The self-defining characteristics that Americans hold dear include their racial and cultural heritage, the language they speak and their choice of worship.

Shanto Iyengar

A study co-authored by Stanford Professor Shanto Iyengar finds that Americans, like residents of other countries, have strong dislikes for members of the political opposition. (Image credit: L.A. Cicero)

But the strongest attachment, according to recent research from a Stanford scholar, is Americans’ connection to their political party. And the strength of that partisan bond – stronger than race, religion or ethnicity – has amplified the level of political polarization in the U.S., the researchers said.

The study, co-authored by Shanto Iyengar, a professor of communication and political science at Stanford, and appearing in the European Journal of Political Research, finds this increasing partisan divide present not only in America, but in other well-established democracies as well.

The reasons why

So why does “partyism,” as the researchers refer to it, trump other social identifiers like gender, race, religion, language and ethnicity – affiliations many Americans value highly?

One reason, the researchers find, is that who you support politically is your choice while factors like your race and ethnicity are assigned at birth. Therefore, because support for a political party is a deliberate decision for an individual, it’s viewed as a choice that more accurately reflects who that person truly is. “Because partisan affiliation is voluntary, it is a much more informative measure of attitudes and belief structures than, for example, knowing what skin color someone has,” the study states.

Another reason is that – unlike race, religion and gender, where social norms dictate behavior – there are few, if any, constraints on the expression of hostility toward people who adhere to opposing political ideologies, the researchers said. For example, certain words are out-of-bounds when directed toward people of specific races or genders. But these boundaries don’t really apply in a partisan environment and, in fact, boorish behavior can actually be encouraged by party leaders.

“There are no corresponding pressures to moderate disapproval of political opponents,” the study states. “In fact, the rhetoric and behavior of party leaders suggests to voters that it is perfectly acceptable to treat opponents with disdain.  In this sense, individuals have greater freedom to discriminate against out-party supporters.”

The researchers also cite the frequency of election campaigns and negative attack advertisements as other causes for growth of this partisan divide.

The trust game

To measure levels of partisanship, the researchers used a behavioral game involving donating money to individuals based on profiles that included, among other information, their political affiliation. The study involved more than 4,000 participants from Belgium, Spain, the United Kingdom and the United States.

The game’s results revealed that players from all four countries exhibited strong bonds with politically like-minded players while expressing “significant dislike for members of the political opposition,” the paper states. What the researchers also found significant was that this partisan behavior appeared both in divided societies, like Belgium and Spain, where rifts along social lines run deep, and in integrated societies, like the U.K. and U.S., where those social divides are less prevalent.

Among U.S. participants, the researchers found Americans’ animosity toward players from opposing political viewpoints was stronger than favoritism shown toward politically like-minded players. In other words, they disliked their enemies even more than they liked their friends. American players provided an 8 percent bonus to players with the same partisan affiliation. However, Republican participants were penalized 10 percent by Democrats and Democratic participants were penalized 16 percent by Republicans.  According to Iyengar, “This finding suggests that partisans are motivated more by out-group animosity than in-group favoritism.”

Showing their support

The widespread behavior suggests that Americans are not alone in having their partisan beliefs occupy a major identity role.

But where they did find Americans to be distinct was in their outward display of partisan identities. Americans affix bumper stickers to their cars and place yard signs outside their homes advertising their political preferences, a behavior uncommon in other societies where citizens tend to keep those views to themselves.  “American campaigns feature greater involvement on the part of ordinary citizens,” Iyengar said. “Campaigns also last much longer than in Europe, giving people more opportunities to send signals concerning their political affiliation.”

Nevertheless, the researchers don’t foresee the impact of the partisan divide waning anytime soon in democracies. In fact, Iyengar’s past research shows the rift among opposing parties has widened over the past 30 years.

“Defined in terms of affect, voters’ sense of partisanship seems to represent a dominant divide in modern democracies and the strongest basis for group polarization,” the researchers write.

Co-authors are Sean J. Westwood, Dartmouth College; Stefaan Walgrave, University of Antwerp (Belgium); Rafael Leonisio, University of the Basque Country and University of Deusto (Spain); Luis Miller, University of the Basque Country (Spain); and Oliver Strijbis, Berlin Social Science Center (Germany).

Media Contacts

Shanto Iyengar, Department of Political Science:

Milenko Martinovich, Stanford News Service: (650) 725-9281,