle the cages of the Democrats and Republicans, who have enjoyed their two-party duopoly since the Civil War and particularly since the Progressive Era ended.

My short answer: The use of single-member districts with plurality rule precludes third parties from making substantial progress. Ample cross-national empirical evidence demonstrates that single-member districts with plurality rule tend to reinforce two-party rule.

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Since 1967, all 435 members of the U.S. House of Representatives have been elected from single-member districts (SMDs) and by the plurality (or "first past the post") voting rule. The same holds for U.S. senators: Each state elects its two U.S. senators separately in statewide, plurality contests. Because 48 states assign their electoral votes in a winner-take-all fashion to the candidate who wins a plurality of each state's popular votes, presidential elections are also a series of SMD, plurality-vote contests. So, too, are the overwhelming and rising share of state legislative elections.

But electoral structures also have non-partisan effects. Four decades ago, more than half the states used multiple-member districts (MMDs) to elect some if not all legislators in one or both chambers; today, all but 11 states use SMDs exclusively to elect all state legislators.

A fabulous paper written by Joshua Shear, a student in my political internship course this past spring, examined why most states have abandoned the use of multiple-member state legislative districts. He concluded that states eliminated MMDs for one or both of two reasons: because legislators did not like co-representing constituents with other legislators; or because states wanted to pre-empt judicial challenges to their redistricting maps, especially race-based mapmaking challenges.

I should pause here to clarify the general implications of what's known as "district magnitude" — that is, the number of legislators elected per district. The political science scholarship strongly suggests that single-member districts help to elect racial minorities, whereas multiple-member districts tend to help elect more women. One need not be an electoral expert to understand the differing effects of MMDs for racial minorities and female office-seekers.

Consider a county featuring five council districts and a clustered Latino population of 20 percent. Electing a Latino council member will be more difficult if all five members are elected county-wide from an at-large district than if members are elected one each from five single-member districts — especially if one of those five districts is, of course, drawn to capture most of the county's Latino population. Unlike minorities who tend to be clustered, women and men are dispersed almost evenly across every state, county, district and Census bloc. One never hears of "majority-female" or "majority-male" districts because such districts are impossible to draw.

The share of female state legislators steadily rose from the 1970s through the mid-1990s, when the numbers began to level off. There are many potential reasons for this recent stagnation, most of which have nothing to do with district magnitude. Studies show that the inability to elect women to office has to more to do with "pipeline" issues like the failure to encourage, fund and support women to run for office or even become involved in politics in the first place. No structural change can solve these problems that stem from political socialization that signals to women and even girls that politics is a man's world.

Maryland is one of 11 states that still feature multiple-member districts; most, but not all, of the Maryland General Assembly's 141 house delegates are elected from MMDs. In terms of share of female state legislators, Maryland consistently ranks in the top 10, if not higher. Although its high ranking cannot be attributed solely to district magnitude, MMDs certainly can't hurt the chances of many female office-seekers.

Because state legislatures are a natural pipeline to higher office, the ability of women to win state legislative seats affects their ability to build the voter and donor bases needed to win statewide office or election to Congress. As ever, electoral structure has important consequences for who does — and does not — govern.

Thomas F. Schaller teaches political science at UMBC; his most recent book is "The Stronghold: How Republicans Captured Congress but Surrendered the White House." His column appears every other Wednesday. His email is schaller67@gmail.com. Twitter: @schaller67.

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