Image by memeda_zyy via Pixabay is licensed under CC0 Image by memeda_zyy via Pixabay is licensed under CC0

Imagine you are the leader of a large country. A passenger plane has been hijacked over your country’s airspace and is heading toward the most densely populated city in the nation. The hijackers have made it clear that they plan to crash the plane into the city’s heavily populated downtown area in an attempt to kill as many people as possible. The head of the military informs you the plane is currently flying over a rural region of the country. If fighter jets are dispatched immediately, the passenger plane could be shot down, killing everyone on the plane but saving the lives of the people in the city.

This exact scenario that was put to Germany’s highest court in 2006 in the wake of the terrorist attacks in New York on 9/11. Two moral arguments were presented. The first argued that killing is always wrong no matter the potential positive outcomes (i.e. lives saved). This deontological argument, often attributed to the moral philosopher Immanuel Kant, in general claims that it is morally wrong to use people as a means to an end.

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The opposition argued that the morally correct thing to do in any situation is to take the course of action that leads to the greatest good. This consequentialist argument, often attributed to the philosophers Jeremy Bentham and John Stuart Mill, makes the general claim that the morally correct action is the one that leads to the best possible outcome. In this view, killing is morally justified if it leads to a greater overall good – the ends justify the means. Ultimately, Germany’s high court endorsed the deontological position arguing that shooting down a hijacked plane is “incompatible with the fundamental right to life and … the guarantee of human dignity” of the passengers on the plane.

While many readers will agree with the ruling of the high court, the fact that this case was adjudicated by the highest court in the land illustrates the contentious nature of this debate. This led my colleagues (Xiaowen Xu and Jason Plaks) and I to ask what traits predispose individuals to favor deontological over consequentialist moral arguments.

We conducted three studies where participants completed a battery of questionnaires. We found that individuals who were more disgust sensitive (i.e. individuals who are more easily disgusted) were more likely to endorse deontological moral judgments and principles. In addition, the relationship between disgust sensitivity was partially explained by how strongly individuals preferred order and were intolerant of ambiguity.

Many readers will find these results curious, specifically the relationship between disgust and deontology. However, research over the last twenty years has stressed the role emotions play in moral thinking. Jonathan Haidt argues that most of our moral judgments are intuitive “gut reactions” driven by emotion. The question still remains: of all emotions, why disgust? A significant body of research has examined the link between disgust sensitivity and moral thinking. Individuals who are more easily disgusted are likely to make more severe moral judgments and recommend more extreme punishment for moral violations.

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Why? Evolutionary theorists argue that the emotion disgust evolved primarily to help organisms avoid dangerous pathogens and disease. When we experience disgust, we are likely to avert our gaze, move away from the object causing the disgust, and are unlikely to consume objects that elicit disgust. By avoiding objects carrying pathogens or disease, our chances of survival increase. Pathogens and disease are not the only forms of danger that should be avoided. Individuals should also avoid people that willingly violate strongly held values (i.e. killing is wrong) or threaten the social order of the group (i.e. deviating from group norms). Avoiding pathogens or dangerous individuals requires the same behavioral response: avoidance. Therefore, evolution has co-opted the disgust response that originated to help us avoid disease to help us avoid social threats that could harm us as individuals or harm our group.

At this point, the reader may think that there may be a link between disgust and morality generally, but why would a disgust sensitive individual favor deontological values? While objects that carry pathogens or disease often have easily identifiable features (i.e. bad smells, the presence of mold, signs of decay), potentially harmful people lack easily identifiable features making them much harder to avoid.

One way to make it easier to identify potential social threats is to hold a set of moral rules that clearly define acceptable and unacceptable actions. This has the effect of reducing ambiguity and increasing the order of the social system – people have a clear understanding of what constitutes a moral violation, and it makes it easier to identify rule violators. Put differently, individuals who are more disgust sensitive have a general preference for order and are intolerant of ambiguity. Living in an orderly society with unambiguous moral rules makes it easy to identify rule violators who should be avoided. A deontological moral system provides an orderly and unambiguous set of rules that outline acceptable and unacceptable behavior.

It should be made clear that the effects we observed were small in nature, meaning that increased disgust sensitivity does not always predict a preference for order and endorsement of deontological values. However, the take-home message is that we often think of our moral beliefs as the result of careful thought and reflection, but, in reality, the types of moral values we come to endorse are in small part influenced by how strongly we experience an emotion that evolved long ago to protect us from pathogens and disease – disgust.