The Chemistry of Trust

A person's level of trust can be changed with a chemical spray

SUSPICION and trust are two sides of the same coin. Over the course of evolution, humans and other animals have walked a line between the need for self-preservation and the benefits and delights of social co-operation. When a swarthy man beckons you into a dimly lit alley, you would do well to walk briskly away, but in reality you might be losing an opportunity to discover a delightful but out-of-the-way little restaurant.

A paper in this week's Nature, by Michael Kosfeld and Markus Heinrichs of the University of Zurich and their colleagues, explores the biological underpinnings of trust in such interactions. The researchers found that trust is surprisingly mechanistic: sniffing a spray containing a hormone called oxytocin increases a person's level of trust in others.

Oxytocin, a hormone produced by part of the brain called the hypothalamus, plays many roles. It stimulates contractions during childbirth and, once a child is born, helps to release milk when its mother feeds it. In some species, notably voles, it has been shown to regulate behaviours such as pair bonding, maternal care and the ease with which an animal will approach a stranger. Dr Kosfeld and Dr Heinrichs therefore had good reason to suspect that it plays a role in trust. They also knew from the work of others that hormones consisting of protein fragments known as peptides can cross into the brain if administered as a nasal spray. Oxytocin is one such peptide.

To probe oxytocin's role in promoting trust between people, the researchers invented a game. This game involved an “investor” and an anonymous “trustee” in whom money, in the form of “monetary units” worth 40 Swiss centimes (32 cents) was invested. Investor and trustee never met, and were allowed to interact only once. In addition to being paid for their time, participants were able to cash their monetary units in at the end of the game, in order to get the proper economic juices flowing. Each investor received 12 units. He could choose to keep all of them, or to give four, eight or all 12 of them to the trustee—which would result in their value being tripled. The trustee then chose whether to reward or abuse the investor's trust by sharing a portion of the proceeds with him.

All the investors and all the trustees had something sprayed up their noses before the experiment started. In some cases, though, there was no oxytocin in this spray. Of the investors who were sprayed with oxytocin, 45% invested the maximum of 12 units, while only 21% of those who received the control spray did so. On average, the oxytocin-sprayed group transferred 17% more money to their trustees than the controls. Oxytocin, therefore, seems to promote trust.

The proof that it is trust that is being promoted, rather than a general bonhomie towards others, or a reduced aversion to risk, comes in two parts. The first is the response of the trustees. These people did not, as some might expect, simply take the money and run. The investors usually got something back, albeit less than half of the trebled amount. But the sum returned did not depend on whether there was oxytocin in the spray a trustee had sniffed—as it might have been expected to if oxytocin promoted generally sociable behaviour, rather than trust specifically.

The second piece of proof that oxytocin is “trust-specific” came when the investors were told that a computer rather than a human trustee would be on the other end of the transaction, and that the amount returned would be decided at random. In this set-up, the oxytocin-sprayed group and the control group invested equal amounts. The researchers thus concluded that oxytocin was not simply lowering a person's risk aversion.

Besides helping to unravel the biological basis of an important emotion, Dr Kosfeld and Dr Heinrichs also raise questions about some of the fundamentals of economics. Studies like these are beginning to shed light on the extent to which humans actually resemble Homo economicus, the proverbial rational economic agent. This particular case raises the possibility that those with different hereditary propensities to produce oxytocin, or different sensitivities towards it, might reach different conclusions when presented with similar economic decisions.

While acknowledging that these results could be put to nefarious use to “induce trusting behaviours that selfish actors subsequently exploit”, the authors hope their findings will instead be used to treat mental disorders such as extreme social phobia. Nevertheless, untrusting readers might beware of strange odours or mysterious vapours in the boardroom.