For years, advocates for open primaries and electoral reform have made a compelling argument: change who votes in primaries, and you change who gets elected. Change who gets elected, and you change how Congress behaves.
It's a logical chain — but until recently, it lacked the kind of rigorous academic evidence needed to move from argument to proof.
That proof now exists. And it comes from one of the most respected research institutions in the country.
A landmark study from the USC Schwarzenegger Institute for State and Global Policy, published in the Journal of Political Institutions and Political Economy, is the first to empirically demonstrate that open primaries — and especially top-two primaries — produce measurably less extreme lawmakers. The implications for independent voters, election reformers, and anyone concerned about the state of American democracy are significant.
The Research: What USC Actually Found
The study, conducted by Christian Grose, academic director of the USC Schwarzenegger Institute, examined the voting behavior of members of the U.S. House of Representatives over a 15-year period from 2003 to 2018. Grose compared lawmakers elected in states with top-two primary systems — California, Louisiana, and Washington — against lawmakers elected in states with traditional closed or partisan primary systems.
To measure ideology, Grose used DW-NOMINATE, a widely respected multidimensional scaling tool developed by political scientists Keith Poole, Howard Rosenthal, and James Lo. The tool assigns each member of Congress a score ranging from -1 (most liberal) to +1 (most conservative) based on their actual roll call votes — not their rhetoric, not their campaign promises, but their real voting behavior in office.
The findings were unambiguous: lawmakers elected in states with top-two primaries cast less extreme ideological votes than their counterparts elected in closed primary states. The effect held for both incumbents and newly elected members — and was even stronger for newer members of Congress who were first elected under the reformed primary system, suggesting the moderating effect compounds over time as the electorate that selects candidates becomes broader and more representative.
Top-Two vs. Open Primaries: Understanding the Difference
Before diving deeper into the findings, it's worth clarifying the distinction between primary election systems, since the research reveals important differences between them.
Closed primaries — the system used in 16 states for congressional primaries — restrict first-round participation to registered party members only. Independent voters are either partially or entirely excluded. The general election then features one candidate from each party.
Open primaries allow any registered voter to choose which party's primary they want to participate in, regardless of their own party registration. Independent voters can participate, but they must choose a partisan ballot.
Top-two primaries go further. In a top-two system, all candidates from all parties appear on a single primary ballot. Every voter — Democrat, Republican, independent, or otherwise — votes among all candidates. The top two vote-getters advance to the general election regardless of party affiliation. That means two Democrats, two Republicans, or any combination could face each other in November.
The USC research found that both open primaries and top-two primaries produce less extreme lawmakers compared to closed primaries — but top-two primaries have a significantly larger effect. As Grose noted, top-two primaries reduce legislator extremity by nearly double the amount that open primaries do. The more inclusive the primary system, the more moderate the lawmakers it produces.
Why Does This Happen? The Incentive Structure Explanation
To understand why open and top-two primaries produce less extreme lawmakers, it helps to think about the incentives each system creates for candidates.
In a closed primary, a candidate's path to victory runs through their party's most committed, most ideologically consistent voters. These are the voters who show up reliably in low-turnout primaries. They tend to be more partisan, more ideologically motivated, and more demanding of ideological purity from candidates. In this environment, the rational strategy for a candidate is to energize the base — not to appeal to the center.
The result is a Congress full of lawmakers who were selected by, and remain accountable to, the most extreme members of their respective parties. Compromise becomes politically dangerous. Reaching across the aisle becomes a liability. Governing becomes almost impossible.
Open and top-two primaries fundamentally change those incentives. When the primary electorate expands to include independent voters — and in the case of top-two, voters from all parties — candidates can no longer win by appealing exclusively to their base. They have to make a case to a broader, more ideologically diverse group of voters. The candidate who can appeal to same-party voters, different-party voters, and independent voters simultaneously has a structural advantage.
That broader appeal doesn't disappear when the candidate wins and takes office. Lawmakers elected in open primary systems remain accountable to a wider electorate. They have less reason to perform for the partisan base and more reason to actually govern — to find workable solutions, build coalitions, and deliver results.
As Grose put it: "This study demonstrates that the type of primary and electoral systems we choose can reduce extremism and ultimately political polarization. Those interested in reducing legislator extremity in the U.S. House should consider adopting new primary systems that encourage legislator moderation."
Political Polarization in Congress: How Bad Is It?
To appreciate the significance of these findings, it helps to understand the scale of the polarization problem the USC study is addressing.
Political scientists widely agree that partisan polarization in Congress is at its highest levels in more than a century. The ideological overlap between the two parties — once substantial — has effectively disappeared. The most conservative Democrat in Congress is now typically more liberal than the most liberal Republican. There is virtually no ideological middle ground left in the institution.
The consequences are visible and concrete. Legislative gridlock has become the default state. Basic governing functions — passing budgets, confirming judges, responding to crises — have become grinding partisan battles. The ability to negotiate, compromise, and build the coalitions necessary to address complex national challenges has broken down.
A recent national poll found that 70% of voters say Congress is not effectively addressing America's biggest problems, including the cost of living, immigration, and healthcare. Only 29% say Congress represents the American people well. These aren't just abstract complaints — they're the real-world consequences of a political system in which the incentive structure has been captured by partisan extremes.
The USC research suggests a concrete, structural path out of that trap. It's not about asking politicians to be better people or voters to be more engaged. It's about changing the rules of the game in ways that change the incentives — and, as the data shows, the outcomes.
Open Primaries and Independent Voters: The Connection
The USC findings have direct implications for independent voters and the broader movement to open primaries to all eligible Americans.
Currently, tens of millions of independent voters are excluded from primary elections in states with closed primary systems. This isn't just unfair to those voters — the USC research suggests it's actively harmful to the quality of governance those voters receive. When independent voters are locked out of primaries, the electorate that selects candidates becomes smaller, more partisan, and more ideologically extreme. The lawmakers that electorate produces are correspondingly more extreme and less capable of effective governance.
Conversely, when independent voters are included in the primary process — as they are in open and top-two primary states — the research shows that the lawmakers produced are measurably less extreme and more capable of functioning in a representative democracy.
This is a critical point for independent voters to understand: the push to open primaries isn't just about your right to vote. It's about your right to be represented by lawmakers who were selected by a process that included you. When you're excluded from primaries, the candidates who emerge are optimized for an electorate that doesn't include your voice. And the research shows those candidates govern differently — more extremely — as a result.
The Evidence Is In: Open Primaries Work
One of the most important aspects of the USC study is what it represents methodologically. This wasn't a survey of voter preferences or a theoretical argument about how electoral systems should work. It was a rigorous empirical analysis of actual legislative behavior — real roll call votes cast by real members of Congress — over a 15-year period.
That kind of evidence is rare and valuable. It allows us to move the conversation about open primaries from "here's what we think would happen" to "here's what the data shows actually happened."
What the data shows is clear:
States that moved to top-two primary systems produced lawmakers who cast less extreme votes. States that adopted open primaries showed similar — if somewhat smaller — effects. The moderation was evident among both incumbent lawmakers and newly elected members, suggesting the effect operates through both changed incentives and changed electoral selection. Newer members elected under reformed systems showed even stronger effects, pointing to long-term compounding benefits as the system matures.
This body of evidence, combined with recent polling showing that 71% of voters support open primaries and 73% believe open primaries would produce better candidates who appeal to a majority of voters rather than just their base, paints a consistent picture: open primaries are not just popular — they work.
What This Means for the Future of Election Reform
The USC research enters a political landscape that is, in many ways, ripe for the reforms it recommends.
At the federal level, the Let America Vote Act — introduced with bipartisan support in Congress — would require states to open their primaries to all registered voters, including independents. The legislation would affect the most consequential elections in the country and bring the benefits documented by the USC research to tens of millions of voters currently shut out of the primary process.
At the state level, reform efforts are active across the country. Florida voters are being asked to consider opening the state's closed primary system to its more than 13 million voters. Other states are exploring top-two and top-four primary models. The momentum is real — and the USC research gives reformers a powerful empirical argument to make.
Both major parties have historically resisted these reforms, working to oppose ballot initiatives supporting top-two primaries and challenging them in courts. Their resistance is understandable from a narrow partisan perspective — open primaries reduce the ability of each party to control its own candidate selection. But from the perspective of voters and the broader health of democracy, the USC findings suggest that resistance is coming at a significant cost. The partisan primary system, as currently structured, is producing more extreme lawmakers and a less functional Congress. That's not an opinion. It's what the data shows.
The Bottom Line for Independent Voters
If you're an independent voter who has felt unheard, underrepresented, or shut out of the democratic process, the USC research offers something important: validation backed by evidence.
The instinct that drove you to register as independent — the sense that the two-party, base-first system isn't working for most Americans — is correct. And the reform that would fix it — opening primaries to all voters — has now been shown, empirically, to produce measurably better outcomes.
Less extremism. Less polarization. More competent, functional lawmakers. A Congress more capable of actually addressing the problems Americans care about.
That's what the science shows. That's what the polling confirms. And that's what the Independent Center is fighting for.
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