Trying to fight against your own psychological biases has a bizarre side-effect, according to new research published in Psychological Science. The study found the perception of time slows down when white people who are concerned about appearing racially biased are perceiving black faces.

“The current research is the first to illustrate that time perception is affected by race,” wrote Gordon B. Moskowitz and his colleagues, Irmak Olcaysoy Okten and Cynthia M. Gooch, in their study.

“An example of time slowing is the experience that a full second has transpired after only half a second, which makes the duration of an actual second feel longer or slower,” the researchers explained. “The implications of a time-slowing bias for interpersonal interactions are profound—imagine a police officer needing to gauge the time in which a minority suspect must respond before force is exerted. The perceived difference of a half second could determine whether shots are fired.”

For their study, the researchers recruited 40 white students (24 women and 16 men) from Lehigh University in Pennsylvania. The participants completed a survey to measure their motivation to control bias and then participated in a temporal-discrimination task.

In the temporal-discrimination task, the participants viewed a computer screen that displayed a geometric shape followed by another geometric shape, a black face, or a white face. All of the faces had neutral expressions. The first image was displayed for 600 milliseconds, while the second image was displayed for 300 to 1,200 milliseconds. The participant then indicated whether the second image had been displayed for a longer or shorter time than the first image.

The researchers found that short amounts of time were mistaken for longer ones when participants who were motivated to reduce bias viewed black faces. But there was no effect among participants who were not concerned about appearing biased.

“What felt like the standard duration was in reality a significantly shorter period of time; therefore, time was perceived to slow. However, this effect is contingent on motivation,” the researchers said.

A second experiment of 36 white male students used a slightly different procedure to test time perception but came to the same conclusion.

The motivation to not seem to be biased affected participants’ perception of time by increasing arousal, the researchers said.

“Ironically, people trying to suppress the appearance of bias are most likely to display this form of implicit bias because their motivation to control prejudice induces race-related arousal,” Moskowitz and his colleagues wrote.

Previous studies have established that heightened physiological arousal actually slows experience of time’s passage. For example, research from 2011 found the perception of time slows down when people view faces with angry expressions.