For a politician who wants to trigger nativist revulsion, a metaphor can be as effective as an explicit insult.

President Donald Trump with Department of Homeland Security Secretary Kirstjen Nielsen during his visit to U.S. Border Patrol McAllen Station in McAllen, Texas, on January 10th, 2019. (Photo: Jim Watson/AFP/Getty Images)

President Donald Trump is fond of describing undocumented immigrants in terms usually reserved for vermin. Just last summer, he warned they would "pour into and infest our country."

New research finds that this sort of language has a pernicious effect on a specific group of people: Those for whom being an American is a central part of their identity. When such people read these remarks, they feel increased levels of disgust toward immigrants, and support harsher measures to keep them out of the United States.

"This research shows the power of metaphor to shape intergroup attitudes and support for government policies," write psychologists Shantal Marshall of Nevada State College and Jenessa Shapiro of the University of California–Los Angeles. Their study is published in the Journal of Social Issues.

Evidence linking anti-immigrant attitudes with a fear of disease has been accumulating in recent years. A 2017 study found that people who are easily disgusted, and are hypersensitive to contamination, are more likely to support anti-immigration policies. Consciously or not, their prejudices are at least partly rooted in the fear that newcomers are carrying with them pathogens from their home countries.

Vermin, of course, are well-known spreaders of disease. So if an unscrupulous politician wants to trigger the above response, comparing immigrants to rodents is an effective strategy. This new research finds that such an association needn't be overt to achieve its goal.

One study featured 114 white Americans who read one of two versions of a news article about illegal immigration—one with neutral language, or another that used metaphors commonly associated with vermin. For example, the first refereed to "large groups" of immigrants who "hurry" across the border; the second referenced "swarms" of immigrants who "scurry" into U.S. territory.

Participants indicated how important being a U.S. citizen is to their sense of identity. Finally, they noted their level of support for four immigration-related policies, such as: "Officers should be able to check for citizenship in case an individual appears illegal."

"The more participants felt being American was central to their identity, the more likely they were to support stringent policies," the researchers report. Crucially, however, this association was limited to those who had read the version with the vermin metaphors.

Another study found that "the higher one's American identity, the more these metaphors increased disgust sensibility." That finding supports the view that anti-immigrant attitudes, at least in part, reflect "real concerns about vulnerability to disease," which specific language can trigger.

"These findings emerged above and beyond participants' political ideology," the researchers add. "American news media consumers may not only be reacting to information about immigration, but to how that information is framed with the use of metaphor."

The findings should serve as a cautionary note to journalists. It seems that, when certain inherently dehumanizing terms are used in relation to immigrants, it triggers a disgust reaction in some people, which in turn hardens their anti-immigrant attitudes.

Perhaps tweets that use such language are best paraphrased—or ignored.