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Commonsense explanations of neuroscience

How the Brain Feels Betrayed

Insula activity may signal the likelihood of betrayal.

According to Shakespeare, the greatest loss Julius Caesar suffered upon assassination wasn’t his riches or his life, it was the trust of his best friend—“Et tu, Brute?” Betrayal hurts.

A recent study in the Proceedings of the Royal Society: Biological Sciences asked why; how does the brain process the possibility of betrayal? In addition to great tragedy, betrayal also makes great science.   

The study, by economists Jason Aimone and Daniel Houser and neuroeconomist Bernd Weber, used a new task to expose 30 participants to the possibility of betrayal while they completed an MRI scan that measured changes in blood flow, a proxy of brain activity.

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The task was a trust game between two players. Thirty participants were investors. At the beginning of a trial, the investor received between 3 and 7 euros. They then chose if they wanted to trust the other player. If they didn’t trust the other player, the money was split evenly between the two players and the trial ended. If they trusted the other player, the money was tripled and then the trustee got to divide the money however they pleased. They could share fairly to encourage further trust, they could take a little more than half, or they could take the whole sum and swindle the investor.

After completing 41 rounds playing with the trustee, the investor got to play an additional 41 rounds, but this time the trustee did not get to make any decisions. Instead, each of the trustee’s 41 decisions were entered into a computer program and drawn randomly for each trial. The possibility for loss remained, but now, if the investor got swindled, only the computer could be blamed.

When Aimone looked at decisions, he saw that investors changed their behavior depending on if they were playing against the person or computer. Investors trusted on 62% of trials when the computer randomly drew the split, but only on 50% of trials when they were playing against a person. We are more likely to trust a computer than another person.

Interestingly, the differences between computer and person depended on gender. Males trusted on 50% of trials when playing a person, but their trust jumped to 70% of trials when they played the computer. Females didn’t change. They trusted in 47% of trials when the person decided, and blipped up to 54% when the computer picked the split. Guys could be more willing to trust gadgets, or they may like to gamble.  

When the authors looked at the brain, they found that activity depended on if the investor trusted or not. Investors showed more activation in the right anterior insula when choosing to trust—and risk losing—relative to playing safe.This is no surprise, because previous studies show that insula activity is greater when there's a chance to lose money.

Insula activation also depended on whether the investor was playing the person or the computer. Activation was greatest when trusting another person, less when trusting the computer, and least when playing safe. Participants who trusted more recruited the insula less when anticipating the possibility of betrayal.  The authors interpreted this as evidence that the patterns of insula activity could be brain code for disliking betrayal.

We know that social exclusion recruits similar brain regions as feeling physical pain, such as the insula. We also know that the insula plays a role in risky decisions, where activation increases in proportion to the size of the risk. So it’s not surprising that choosing to trust, a situation where you anticipate the chance to get stung, would recruit the insula.

It’s an interesting task and the results are novel and intriguing. My only concern is that it’s still possible that the insula is anticipating loss in general, rather than betrayal specifically. Insula function could be additive. It’s possible that being betrayed in a financial game could represent the combination of losing money and trust. Social loss could be more painful than monetary loss alone, but perhaps losing double the money would produce the same effects.    

It would be interesting to see a study where two groups of participants are recruited, one with high and the other with low aversion to betrayal. It would also be interesting to disentangle the financial from the social risk.

Nonetheless, the study provides evidence that the insula might serve as an alarm bell to alert you to the Brutus in your life.

Joshua Gowin, Ph.D., earned his doctorate in behavioral neuroscience at the University of Texas Health Science Center in Houston.

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