WASHINGTON—Think you wouldn’t tolerate a racist act? Think again, says a surprising experiment that exposed some college students to one and found indifference at best.

Here’s the scene: Researchers in Toronto recruited 120 non-black York University students for what purported to be a psychology study. A participant was directed to a room where two actors posing as fellow participants—one black, one white—waited. The black person said he needed to retrieve a cell phone and left, gently bumping the white person’s leg on the way out. The white actor then did one of three things: Nothing. Said, “I hate when black people do that.” Or used the N-word.

Then a researcher entered and said the “psychology study” was starting and that the student should pick one of the two others as a partner for the testing.

Half the participants just read about that scene, and half actually experienced it.

Those asked to predict their reaction to either comment said they’d be highly upset and wouldn’t choose the white actor as their partner.

Yet students who actually experienced the event didn’t seem bothered by it—and nearly two-thirds chose the white actor as a partner, the researchers report Friday in the journal Science.