If I were to say that I’m thinking about having sex with my stepbrother, I guess you’d tell me to think again: sex with a sibling or even a stepsibling is just plain wrong – it’s not a morally acceptable action. The reason I’m posing this hypothetical proposition is because it’s worth considering why we find this kind of behaviour so wrong. Is this judgment based on a rationally derived principle about maximising good and minimising harm? Surely sex with my sibling would harm our relationship, not to mention the rest of our family’s relationship with each of us. Or is the moral judgment here based simply on the fact that sibling sex makes us more than a little queasy? In other words, are our moral beliefs merely gut feelings – quite literally stemming from our body’s tendency to become repulsed by certain human behaviours?

There are, after all, practices that many of us deem morally wrong and disgusting, including sex with a close relative, but also touching a dead body, or eating a recently deceased pet. And the more disgusting we find these behaviours, the more wrong they seem (sibling sex is obviously worse than first-cousin sex, which is worse than second-cousin sex, etc). This association begs the question: might our moral judgments come from the sickened way that morally improper behaviours make us feel? And if feelings of nausea cause our moral beliefs, could that explain why certain objectively blameless practices – homelessness – are considered by many to be morally taboo?

Until recently, no research study had been able to figure out if the disgust felt upon encountering a morally troubling situation is what makes us decide that the situation is wrong. In fact, no study had even determined whether that feeling is real – whether, when we say we are disgusted by some morally reprehensible event, we mean it literally: we feel nauseous.

This gap in scientific knowledge led my former graduate student Conor Steckler to come up with a brilliant idea. As those prone to motion sickness might know, ginger root can reduce nausea. Steckler suggested we feed people ginger pills, then ask them to weigh in on morally questionable scenarios – behaviours such as peeing in a public pool, or buying a sex doll that looks like one’s receptionist. If people’s moral beliefs are wrapped up in their bodily sensations, then giving them a pill that reduces some of those sensations might reduce how wrong those behaviours seem.

In my psychology lab at the University of British Columbia, we filled empty gel capsules with either ginger powder or sugar (for randomly assigned control participants); in a double-blind design, neither the participants nor the researchers running the study knew who received which pill. After swallowing their pills and waiting 40 minutes for them to metabolise, participants were asked to read scenarios describing a range of possible moral infractions, and tell us how morally wrong they believed each to be. Sure enough, as we reported in an article in the Journal of Personality and Social Psychology in 2019, we found the predicted difference. Those who ingested ginger decided that some of those violations, such as someone peeing in your swimming pool, were not so wrong after all. Blocking their nausea changed our participants’ moral beliefs.

Importantly, these effects didn’t emerge for all the moral dilemmas we presented. Prior to conducting the research, we had categorised hypothetical moral situations as either highly severe or only moderately problematic, based on research assistants’ judgments of wrongness. Having sex with a sibling and eating one’s dead dog were considered highly severe, but touching the eyeball of a corpse, eating faeces that had been fully sanitised, and buying an inflatable sex doll that looks like one’s receptionist were seen as more moderate. In our studies, ginger had no effect on participants’ responses to highly severe infractions. Apparently, most people think it’s so obviously wrong to eat your own dog or sleep with a close relative that any disgust they might feel at these behaviours had no impact on their beliefs.

In contrast, for the more ambiguous infractions – such as buying that sex doll or eating (totally clean!) faeces – people’s moral judgments were partly shaped by their disgust feelings. In such cases, where disgust is elicited but wrongness is uncertain, people seem to lean on their gut emotions to make moral judgments. If those feelings are inhibited, so that people can think about the possibility of eating clean faeces without wanting to throw up, the objectionable behaviours become less morally problematic.

We also found that ginger had no effect on people’s beliefs about other kinds of moral violations: those that involve harm to others, such as drinking and driving, or those that involve fairness, such as failing to tip a server. The violations that were affected by ginger, in contrast, centred on maintaining the purity of one’s own body. These transgressions are ones that have, historically, carried a high likelihood of transmitting disease. As a result, it is evolutionarily adaptive for us to feel disgusted by, and consequently avoid, close contact with dead bodies, human faeces and certain unsafe sex practices. Throughout human evolutionary history, moralising these behaviours, along with others that protect the sanctity of the body, might have been a useful way for societies to shield their members from dangerous germs they had no cognitive awareness of. According to the psychologist Jonathan Haidt and his colleagues, in many cultures this presumably adaptive tendency morphed into a broader ethic that uses concepts such as purity, sanctity and sin to discourage behaviours perceived to cause some manner of bodily degradation. In many cultures, these rules have stretched far beyond their original adaptive purposes; today, across the globe, societies regulate individuals’ purity-related behaviours by invoking morality in ways that sometimes do – but just as often do not – lead to actual health or social benefits.

In fact, much of the socially proscribed moralisation of sanctity that occurs now is, itself, wrong. It is appropriate, and useful, for people to feel disgusted by spoiled foods, faeces, dead bodies and sibling sex. But that doesn’t mean that we should moralise these emotional responses. We don’t have to extend our beliefs about right and wrong to behaviours that don’t actually hurt others, even if we find them disgusting. The tendency to do so is an ancient evolutionary holdover and, with the help of modern sanitation and safe sex practices, it’s one we can afford to set aside.

Yet this kind of moralisation is manifested frequently in response to a number of behaviours that, to some, appear to tarnish the presumed purity of the human body. The belief – held by 51 per cent of people in the United States – that it is wrong to engage in gay sex is shaped by the moralisation of sanctity. Some people might feel disgust in response to certain sexual behaviours (in the same way that most children do to all sexual behaviours) but, for adults, that emotional reaction is a misfire. Their disgust is not a valid signal of danger. And our research shows that moral beliefs based on sanctity concerns represent a different category of morality than those based on harm and fairness. We were able to shift people’s sanctity beliefs simply by giving them ginger. A moral view that changes on the basis of how nauseous we feel is probably not one that we want to put a lot of stake in.

Instead, many of us would prefer to hew to a set of moral standards that come from a coherent, rationally derived philosophy about enhancing justice and mitigating harms. Certain human behaviours do make us feel sick. But we need not rely on those feelings as a basis for our moral principles, or when judging others for what we feel to be immoral.

Before deciding that something is wrong, we might ask ourselves, is it just that I’m disgusted by it? Or, when encountering what appears to be a moral dilemma, we could play it safe and reach for a ginger ale.