Valerie Fridland writes in her new book, Why We Talk Funny: the Real Story Behind Our Accents, that humans instinctively to use accents to categorize those around us. “We learn to recognize other people as being like us through the way that they sound,” Fridland says. It happens early: studies suggest small children, when choosing friends, favor those who share their accent.

In one study, for instance, five- and six-year-olds were shown pairs of kids on a computer screen, one with a local Canadian accent and one with a British accent. Asked who they wanted to be friends with, they picked the kid with the local accent – even though they lived in Toronto and are exposed to a huge range of accents every day.

Our accent-based judgments lead to serious problems, fueling stereotypes about class, ethnicity and regional background. That can take a toll in a range of high-stakes scenarios, including job interviews, when someone with a posher accent might be deemed more capable than someone with a more working-class one. It can lead to assumptions about how someone thinks, as in a study that found subjects assume politicians with southern accents are making conservative arguments. It can even affect the way juries react to witnesses, as Fridland believes happened in the trial of George Zimmerman.