The political landscape of the United States is marked by extreme political division. Democrats and Republicans battle for control of the government, and with it the direction of the nation’s social and economic policies. Unwilling to compromise or cooperate, the two parties are pitted against each other in a zero-sum game in which the only goal is to attain power—and to keep it. Meanwhile, the business of government—such as maintaining infrastructure and ensuring the well-being of the populace—remains undone.

At the grassroots level, supporters of each political party are just as ferociously divided. Each side has its own news sources, which confirm the narrative they want to hear. As a result, there’s little incentive for political opinions outside their comfort zone. Instead, each side hurls insults at the other, with conservatives calling their opponents “libtards,” and liberals dismissing their counterparts as “deplorables.”

What accounts for such political divisiveness? According to University of Cambridge psychologist Leor Zmigrod and her colleagues, one explanation that has been current among political psychologists for more than half a century has to do with the notion of cognitive inflexibility. This is the inability to adapt the way you perceive or think about changing circumstances in the world. Yet even among political psychologists, there’s a sharp division in how they view the role of cognitive inflexibility in driving extreme political partisanship.

According to the rigidity-of-the-right hypothesis, it’s precisely cognitive inflexibility that defines the division between the political right and left. After all, conservatives don’t like change, and they prefer to maintain the status quo. This, at least by some accounts, is the very definition of cognitive inflexibility. In contrast, liberals see change as good and promote progress, especially in the social realm, and it follows that they must be more cognitively flexible than their conservative counterparts.

The alternative is the ideological extremity hypothesis. According to this view, political extremism on either side stems from cognitive inflexibility. That is, the decision to go left or right may be due to other factors, but the choice to go all the way left or right is driven by the degree to which people are unwilling to even consider other perspectives once they’ve adopted a particular worldview.

Over the decades, plenty of support has been proffered for both hypotheses. According to Zmigrod and her colleagues, however, these data are difficult to interpret because each group of researchers has defined and measured cognitive inflexibility in their own way. What’s needed instead is a valid measure of cognitive flexibility that’s widely accepted and devoid of political overtones. This is exactly the approach that Zmigrod and her colleagues have taken in a study they recently published in the Journal of Experimental Psychology.

In the field of cognitive , cognitive flexibility is defined as persistence in a particular mode of thought or behavior even when it's against the person’s own best interest. Cognitive inflexibility is a symptom of several disorders, such as , , and damage to the frontal lobe of the brain. Thus, clinicians have developed a number of techniques for assessing cognitive inflexibility.

Even among the normal population, people vary widely in their degree of cognitive flexibility, and this can be measured with the same instruments that are used to assess patients. Thus, it’s important to understand that cognitive flexibility isn’t a dichotomy between “flexible” and “inflexible.” Rather, it’s a continuum with these labels as the extremes.

Zmigrod and her colleagues relied on three widely used clinical assessments of cognitive flexibility in their study. The first was the Remote Associates Test, in which the respondent is given three words, such as cottage, Swiss, and cake, and then asked to name a fourth word related to these three. The faster the person’s response, the greater their cognitive flexibility. (The answer, by the way, is cheese.)

The second assessment of cognitive flexibility was the Wisconsin Card Sorting Test. In this task, respondents are asked to sort a deck of cards according to a rule they have to figure out through the “correct” and “incorrect” feedback they get after laying down each card. People eventually discern the rule, but once they do so, the experimenter changes the rule without telling them. Cognitive flexibility is determined by observing whether the person adapts to the new rule—and how long it takes them to do so.

The third assessment was the Alternative Uses Test, in which participants are given two minutes to think of as many possible uses for a common household item such as a newspaper. The more unique ideas they can generate, the greater their cognitive flexibility.

For the study, Zmigrod and her colleagues recruited more than 700 American citizens who identified as either Republican, Democrat, or Independent to fill out a series of surveys and to complete the assessment tasks online. The results showed that cognitive inflexibility—the inability to adapt one’s thinking and behavior in response to a changing world—was a characteristic of people exhibiting extreme political partisanship, whether on the left or on the right. Meanwhile, those who considered themselves moderates or independents displayed the most cognitive flexibility. Thus, the ideological extremity hypothesis was supported by the data.

However, Zmigrod and colleagues also found some support for the rigidity-of-the-right hypothesis. Specifically, when they only looked at responses concerning social issues, they found that cognitive inflexibility was more characteristic of conservatives. Rigidity in thinking and perceiving the world were hallmarks of political extremism on either side of the aisle, but cognitive inflexibility characterized those with strong conservative social views.

In the U.S., we tend to view political affiliation as a single dimension, ranging from Democratic liberals on one end to Republican conservatives on the other end, with moderate Independents in the middle. However, European democracies typically have multiple parties, none with a majority, forcing them to form (often temporary) alliances. This pressure to cooperate can help keep a democracy from sinking into the perpetual gridlock characteristic of American .

Political psychologists who study these multi-party systems often propose a two-dimensional model of political partisanship. One dimension is the liberal-conservative continuum on social issues, while the other is the degree of extremism or moderation in one’s political views, regardless of what they are. The data from this study by Zmigrod and colleagues suggest that Americans similarly range along these two dimensions, despite living in a two-party system that’s so culturally ingrained in U.S. society.