Scientific Understanding of Consciousness
Human Preferences Sensitive to Inequality
Nature 463, 1089-1091 (25 February 2010)
Neural evidence for inequality-averse social preferences
Elizabeth Tricomi, Antonio Rangel, Colin F. Camerer & John P. O’Doherty
Psychology Department, Rutgers University, Newark, New Jersey 07102, USA
Division of the Humanities and Social Sciences,
Computational and Neural Systems, California Institute of Technology, Pasadena, California 91125, USA
School of Psychology and Trinity College Institute of Neuroscience, Trinity College, Dublin 2, Ireland
A popular hypothesis in the social sciences is that humans have social preferences to reduce inequality in outcome distributions because it has a negative impact on their experienced reward. Although there is a large body of behavioural and anthropological evidence consistent with the predictions of these theories, there is no direct neural evidence for the existence of inequality-averse preferences. Such evidence would be especially useful because some behaviours that are consistent with a dislike for unequal outcomes could also be explained by concerns for social image or reciprocity, which do not require a direct aversion towards inequality. Here we use functional MRI to test directly for the existence of inequality-averse social preferences in the human brain. Inequality was created by recruiting pairs of subjects and giving one of them a large monetary endowment. While both subjects evaluated further monetary transfers from the experimenter to themselves and to the other participant, we measured neural responses in the ventral striatum and ventromedial prefrontal cortex, two areas that have been shown to be involved in the valuation of monetary and primary rewards in both social and non-social contexts. Consistent with inequality-averse models of social preferences, we find that activity in these areas was more responsive to transfers to others than to self in the ‘high-pay’ subject, whereas the activity of the ‘low-pay’ subject showed the opposite pattern. These results provide direct evidence for the validity of this class of models, and also show that the brain’s reward circuitry is sensitive to both advantageous and disadvantageous inequality.
A pervasive notion in social science is that human preferences and behaviour are sensitive to inequality considerations. This suggestion is based on considerable experimental and field evidence. Small-scale societies often share through norms of informal social insurance, and large-scale societies share through governmental welfare transfers and progressive taxation. In corporations, wages are typically secret and vary less than productivity does, as though workers have a strong aversion for earning less than others do. Workers also seem to reciprocate when they feel companies have treated them well. but withdraw effort when they feel wronged. These social patterns have been replicated under controlled conditions in many behavioural economics experiments: participants regularly share wealth with strangers, punish non-cooperators at a cost to themselves. and reject unfair divisions of a pool of money.
Subjective ratings, and preferences inferred from choices, provide evidence that subjects like transfers that reduce inequality. However, it is unknown whether reward structures in the brain respond to self–other monetary gains in ways that reflect a preference for reducing inequality. The missing neural evidence is important because most of the behavioural observations consistent with a dislike for unequal distributions can also be explained by concerns for social image or reciprocity, rather than an aversion to inequality. Furthermore, the behavioural evidence is mixed about people’s dislike of advantageous inequality (that is, a willingness to decrease their own payoff to improve those of people who are worse off). Some studies indicate that the better-off will pay to reduce an outcome gap, but other studies suggest they will pay only to maintain or to increase their relative status
We used functional magnetic resonance imaging (fMRI) to test directly for the presence of inequality-averse social preferences in the human brain, for both positive and negative inequality.
Our test focused on the response of the ventral striatum and ventromedial prefrontal cortex (vmPFC), areas known to be involved in the valuation of stimuli at the time of decision making and with processing the experienced subjective value of receiving monetary reward and other rewards
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