| |
Brain chemical serotonin helps us tolerate foul play and appears to cause aggressive angry behavior response
|
| |
| |
Serotonin allows us to keep our cool when faced with life's unfairness.
Daniel Cressey
Foul! Without serotonin, we'd find it harder to deal with others who treat us harshly.Punchstock
Controlling your anger and reacting sensibly when someone treats you badly can be a problem. And if you have low levels of serotonin, it can be even more of a problem, a new study has found.
Molly Crockett at the University of Cambridge, UK, and her colleagues gave volunteers a drink that temporarily lowered their levels of serotonin, a brain 'neurotransmitter' linked to happy mood. They then had them play ethe Ultimatum Game', which involves accepting or rejecting offers of money.
Those with lower serotonin levels showed increased retaliation to offers that they perceived to be unfair.
"We've suspected for years that there's a link between serotonin and impulsive aggression and emotional regulation," says Crockett. "Until this study it wasn't clear whether serotonin was playing a causal role."
It has long been known that low serotonin levels are associated with groups of people prone to impulsiveness and problems with emotional control, such as alcoholics, violent criminals and suicide attempters. Low serotonin is also found in clinical conditions such as depression and anxiety.
"We've known for 30 years that low serotonin is associated with impulsivity, inwardly directed aggression and outwardly directed aggression," says David Nutt, head of the Psychopharmacology Unit at the University of Bristol's Faculty of Medicine and Dentistry, who was not involved in the new study.
"What we are doing now is externally manipulating it. We need to study it in a more controlled environment."
The Ultimatum Game
So how does the Ultimatum Game work? Well, imagine what you would do if someone offered you 6. But what if they offered you 6 out of a total of 13 that they'd been given to split with you?
Most people would take the money, especially if rejecting the money meant getting nothing.
But what if they offered you 6 out of 30 they'd been given to split with you? That is a clearly unfair split, but logically it is still better than nothing.
In the Ultimatum Game, you can punish the other player by rejecting the offer, meaning they get nothing. Of course, you also get nothing.
In Crockett's study, detailed this week in Science 1, 20 participants were given a number of attempts at the game, with fair offers, defined as 45% of the stake, unfair offers, defined as 30% of the stake, and very unfair offers, defined as 20%. Participants were randomized to get the serotonin-lowering treatment or a placebo.
While placebo participants rejected about 65% of very unfair offers, those with low serotonin rejected more than 80%.
Researchers also measured the mood, fairness judgement and reward processing of participants. They found these to be unaffected by lower serotonin, clearly implicating the neurotransmitter in the more aggressive response to injustice.
Our desire for fairness also seems to be a universal trait.
Crockett says even when the Ultimatum Game has been tested in poor countries with amounts of money equivalent to a week's or a month's wage, people still reject unfair offers. "The motivation for fairness is a strong one," she says.
*
References
1. Crockett, M. et al., Science, early online publication doi: 10.1126/science.1155577 (2008).
Serotonin Modulates Behavioral Reactions to Unfairness
www.sciencexpress.org / 5 June 2008/
Molly J. Crockett,1,2* Luke Clark,1,2 Golnaz Tabibnia,3 Matthew D. Lieberman,3 Trevor W. Robbins1,2
1Department of Experimental Psychology, University of Cambridge, UK.
2Behavioural and Clinical Neuroscience Institute, University of Cambridge, UK.
3Department of Psychology, University of California Los Angeles, USA.
*To whom correspondence should be addressed. E-mail: mc536@cam.ac.uk
One of the first social rules we learn as children is the golden one: treat others as you wish to be treated. Unfortunately, our peers do not always deserve gold stars for their behavior, which tempts us to retaliate. Resisting aggressive impulses may be difficult, but successfully navigating social life sometimes requires self-regulation in the face of perceived injustice.
Serotonin (5-HT) has long been implicated in social behavior, including impulsive aggression, but its precise involvement in impulse control is controversial (1). Since social interactions can evoke strong emotions, it is plausible that 5-HT modulates impulsivity via emotion regulation mechanisms. Emotion regulation during social interactions has been studied with the Ultimatum Game (UG), in which one player (the proposer) proposes a way to split a sum of money with another player (the responder). If the responder accepts the offer, both players are paid accordingly. If the responder rejects the offer, neither player is paid. Responders tend to reject offers less than 20-30% of the total stake, despite the fact that such retaliation is costly (2), and rejection decisions are predicted by the intensity of the aversive response to the unfair offer (3,4).
We investigated the effects of manipulating 5-HT function on rejection behavior in the UG. We used a double-blind, placebo-controlled acute tryptophan depletion (ATD) procedure to temporarily lower 5-HT levels in 20 healthy volunteers (5). Once following ATD and once following placebo, participants played the role of responder during several one-shot UGs (Fig. 1A, 5). Offers fell into one of three "fairness" categories: 45% of stake (fair), 30% of stake (unfair), or 20% of stake (most unfair). We independently manipulated social reward (fairness) and basic monetary reward (offer size) by varying both the offer amount and the stake size across trials (Fig. 1B, 5).
Rejection rates (% offers rejected) were calculated for each subject at each level of fairness, during ATD and placebo treatments. Repeated-measures analysis of variance revealed a highly significant interaction between treatment and fairness (F=6.891, P=0.003). Compared to placebo, ATD significantly increased rejection rates, and this effect was restricted to unfair offers (Fig. 1C, supporting online text). In contrast, ATD did not interact significantly with offer size (F=1.164, P=0.294). Controlling for fairness, participants tended to reject low offers more frequently than high offers, regardless of treatment (supporting online text).
The increased rejection of unfair offers following ATD cannot easily be attributed to changes in mood, fairness judgments, or basic response inhibition. As found previously (1), there was no effect of ATD on self-reported mood (5, supporting online text). On each session, we asked participants to indicate the size of a fair offer for each stake, and ATD did not affect these judgments (F=0.648, P=0.431). Finally, consistent with past research (1), we found no effect of ATD on Go/No-go performance, a standard test of response inhibition (5, supporting online text).
These results show that manipulating 5-HT function can selectively alter reactions to unfairness in a laboratory model of self-regulation. Temporarily lowering 5-HT levels increased retaliation to perceived unfairness without affecting mood, fairness judgments, basic reward processing or response inhibition. Our results illuminate the neural mechanisms underlying emotion regulation in the UG. Neuroimaging studies of the UG have implicated both dorsolateral prefrontal cortex (DLPFC) and ventral PFC (VPFC) in regulating reactions to unfair offers (3, 4). While disrupting DLPFC function with transcranial magnetic stimulation leads to decreased rejection of unfair offers (6), patients with VPFC damage reject a higher proportion of unfair offers than controls (7). The present effects of ATD mirror those of VPFC lesions, and are consistent with other data (8) indicating a critical neuromodulatory role for 5-HT in this region.
References and Notes
1. E.A.T. Evers et al., Psychopharmacology 187, 200 (2006).
2. W. Guth, R. Schmittberger, B. Schwarze, J Econ Behav
Org 3, 367 (1982).
3. A. G. Sanfey, J. K. Rilling, J. A. Aronson, L. E. Nystrom,
J. D. Cohen, Science 300, 1755 (Jun 13, 2003).
4. G. Tabibnia, A. B. Satpute, M. D. Lieberman,
Psychological Science 19, 339 (2008).
5. Materials and methods are available on Science online.
6. D. Knoch, A. Pascual-Leone, K.Meyer, V. Treyer, E. Fehr,
Science 314, 829 (2006).
7. M. Koenigs, D. Tranel, J Neurosci 27, 951 (Jan 24, 2007).
8. H. Clarke et al., J Neurosci 25, 532 (Jan 12, 2005).
9. This work was completed within the University of
Cambridge Behavioural and Clinical Neuroscience
Institute, funded by a joint award from the Medical
Research Council and the Wellcome Trust.
|
|
| |
| |
|
|
|
