New research shows that U.S. political parties are becoming increasingly polarized due to their quest for voters—not because voters themselves are becoming more extremist.

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 senior author of the study Daniel Abrams, an associate professor of engineering sciences and applied mathematics in Northwestern’s McCormick School of Engineering.

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,” Dr. Abrams said.

The team found that, to differentiate themselves toward voters, politicians of the two major parties moved 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,” Adilson Motter, co-author of the study and a professor of physics and astronomy at Northwestern, stated.

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 do not necessarily follow those patterns, and the parties’ positions have become dramatically polarized.

“People aren’t perfectly rational, but they’re not totally irrational either,” Dr. 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.”