Study reveals two brain regions linked to judging trustworthiness
TUESDAY, Feb. 19, 2002 (HealthDayNews) -- "Love all, trust a few," wrote Shakespeare, but deciding who you trust is a tricky matter.
Judging who is worthy of your faith is a crucial ability, influencing everything from financial investments to who will look after your family. To make those decisions, we draw on a variety of sources, not the least of which is visual information, including emotional cues on a person's face.
Now, new research points to the areas of the brain that receive and decipher that particular information, giving us our "gut instinct" about who is worthy of our trust, and who isn't.
The study, which appears in the March issue of Nature Neuroscience, focused on two brain regions. One was the amygdala, a brain structure made up of two fingernail-sized structures that respond to potentially dangerous stimuli and trigger the body systems that protect us. The other was a region called the right superior temporal sulcus (STS).
Previous studies of patients who had suffered damage to their amygdala revealed that while they could understand the differences between various emotions, they were blind to the visual cues of those emotions found on images of human faces.
"They tend to be more easily led or taken in," says study co-author Bryan A. Strange, a researcher at University College London's Institute of Cognitive Neuroscience and the Institute of Neurology in London.
However, it wasn't clear whether the amygdala was making the judgment about trustworthiness or if was responding to a judgment made elsewhere in the brain. The British team decided to use brain imaging to decipher how the mind responds to various faces.
In the study, eight men and six women between the ages of 18 and 30 were shown images of 120 white male faces, half of them high school students and half of them university students. The study subjects were shown the faces while undergoing a brain imaging scan.
During the first stage of the experiment, the volunteers were shown 60 faces for one second each and asked to indicate via push buttons whether the person in the image was in high school or college essentially to make a judgment about age. In the second part of the experiment, they were told to indicate whether 60 different faces appeared trustworthy or untrustworthy.
Afterward, the volunteers judged each face on a scale of one to seven, with high scores indicating the most trustworthy image. Then, they were asked to list which of seven emotions neutral, happy, sad, angry, disgust, fear, surprise they had perceived in each face.
On average, the volunteers judged more than half of the faces as having neutral expressions. Faces that were commonly judged as happy were significantly more likely to be rated as trustworthy, while neutral, angry or sad faces were not judged to be as trustworthy.
"The amygdala is sensitive to the trustworthiness independently of what you're doing," says Strange, adding the brain scans revealed the amygdala was more active when shown "untrustworthy faces than the trustworthy ones." This was also true when study participants were asked to judge either trustworthiness or the age of the faces contained in the photos.
"It seems to be an automatic mechanism going on in this particular part of the brain, deciding whether that person is approachable or not," says Strange. "It doesn't matter what you're doing or what aspect of the face you're attending to."
However, says Strange, when the volunteers were told explicitly to judge whether the faces they were examining were trustworthy, the imaging scans also showed activity in the right STS.
Ralph Adolphs, the author of a commentary accompanying the study and an assistant professor of neurology at the University of Iowa, says the brain imaging helps to make distinctions about the brain regions involved in judging trustworthiness.
"It adds more detail to the picture of how the brain links what we see in the world our perception, in this case, of other peoples' faces to the kinds of judgments that we make about them," says Adolphs.
"It's a complex issue," he notes, because the brain uses a variety of regions and strategies to perform this task.
It's not yet clear what facial features signal trustworthiness, says Strange, but the researchers plan to study this. There is some preliminary evidence that distance between the eyes is also an important factor.
In addition, there are other factors that deserve more study, he adds.
"When men look at women, they tend to be more trusting," says Strange, adding that previous studies have shown that older people are generally judged as seeming more trustworthy. He adds that differences in gender and age will be examined in greater detail in future studies.