Curr Biol. 2015 Jul 1. pii: S0960-9822(15)00595-3. doi: 10.1016/j.cub.2015.05.021. [Epub ahead of print]

Dissociable Effects of Serotonin and Dopamine on the Valuation of Harm in Moral Decision Making.

Crockett MJ1, Siegel JZ2, Kurth-Nelson Z3, Ousdal OT4, Story G5, Frieband C5, Grosse-Rueskamp JM5, Dayan P6, Dolan RJ3.

Author information

• 1Department of Experimental Psychology, University of Oxford, 9 South Parks Road, Oxford OX1 3UD, UK; Wellcome Trust Centre for Neuroimaging, University College London, 12 Queen Square, London WC1N 3BG, UK. Electronic address: molly.crockett@psy.ox.ac.uk.
• 2Department of Experimental Psychology, University of Oxford, 9 South Parks Road, Oxford OX1 3UD, UK.
• 3Wellcome Trust Centre for Neuroimaging, University College London, 12 Queen Square, London WC1N 3BG, UK; Max Planck-UCL Centre for Computational Psychiatry and Ageing, University College London, 12 Queen Square, London WC1N 3BG, UK.
• 4Wellcome Trust Centre for Neuroimaging, University College London, 12 Queen Square, London WC1N 3BG, UK; Department of Radiology, Haukeland University Hospital, Jonas Lies vei 65, 5021 Bergen, Norway.
• 5Wellcome Trust Centre for Neuroimaging, University College London, 12 Queen Square, London WC1N 3BG, UK.
• 6Gatsby Computational Neuroscience Unit, University College London, Alexandra House, 17 Queen Square, London WC1N 3AR, UK.

Abstract

An aversion to harming others is a core component of human morality and is disturbed in antisocial behavior [1-4]. Deficient harm aversion may underlie instrumental and reactive aggression, which both feature in psychopathy [5]. Past work has highlighted monoaminergic influences on aggression [6-11], but a mechanistic account of how monoamines regulate antisocial motives remains elusive. We previously observed that most people show a greater aversion to inflicting pain on others than themselves [12]. Here, we investigated whether this hyperaltruistic disposition is susceptible to monoaminergic control. We observed dissociable effects of the serotonin reuptake inhibitor citalopram and the dopamine precursor levodopa on decisions to inflict pain on oneself and others for financial gain. Computational models of choice behavior showed that citalopram increased harm aversion for both self and others, while levodopa reduced hyperaltruism. The effects of citalopram were stronger than those of levodopa. Crucially, neither drug influenced the physical perception of pain or other components of choice such as motor impulsivity or loss aversion [13, 14], suggesting a direct and specific influence of serotonin and dopamine on the valuation of harm. We also found evidence for dose dependency of these effects. Finally, the drugs had dissociable effects on response times, with citalopram enhancing behavioral inhibition and levodopa reducing slowing related to being responsible for another's fate. These distinct roles of serotonin and dopamine in modulating moral behavior have implications for potential treatments of social dysfunction that is a common feature as well as a risk factor for many psychiatric disorders.