Social equality in the number of choice options is represented in the ventromedial prefrontal cortex

Aoki, Ryuta, Matsumoto, Madoka, Yomogida, Yukihito, Izuma, Keise, Murayama, Kou, Sugiura, Ayaka, Camerer, Colin F., Adolphs, Ralph and Matsumoto, Kenji (2014) Social equality in the number of choice options is represented in the ventromedial prefrontal cortex. Journal of Neuroscience, 34 (18), 6413-6421. (doi:10.1523/JNEUROSCI.4427-13.2014).

Abstract

A distinct aspect of the sense of fairness in humans is that we care not only about equality in material rewards but also about equality in nonmaterial values. One such value is the opportunity to choose freely among many options, often regarded as a fundamental right to economic freedom. In modern developed societies, equal opportunities in work, living, and lifestyle are enforced by antidiscrimination laws. Despite the widespread endorsement of equal opportunity, no studies have explored how people assign value to it. We used functional magnetic resonance imaging to identify the neural substrates for subjective valuation of equality in choice opportunity. Participants performed a two-person choice task in which the number of choices available was varied across trials independently of choice outcomes. By using this procedure, we manipulated the degree of equality in choice opportunity between players and dissociated it from the value of reward outcomes and their equality. We found that activation in the ventromedial prefrontal cortex (vmPFC) tracked the degree to which the number of options between the two players was equal. In contrast, activation in the ventral striatum tracked the number of options available to participants themselves but not the equality between players. Our results demonstrate that the vmPFC, a key brain region previously implicated in the processing of social values, is also involved in valuation of equality in choice opportunity between individuals. These findings may provide valuable insight into the human ability to value equal opportunity, a characteristic long emphasized in politics, economics, and philosophy.

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