U.S. political parties become extremist to get more votes

The research team, which includes Northwestern University researchers, found that extremism is a strategy that has worked over the years even if voters' views remain in the center. Voters are not looking for a perfect representative but a "satisficing," meaning "good enough," candidate.

"Our assumption is not that people aren't trying to make the perfect choice, but in the presence of uncertainty, misinformation or a lack of information, voters move toward satisficing," said Northwestern's Daniel Abrams, a senior author of the study.

The study is now available online and will be published in SIAM Review's printed edition on Sept. 1.

Abrams is an associate professor of engineering sciences and applied mathematics in Northwestern's McCormick School of Engineering. Co-authors include Adilson Motter, the Morrison Professor of Physics and Astronomy in Northwestern's Weinberg College of Arts and Sciences, and Vicky Chuqiao Yang, a postdoctoral fellow at the Santa Fe Institute and former student in Abrams' laboratory.

To accommodate voters' "satisficing" behavior, the team developed a mathematical model using differential equations to understand how a rational political party would position itself to get the most votes. The tool is reactive, with the past influencing future behaviors of the parties.

The team tested 150 years of U.S. Congressional voting data and found the model's predictions are consistent with the political parties' historical trajectories: Congressional voting has shifted to the margins, but voters' positions have not changed much.

"The two major political parties have been getting more and more polarized since World War II, while historical data indicates the average American voter remains just as moderate on key issues and policies as they always have been," Abrams said.

The team found that polarization is instead tied to the ideological homogeneity within the constituencies of the two major parties. To differentiate themselves, the politicians of the parties move further away from the center.

The new model helps explains why. The moves to the extremes can be interpreted as attempts by the Democratic and Republican parties to minimize an overlap of constituency. Test runs of the model show how staying within the party lines creates a winning strategy.

"Right now, we have one party with a lot of support from minorities and women, and another party with a lot of support from white men," Motter said.

Why not have both parties appeal to everyone? "Because of the perception that if you gain support from one group, it comes at the expense of the other group," he added. "The model shows that the increased polarization is not voters' fault. It is a way to get votes. This study shows that we don't need to assume that voters have a hidden agenda driving polarization in Congress. There is no mastermind behind the policy. It is an emergent phenomenon."

The researchers caution that many other factors -- political contributions, gerrymandering and party primaries -- also contribute to election outcomes, which future work can examine.

The work challenges a model introduced in the late 1950s by economist Anthony Downs, which assumes everyone votes and makes well-informed, completely rational choices, picking the candidate closest to their opinions. The Downsian model predicts that political parties over time would move closer to the center.

However, U.S. voters' behaviors don't necessarily follow those patterns, and the parties' positions have become dramatically polarized.

"People aren't perfectly rational, but they're not totally irrational either," Abrams said. "They'll vote for the candidate that's good enough -- or not too bad -- without making fine distinctions among those that meet their perhaps low bar for good enough. If we want to reduce political polarization between the parties, we need both parties to be more tolerant of the diversity within their own ranks."