A man in his early twenties walks into a public toilet in Los Angeles. He heads for a cubicle, kicking it open. The camera follows as he kneels, lowers his head and begins to lick the toilet bowl. He looks up at the camera, smiles, and says “coronavirus challenge” before licking the bowl again.

On the social media platform TikTok, this young man calls himself Larz. He posted the 13-second video on 20 March, the day that the global death toll from Covid-19 passed 10,000. It was accompanied by the message, “RT to spread awareness of coronavirus”, with a smiley face. Two days later, Larz posted another video of himself, this time from a hospital bed. “I tested positive for coronavirus”, the post read, with a crying face added.

The “coronavirus challenge” videos began with a young woman licking the seat of an aeroplane toilet, and continued with a man licking the handle of a car door. Someone licked the grips of a child’s scooter; a man licked the handrail of a New York metro carriage. The videos triggered a moral shock. “Why,” raged Piers Morgan after watching the Larz video, “is this moron getting treated? He should be in jail, not taking up a vital hospital bed.”

The outrage seems justifiable; young narcissists are endangering themselves and others. But it is worth looking harder at this anger and revulsion, because beneath it there is something much more primal. Disgust and the fear of disease are strong forces, powerful enough to radically change our behaviour – and our politics.

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In many cases, it is clear how our instinctive revulsion when confronted by rotting food or excrement helps us navigate the world more safely. Mark Schaller, of the University of British Columbia, calls our fear of disease part of the “behavioural immune system” that helps us to avoid things which might infect or poison us. But this basic impulse is easily fooled. One experiment by Val Curtis, director of the Environmental Health Group at the London School of Hygiene and Tropical Medicine, found that changing the colour of a harmless liquid from a clinical blue to a pus-like yellow was enough to trigger feelings of disgust.

Disgust is also provoked by the behaviour of others. Again, there are cases in which the reasons for this are obvious – spitting, nose-picking and licking toilet bowls have obvious consequences for the hygiene of one’s environment – but also more complex causes. Paul Rozin, a professor at the University of Pennsylvania who specialises in the psychology of food, has shown that when people see others eating something they consider disgusting, that sense of disgust is conferred onto the diners themselves.

Our disgust at the behaviour of others can also be a moral revulsion. Tabloid newspapers describe everything from panic-buying to theft as “disgusting”. Indeed, our intuitive sense of disgust is at the root of many moral assessments. The New York University psychologist Jonathan Haidt illustrated this when, as part of the experiment, he told subjects about a man who buys a frozen chicken, takes it home, thaws it, has sex with it, then cooks it and eats it. Most of Haidt’s subjects were disgusted by the man’s behaviour, but few could explain why; though perverse, the only harm caused by his actions was to his own dignity.

Nevertheless, feelings of disgust have helped to form the principles of most cultures throughout history. The late British anthropologist Mary Douglas argued that societies categorise the world as “pure” or “dangerous”. The can be seen in the dietary taboos that state, for example, that vegetables are fine but pork is dirty and dangerous.

While some of these conventions may be arbitrary, our sense of disgust in the presence of disease is millennia older than our knowledge of microbes and infection control. Infectious diseases such as influenza and smallpox are thought to have killed more humans throughout history than anything else, and this brutal fact is etched into our collective behaviour. It has attuned us to sources of infection, such as dirty water (cholera) and rats (bubonic plague).

But we are now better informed, and more sensitive, to the causes of disease than ever before. Even subtle reminders of disease are enough to trigger distaste; one experiment found that simply putting a bottle of hand sanitiser in a hallway was enough to provoke thoughts about germs and a sense of disgust. In doing so, even innocuous triggers such as this can influence the decisions we make in our own behaviour, and our politics.

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The presence of pathogens can have a profound impact on how a society works. A growing body of research suggests that societies in which pathogens are more common (such as Nigeria) differ from societies where pathogens are less present (such as Norway). A 2011 study of 33 different societies around the world found that places with more pathogens tended to have to have “tighter” cultures, which enforced strong social norms and supported harsh punishments for those who broke these rules. Places which had low levels of pathogens correspondingly tended to have “looser” cultures, with weaker social norms and much greater tolerance for a diversity of behaviour.

Other research has shown that changes in the levels of pathogens within a country over time correlate to gender equality. A study published in Nature in 2016 tracked levels of pathogens in the USA between 1953 and 2013, and found that when levels of pathogens went down, equality between men and women went up. Economies, too, can be shaped by disease. A 2014 study of 185 countries by Tulane University’s Damian Murray found that countries where diseases are less common tended to be more innovative, filing more patents and producing more Nobel Prize winners.

These changes are not brought about by pathogens themselves, but our desire to avoid them. And just as the disgust that keeps us from eating rotten food can seep into our opinions of other people’s behaviour, a society that thinks more about disease may well become more conservative, more insular, and less tolerant.

Perhaps the biggest study of this effect so far was led by Tilberg University’s Yoel Inbar. A survey of more than 30,000 people found that those who are more prone to feeling disgusted are more likely to vote for conservative parties. In another study, Inbar found that people who were more easily disgusted were also more likely to agree with statements that condemned the building of new mosques, gay marriage or the political influence of the EU.

Other researchers have asked whether xenophobia is linked to an underlying fear of disease. A team led by the University of British Columbia’s Jason Faulkner showed one group of subjects a series of pictures of filthy kitchen sponges, which they were told could spread bacteria, while another group was shown pictures of accidents in everyday life. When they measured the attitudes of each group towards immigrants and immigration policy, they found the group who had been indicted to think about germs and disease tended to have more negative views of unfamiliar immigrant groups, and preferred stricter immigration policies.

This effect has been observed in real voting behaviour, too. At the University of British Columbia, psychology professor Mark Schaller and his students tracked voter preferences during the 2014 US federal elections, which occurred at the same time as a major outbreak of Ebola in West Africa and a small number of cases in the US. After each major news story related to the Ebola outbreak, polls recorded a more conservative tendency in voters. The US government’s response to Ebola in 2014 was rapid, decisive and is generally held to have been very effective in containing the disease, but voters did not rewards the incumbent Democrats for it. The Republicans won the Senate, making the greatest gains of either party for more than 30 years.

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How does this happen? Disgust is primal, but its effects are varied and complex. By combining them, we can begin to see how this basic impulse translates into political preferences.

One important effect is that when we worry about disease, we also start to value conformity. In an experiment by Damian Murray and Mark Schaller, a group of university students was asked to think about a time they had felt vulnerable to germs or disease, and then to vote on an unrelated matter. They voted by putting a penny into a clear plastic cup. Unbeknown to the students, the researchers had already placed 25 pennies into one of the cups, and only three pennies into the other. The students who had been reminded about diseases were much more likely to vote with what they thought was the majority. Murray and Schaller also found that students who were reminded of diseases also tended to like people who were described as more conformist. “A disease epidemic,” they observed, “may lead to temporarily higher levels of conformity within populations.”

Another effect of thinking about infections and germs is that it can make us more discriminatory. In one study led by Justin Park of Bristol University, subject were shown either a film about disease, or a film about traffic accidents. When they were asked for their opinions about obesity, the group who had been thinking about disease were much more negative in their opinions of obese people.

Thinking about infection can even change how socially outgoing we are. In a study led by Chad Mortensen of Arizona State University, students who had been shown pictures of infectious diseases not only rated themselves as being more introverted but actually behaved in a more introverted way, using measurably fewer gestures. A sense of vulnerability to disease makes us, emotionally and physically, less likely to reach out.

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Does this mean Covid-19 will create a future that is more conformist, inward-looking and discriminatory, less tolerant and open? Perhaps not. Disgust, for all its influence on our behaviour, is surprisingly easy to overcome. Other urges, such as hunger and sexual desire, have been shown to be more powerful. When we are biologically closer to someone, too, we are less likely to be disgusted by them. Any parent will testify that their children’s soiled nappies are, to them, less foul-smelling than those of other babies (this, too, has been confirmed by research).

The American psychologists Martie Haselton and David Buss have suggested that our sense of disgust has evolved to be oversensitive: an in-built precautionary principle that, like a highly-tuned smoke alarm, goes off at the smallest whiff of danger. Buss argues that we are willing to tolerate lots of false alarms because the costs of not identifying a source of infection can be very high indeed. The price for this fine sense is the need to identify these false positives, and refrain from acting on them.

This is necessary not just because we do not want our political positions to be dictated by disgust, but because disgust is not always rational. In 2014, some communities in West Africa refused to allow healthcare workers access. Understandably, they worried that the outsiders would bring in disease, but their refusal to allow testing and treatment accelerated the spread of the virus. The current pandemic has led political leaders to blame other countries and close borders, when international co-operation is the rational response and imposing travel restrictions is, says Ashish Jah of Harvard University, ”a semblance of action” that offers a dangerously unrealistic sense of safety.

In the US, the political far right has for some time made Covid-19 the subject of conspiracy theories that it was created by the Chinese government, or by the Democratic Party, or even by Bill Gates. In Hungary, Viktor Orban’s government has used the pandemic to further ramp up anti-immigrant sentiment and to tighten its grip on power.

But disgust is not in itself a negative emotion, and it can drive positive behaviour. It can encourage healthy practices such as hand-washing, and the universal nature of a pandemic can help people to remind people that for a disease, borders, social classes and other human barriers simply do not exist.

In truly large-scale emergencies such as the current pandemic, another powerful force comes into play: our desire to help others. Jonathan Haidt, one of the world authorities on disgust, puts it like this: “Moral evaluation will greatly exceed moral disgust, in the coming months. Disasters bring out the best in people”.

Evidence gathered by Enrico Qunatralli at the University of Delaware supports the argument that during disasters, cool heads, emotional resilience and co-operation prevail. So far, the Covid-19 crisis seems to follow this pattern; beneath the media outrage over a few instances of bad behaviour, the more widespread reality has been people making rational decisions and helping the vulnerable.

It is a fascinating, if worrying, time to study human behaviour. The Covid-19 pandemic could be seen as a psychological experiment on a vast scale, in which the population is reminded about disease dozens of times per day. The big question is how it will react.

André Spicer is professor of organisational behaviour at Cass Business School, City, University of London.