What Your Brain Knows About Trust

'Intention to Trust' Mapped Out by Scientists

From the WebMD Archives

March 31, 2005 -- Want to build trust with someone? Watch how you treat them and be more generous than they expect -- the sooner, the better.

That nugget of common sense has scientific backing. Brain scans reveal what people's minds were up to during a game of trust.

The results hint at what makes or breaks trust -- a complex process in the "real world" outside the lab. The findings might also yield new leads on conditions like schizophrenia, autism, and borderline personality disorder, say the researchers. Those conditions can make it tough to size people up correctly, they note.

Trust Test

The experiment paired up students from Baylor College of Medicine in Houston, and the California Institute of Technology.

The partners were anonymous. Each pair consisted of an "investor" and a "trustee." Here's how it worked:

The investor could give the trustee any amount out of $20. That money tripled in value with the trustee. Then, the trustee could decide how much money to give back to the investor. They didn't chit-chat, shake hands, sign contracts, or anything else.

All 48 pairs played the game 10 times. That gave each person time to build a reputation with their partner. Meanwhile, their brains were scanned as they played.

Burned or Benefited by Trust?

It didn't take long for the students to form "intentions to trust," say the researchers. That intention centered on reciprocity. In other words, if the investor short-changed the trustee, they could trust that their partner would do the same when they had the chance.

When someone was more generous than expected, the brain lit up with surprise, say the researchers. For instance, if the trustee gave back more money than expected early on, the investor often trusted them with more money in the next round. Generosity increased trust.

One brain area -- the caudate nucleus -- was particularly important. That's where generosity's surprise effect was noted by the researchers, who included P. Read Montague, PhD, associate professor of neuroscience at Baylor College of Medicine.

Fair play or being sold short didn't have the same effect on the brain, the study shows. The study appears in the April 1 issue of Science.

Show Sources

SOURCES: King-Casas, B. Science, April 1, 2008; vol 308: pp 78-83. News release, Science.
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