A co-worker tells you of their plans to go on a killing spree. You can tell the police, but your co-worker is likely to kill many people before they catch him. You have a gun, and your only option is to kill the co-worker in order to prevent many more deaths. Do you pull the trigger?

It is a common human conceit to think of ourselves as rational and logical beings – we do not like to see ourselves as being slaves to our emotions. The uncomfortable truth is that we make most of our decisions for purely emotional reasons, and then use our vast reasoning capability to justify or rationalize those decisions. It is therefore helpful to understand the nature of human emotions (especially those that are universal and not individual quirks) as well as the relationship between the reasoning and feeling parts of our brains.

A recent study by Ralph Adolphs at Caltech, Antonio Damasio at USC, and others looked at this relationship with regard to the type of moral dilemma I presented above. In this type of dilemma the rational frontal lobes will calculate that killing the co-worker will result in the fewest deaths overall and is therefore the logical and ethical course of action. However, a specific part of the brain called the ventromedial prefrontal cortex (VMPC) creates a negative emotional reaction to the notion of killing another person. These two parts of the brain fight it out, and in most people the emotional inhibition against killing wins out over the cold calculation of body count.

Adolphs and Damasio looked at people with damage to the VMPC, neurologically intact individuals, and those with other kinds of brain damage. They found that those with an intact VMPC had significant aversion to killing their co-worker and thought they simply could not bring themselves to do it. Those with damage to the VMPC did not have this emotional inhibition – they made the rational decision to minimize the body count.

Emotions serve a purpose – they motivate us to take action which is likely to maximize our evolutionary fitness, to survive and reproduce. So we are disgusted at foods that are likely to make us sick, so we don’t eat them. We are frightened by predators that are likely to kill us. We have an overpowering desire to have sex, for obvious reasons. And, it seems, we have a part of the brain that makes us squeamish about killing others, even when it is justified and makes rational sense. It makes sense that a social creature like Homo sapiens would evolve such an emotion. (That is also probably why in situations such as war we tend to dehumanize our enemies – to minimize the inhibiting effect of the VMPC.)

This study, along with the many like it, is a grim reminder of several aspects of the human condition. First, our sense of ourselves is uncomfortably dependent upon our neuroanatomy. We may think of ourselves as good, kind, and reasonable people, and this may be true. But all of us have only a small piece of gray matter in our prefrontal cortex standing between us and being psychopathic killers.

This brings up many interesting questions about guilt and responsibility. If someone who has demonstrable damage to their VMPC in fact kills someone (let’s say because they unjustly stole their parking space), should they be considered innocent by means of neurological damage – similar to the insanity defense? From a practical point of view society needs to be protected from such individuals, but do they really deserve to be punished? Does the brain damage mitigate their guilt?

Further, studies like this remind us that we are emotional creatures, and we evolved our emotions in a very different environment that the one in which we live now. It is useful, therefore, to understand the human emotional landscape, to see where it works and where it doesn’t, and to consciously mitigate the negative consequences of our now maladaptive emotions. For example, the emotion of anger and rage was likely necessary in a primitive tribe for self-defense and defense of our genetic relatives. But such emotions can be dangerous in a world with cars and automatic weapons. We evolved our hunger in a situation where life was largely calorie limited, but now we live with a surfeit of easy calories and as a consequence, obesity.

In this case, however, the VMPC gives us a useful emotion. All things considered it’s a good thing that we feel hesitant about directly harming or killing others.