Voters make choices based less on only economic self-interest than was once thought and more on political identity, according to a growing body of recent research. London Business School’s Stephanie Y. Chen, a recent graduate of Chicago Booth’s PhD Program, and Booth’s Oleg Urminsky add to mounting evidence that party affiliation is key, finding that people who consider their political party more central to their identity tend to vote along party lines more than those who see it as more peripheral.

This role of an affiliation in a person’s identity is known as “causal centrality.” Consider the hypothetical example of two Republican women in the United States. One says that being a Republican doesn’t inform other aspects of her identity, such as her beliefs and convictions, or even her choice of profession. The other says being a Republican does inform these things. For the second person, being a Republican is more causally central.

In an online survey in the US, Chen and Urminsky directed 355 people, who had already disclosed their political affiliations, to think about how being a Republican or a Democrat related to other aspects of their identity. They asked Democrats, “Which of the other features of your personal identity listed below, if any, are caused by you being a Democrat?” The choices included: “being female/male,” “being pro-gun control,” “being pro-choice,” and “being a college graduate.” Participants checked the aspects they agreed with. In some cases, the researchers gave participants the list of options, and in other cases, participants generated their own. They posed a similar set of questions to Republicans.

Chen and Urminsky find that people for whom political identity was more causally central were more likely to vote along party lines, even if they disliked their party’s candidate.

They find that national identity, when it is causally central, can also predict political behavior. A poll they conducted of 243 UK residents about the Brexit referendum reveals that people who evaluated being British or English as more causally central to their identities were more likely to support Brexit than those for whom national identity was less central.

The findings could be used to further segment voters, Urminsky says, but he hopes they will help people better understand those with different politics. A Democrat trying to understand a Republican’s votes might, for example, look at how a person’s family history or political role models led someone to become a Republican.

“Identity is really the roadmap of our lives,” he says. “A more nuanced understanding of why political parties are important to people may give us strategies for talking to others.”

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