Trust in the brain

In one of the first ever studies to simultaneously scan the brains of two people, researchers at the Human Neuroimaging Lab in Texas have furthered our understanding of how the brain represents trust.

Forty-eight pairs of participants took part in an economics game. Groundbreaking 'hyperscanning' software meant that while one person lay in a scanner in Texas, their partner lay in a similar machine hundreds of miles away in California, and yet their brain images could be analysed together.

One partner was given $20 and was the investor. Each round they chose how much money to pass to their partner, the 'trustee'. Both players knew that according to the rules, the money tripled once it was passed to the trustee. The trustee then chose how much of that profit to share back with the investor before the next round began. There were ten rounds.

When an investor raised their next investment despite the trustee having been unfair the previous round, the researchers found that the trustee's brain then reacted in a characteristic way. Their caudate nucleus - a neural area well-connected to the brain's reward pathways - became more active, probably reflecting their increasing 'trust' in the investor's good will. Activity in this region of their brain then also predicted how generously they shared that round's profit.

Moreover, as the game wore on, this differential activity in the caudate began occurring earlier by several seconds, suggesting the trustee was starting to anticipate a generous investment (or not) from their partner as they learned their playing style.

The researchers said: "Taken together, these results suggest that the head of the caudate nucleus receives or computes information about (i) the fairness of a social partner's decision, and (ii) the intention to repay that decision with trust".
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King-Casas, B., Tomlin, D., Anen, C., Camerer, C.F. Quartz, S.R. & Montague, P.R. (2005). Getting to know you: reputation and trust in a two-person economic exchange. Science, 308, 78-83.

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