A neural substrate for moral decisions

An advance online publication in Nature shows that damage to a specific region of the frontal lobe alters people’s ability to make moral judgments.

In the study, whose authors include Marc Hauser and Antonio Damasio, six patients with damage to the ventromedial prefrontal cortex (VMPC) were compared with twelve patients with damage to other parts of the brain, and a control group of twelve people with no brain damage. The participants were presented with a series of 50 situations in which non-moral, personal moral or impersonal moral decisions had to be made. their decisions in these situations were compared to those of controls who had no brain damage. 

For example, the participants were asked imagine that they are in control of a runaway boxcar trolley approaching a fork in the track. On the left side is a group of five workmen, while on the right is a single railway worker. The patients had to decide whether to do nothing, in which case the trolley will turn leftwards at the fork, killing the five workers, or to hit a switch, sending the trolley to the right when it reached the fork. In this hypothetical impersonal situation, the decisions of the six brain-damaged patients were no different from those of the controls or of patients with damage to other regions of the brain - all three groups opted to hit the switch.

There was also little difference in the responses of the groups to non-moral situations. But when asked to make personal moral decisions, there was a significant difference. When presented with another fictitious scenario, the brain-damaged patients were about twice as likely to suffocate their baby in order to prevent it from crying and revealing the whereabouts of the subject and his or her neighbours to enemy troops instructed to kill all the remaining civilians in a village. It was also found that the differences between brain-damaged patients and controls was greatest in high-conflict personal situations, which involved a trade-off between harming a single person and the collective welfare of a group of people. The time taken to make decisions in these high-conflict situations was significantly longer in both groups than the reaction time when making decisions about low-conflict situations.   

The six patients had incurred brain damage from an aneurysm or during surgical removal of tumours (orbital meningiomas). Neuroimaging showed that they had damage in different parts of the frontal cortex, but all of them had damage in an overlapping region which included the VMPC. (This overlapping region is marked in red on the image above). Neurons in the VMPC are believed to be involved in encoding the emotional value of sensory stimuli, and project to base of the forebrain and regions of the brainstem which generate the physiological responses to emotions mediated by the autonomic nervous system. Previous studies have shown that damage to the VMPC results in reduced responses to subtle social cues, and a diminished sense of compassion, shame and guilt. In the current study, the intelligence and logical reasoning of the six patients with VMPC damage was unaffected, and they all had full knowledge of social norms. However, they all displayed impaired autonomic responses to emotionally-charged images, and, in line with the previous findings, had a significantly diminished sense of empathy, embarrassment and guilt. Thus, the findings confirm the notion that there are at least two neural systems involved in making moral decisions: one in which emotions are involved, and one which performs a cost-benefit analysis. The former appears to be disrupted in the six patients with VMPC cortex, while the latter is intact. It is believed that the emotion-based system for making moral decisions evolved first, perhaps in a situation where small numbers of people lived in kin groups. Damasio says, “A nice way to think about it is that we have this emotional system built in, and over the years culture has worked on it to make it even better”. Because the study involved a small number of participants making hypothetical moral decisions, Damasio and his colleagues stress that the findings cannot be used to predict how people might act in real situations. Nevertheless, it provides evidence for the role of emotions, particularly social emotions, in making moral decisions, as the differences in the responses of the three groups were greatest when the decisions being made were emotionally charged. Reference: Koenigs, M. et al. (2007). Damage to the prefrontal cortex increases utilitarian moral judgements. Nature doi: 10.1038/nature05631. [Abstract]
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