MutaLab - Laboratoire de la Mutation

Cerveau, justice et magot : comment une petite zone cérébrale nous fait souvent faire de grosses conneries Une région de cerveau pourrait limiter notre égoïsme naturel et expliquer la naissance de la justice dans les sociétés humaines. C’est en tout cas l’hypothèse lancée par Ernst Fehr, un économiste zurichois. Ce dernier, avec ses collègues, a étudié, via stimulation électromagnétique transcrânienne, l’influence d’une région cérébrale précise, le cortex préfrontal dorsolatéral, dans l’appréhension commune de l’équité et de l’injustice. Les humains sont en effet les seuls animaux, dans l’état actuel de nos connaissances, à agir méchamment afin de rendre la « justice », à punir des gens qui se sont comportés « injustement » et ce même si la punition n’est pas dans l’intérêt du punisseur. Il a été jusqu’alors difficile d'expliquer cette tendance en termes évolutionnaires, parce qu'elle semble n’avoir aucun avantage adaptatif évident, tant la punition de l'injustice peut quelquefois léser le punisseur lui-même. A l'aide du « jeu de l’ultimatum », les chercheurs ont identifié la partie du cerveau responsable dans la punition de l'injustice. Des sujets ont été apérés de façon anonyme, chaque partenaire recevait 20 $ qu’il était invité à partager avec son compagnon. Il pouvait choisir d’offrir n’importe quelle quantité et, si l’autre l’acceptait, chacun gardait son butin. Dans le cas où il refusait, tout le monde perdait. Selon un modèle purement économique où l’intérêt personnel prime toujours, personne ne devait refuser l’offre, même si elle était vraiment basse (inférieure ou égale à 1 $) mais dans la réalité, les receveurs dénigraient presque systématiquement l’offre, empêchant ainsi à l’autre de garder le reste. En désactivant via stimulation électro-magnétique intracrânienne le cortex préfrontal dorsolatéral, les receveurs étaient plus enclins à accepter toute sorte d’offre. *** Sense of justice discovered in the brain • 19:00 05 October 2006 • NewScientist.com news service • Helen Phillips A brain region that curbs our natural self interest has been identified. The studies could explain how we control fairness in our society, researchers say. Humans are the only animals to act spitefully or to mete out "justice", dishing out punishment to people seen to be behaving unfairly – even if it is not in the punisher's own best interests. This tendency has been hard to explain in evolutionary terms, because it has no obvious reproductive advantage and punishing unfairness can actually lead to the punisher being harmed. Now, using a tool called the “ultimatum game”, researchers have identified the part of the brain responsible for punishing unfairness. Subjects were put into anonymous pairs, and one person in each pair was given $20 and asked to share it with the other. They could choose to offer any amount – if the second partner accepted it, they both got to keep their share. In purely economic terms, the second partner should never reject an offer, even a really low one, such as $1, as they are still $1 better off than if they rejected it. Most people offered half of the money. But in cases where only a very small share was offered, the vast majority of "receivers" spitefully rejected the offer, ensuring that neither partner got paid. Fehr's fair Previous brain imaging studies have revealed that part of the frontal lobes known as the dorsolateral prefrontal cortex, or DLPFC, becomes active when people face an unfair offer and have to decide what to do. Researchers had suggested this was because the region somehow suppresses our judgement of fairness. But now, Ernst Fehr, an economist at the University of Zurich, and colleagues have come to the opposite conclusion – that the region suppresses our natural tendency to act in our own self interest. They used a burst of magnetic pulses called transcranial magnetic stimulation (TMS) – produced by coils held over the scalp – to temporarily shut off activity in the DLPFC. Now, when faced with the opportunity to spitefully reject a cheeky low cash offer, subjects were actually more likely to take the money. The researchers found that the DLPFC region's activity on the right side of the brain, but not the left, is vital for people to be able to dish out such punishment. "The DLPFC is really causal in this decision. Its activity is crucial for overriding self interest," says Fehr. When the region is not working, people still know the offer is unfair, he says, but they do not act to punish the unfairness. Moral centre? "Self interest is one important motive in every human," says Fehr, "but there are also fairness concerns in most people." "In other words, this is the part of the brain dealing with morality," says Herb Gintis, an economist at the University of Massachusetts in Amherst, US. "[It] is involved in comparing the costs and benefits of the material in terms of its fairness. It represses the basic instincts." Psychologist Laurie Santos, at Yale University in Connecticut, US, comments: "This form of spite is a bit of an evolutionary puzzle. There are few examples in the animal kingdom." The new finding is really exciting, Santos says, as the DLPFC brain area is expanded only in humans, and it could explain why this type of behaviour exists only in humans. Fehr says the research has interesting implications for how we treat young offenders. "This region of the brain matures last, so if it is truly overriding our own self interest then adolescents are less endowed to comply with social norms than adults," he suggests. The criminal justice system takes into account differences for under-16s or under-18s, but this area fully matures around the age of 20 or 22, he says. Journal reference: Science (DOI: 10.1126/science.1129156)