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Your Brain Decides Whether to Trust Someone in Milliseconds

We can determine trustworthiness even when we’re only subliminally aware of the other person.
(Photo: jfchenier/Flickr)

(Photo: jfchenier/Flickr)

Say you’re in the supermarket parking lot holding your infant, bags of groceries, and fumbling to open your car. A stranger walks up and says, “Here, let me hold your baby.” Should you let him?

According to a new New York University study, knowing whether or not to trust someone is so critically important that we can tell whether a face is trustworthy before we even consciously know it's there.

The NYU researchers knew from previous studies that people are fairly similar when it comes to how they judge a face’s trustworthiness. They wanted to find out whether that would hold true if people only saw a face for a quick moment—an amount of time so short, in fact, that it would prevent making a conscious assessment.

Faces with "higher inner eyebrows and pronounced cheekbones," the paper explains, "are seen as trustworthy and lower inner eyebrows and shallower cheekbones are seen as untrustworthy."

They found that the “human amygdala is automatically responsive to a face’s trustworthiness in the absence of perceptual awareness.” In short, we don’t have to think logically about whether we should trust someone—our brains know the instant we encounter them.

To carry out their study, the researchers monitored the amygdalae of 37 volunteers (28 females) ages 18 to 35 while showing them 300 faces for 33 milliseconds each. Those faces had already been tested with a different set of 10 subjects, who saw them for much longer to determine whether they were trustworthy. In those previous tests, people had a strongly uniform opinion about whether to trust each face. Faces with “higher inner eyebrows and pronounced cheekbones,” the paper explains, “are seen as trustworthy and lower inner eyebrows and shallower cheekbones are seen as untrustworthy.” Other studies suggest that we can detect trustworthiness in someone else’s face thanks to subtle cues like the amount of white showing in the eyes.

After the new subjects “saw” each face for 33 milliseconds, the image was replaced by “a neutral face mask for 167 milliseconds that disrupted further visual processing of the target.” Effectively, the researchers were letting their subjects see each face only subliminally.

Fascinatingly, different parts of the amygdala lit up when a subject saw an untrustworthy face versus a trustworthy one—and it lit up more when the face in question was suspicious.

“Faces that appear more untrustworthy and likely to inflict harm,” says Jon Freeman, the study’s senior author, “are spontaneously tracked by the amygdala, so the amygdala could then quickly alter other brain processes and coordinate fast, appropriate responses to people—approach or avoid.”

His research suggests that the amygdala’s role in instantly interpreting social cues is more important than previously thought, which makes sense when you consider human history. Aggression and conflict, the study says, “have had a substantial impact on human evolution.... Automatic evaluation of another’s likelihood to harm or help via facial trustworthiness would facilitate survival.”

Our talents for making snap judgments “could either be hard-wired or learned from the social environment,” Freeman says. “Our results can't speak to that issue.”

So should you trust the guy in the parking lot? Your brain already knows.

Rosie Spinks contributed reporting.