The next time you feel angry at a friend who has let you down, or grateful toward one whose generosity has surprised you, consider this: you may really be bargaining for better treatment from that person in the future. According to a controversial new theory, our emotions have evolved as tools to manipulate others into cooperating with us.

Until now, most psychologists have viewed anger as a way to signal your displeasure when another person does you harm. Similarly, gratitude has been seen as a signal of pleasure when someone does you a favour. In both cases, emotions are seen as short-term reactions to an immediate benefit or cost.

But it’s more cunning than that, says John Tooby, an evolutionary psychologist at the University of California, Santa Barbara. Anger, he says, has as much to do with cooperation as with conflict, and emotions are used to coerce others into cooperating in the long term.

Tooby and his colleagues think that our anger or gratitude reflect our judgement of how much the other person is sacrificing enough for us – and whether they will continue to do so in future.


For instance, you might feel angry towards a friend who broke a dinner date to watch a TV programme, but not at one who did so to take his child to the hospital. Tooby points out that the harm to you is the same in each case, but the first friend’s behaviour indicates his low regard for your interests – triggering anger – while the second friend’s does not.

Motivation matters

As another example, an unpublished study by Tooby’s colleague Julian Lim found that 296 student volunteers were more willing to cooperate with an unseen partner when that partner had forgone a profit to give them money. This gratitude was absent when the partner gave them the same amount of money not as a favour but to avoid paying a penalty, Lim reported last month at a meeting of the Human Behavior and Evolution Society in Eugene, Oregon.

All this suggests that anger and gratitude – and perhaps other emotions, too – may be tools for turning up a partner’s mental cooperation control dial, says Tooby’s colleague Aaron Sell. You get angry not when someone hurts you, but when their actions betray a setting of their cooperation dial that is lower than you expect, and your anger is both a threat to turn down your own dial and an inducement to them to turn theirs up. You show gratitude not when someone benefits you, but when their dial is set higher than you expect, and this signals that you plan to turn yours up in response.

Their hypothesis is not yet fleshed out, and the team still has to show that these emotional reactions really do tune the partner’s cooperation control dial as they predict – but they say they have experiments in the pipeline which do just that.

Strength and beauty

Preliminary evidence is consistent with the idea, however. Psychological tests of 281 university students revealed that those with a stronger sense of entitlement tended to be more anger-prone, as one would predict if they expected others to set their cooperation dials higher.

Stronger men and more attractive women were quicker to anger, too, Sell reported last year (Proceedings of the National Academy of Sciences, DOI: 10.1073/pnas.0904312106). Although stronger men wouldn’t necessarily have any reason to expect better treatment in modern society, in the past they would have been desirable mates – as attractive women may still be – and so may have a stronger sense of entitlement.

And a wealth of evidence – some of it presented at last month’s meeting, and some still unpublished – suggests that the cooperation control dial, or “welfare trade-off ratio”, is a real part of our mental make-up, says Tooby.

Others in the field remain unpersuaded. “Whether [anger] constitutes some form of negotiation is a just-so story,” says Michael Lewis, a developmental psychologist at the Robert Wood Johnson Medical School in New Brunswick, New Jersey. “They certainly don’t have any strong evidence to support that claim.”

And beware of experiments using university students, warns Joe Henrich, an evolutionary anthropologist at the University of British Columbia in Vancouver, Canada. Tests have shown that North American university students are some of the least typical people on the planet, psychologically speaking: “If you’re going to build a theory of human nature, university students might be the worst population to use,” he says.