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The psychology of uncertainty, vaccinations, and protecting the most vulnerable


09 Jul 2018


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Measles is making a comeback in Britain and Europe with numbers rising to record levels this year. Last year in Europe, measles killed 35 people, including young children. The re-emergence of measles can be traced to falling rates of vaccination and might make you want to re-think your summer plans. Crowded environments with low levels of hygiene, also known as summer festivals, are something to avoid if unsure about whether you have been properly vaccinated. And maybe re-think going for holidays to Romania, Italy and Greece, the countries with the highest rates of measles outbreaks this year.

But of course, even if you are not vaccinated, your chances of getting measles are low. And if you are infected, dying from measles is rare. The people that die during measles outbreaks are vulnerable babies that are too young to be vaccinated and unvaccinated people with compromised immune systems. And what are the chances that you infect one of these vulnerable people? Extremely low. Your intuition then might be that even if you are unsure about your vaccination status, the low odds don’t seem to justify the effort to engage with the NHS or any other health care provider. Maximize your benefits, and others will surely be fine. Individually, this feels right, but for the communities and countries we live in, this is disastrous, slowly eroding herd immunity that protects the most vulnerable.

Research in psychology and behavioural economics backs the idea that the uncertainty that surrounds infectious disease such as measles provides the ideal conditions for its spread, transmitted by risk-seeking human decisions in social contexts. For instance, our experimental research conducted as part of the Oxford Martin Programme on Collective Responsibility for Infectious Disease  supports the idea that when people are made aware of the uncertainty about how their decisions might affect others, they tell themselves a self-serving story, suggesting that things will work out just fine. What are the odds that you get infected with measles and then infect somebody else? Our intuitions tell us that things will be fine for them and us.

John Rawls, however, might disagree. He articulated the most influential theory of justice last century. It was dubbed “maximin” and focussed on the plight of the worst off or most vulnerable. According to maximin, the most just social arrangement is the one in which the worst off are as well off as possible. Inequality can be tolerated, but only if it works to raise the plight of the worst off. In the context of vaccination, the most vulnerable are young babies or those who have diseases of the immune system, or immunosuppression; they can’t be vaccinated. Healthy people choosing not to vaccinate put the most vulnerable at risk, including small babies.

In this spirit, Rawls might highlight the harm we could inflict on the most vulnerable among us. In the context of infectious diseases he might urge us to consider those who are too young to have protection against measles, such as young babies, or those who are most vulnerable to the infection, such as those with compromised immune systems. Rather than maximizing your own benefits and gambling with the lives of others, he might suggest minimizing the chance that your actions might impact the most vulnerable in society. Looking at our research – just published in Nature Human Behaviour – and the research of others it appears that people do not share this intuition when deciding between self-interest and the interests of others. At least not in the situations tested in these studies. However, our recent research shows that in some situations, people seems to act just like Rawls hoped they would.

In a recent study, we described for the first time how uncertainty does not always make people selfish. The key to generosity under conditions of uncertainty, it seems, is to draw people’s attention to the possibility that a vulnerable person might be affected by one’s decision, leading people to overweight the worst possible outcome in their decision making. But first, we showed how uncertainty helps people be selfish.

In some of our studies, for instance, we asked hundreds of people whether they would stay away from work when infected with the African flu – a made-up disease – even if this meant potential harm to their career. Sometimes, we highlighted that is was not certain that if they go to work, that they would infect a co-worker, i.e., that there was a 50% chance that they would infect a co-worker. Highlighting the uncertainty about the outcome of participants’ actions for others such as infecting them with the African flu increased participants’ self-interested intentions to go to work. People were willing to take the risk of harming somebody else for their own benefit. This is in line with research showing that if there is a tiny chance that your self-interested actions might produce a good outcome for the other person, even if the chances are low, people optimistically assume that things will work out fine for others, and reap the benefits for themselves. Not like Rawls at all.

But what happens if you direct participants’ attention to the uncertainty surrounding their potential impact on the well-being of another person? Other participants in our studies read that if they infect another co-worker with the African flu, that this co-worker could be young, and the flu would do no harm to them, or the person could be old, and the flu might be harmful for them. Would participants under such uncertainty favour self-interest, assuming that if they infect another person, surely it would be the young co-worker?

No, they didn’t. Rather, they acted as if they adopted John Rawls’ maximin strategy, putting a premium on preventing any potential harm to a vulnerable person. Participants intended to stay home to the same degree as participants that learned that if they wet to work, they will infect a vulnerable co-worker. Participants in our studies showed the same pattern of results not only when making hypothetical decision but also when deciding about how to split money between themselves and another person.

Rawls believed that when we do not know who we would be in society (the veil of ignorance), we would choose maximin. However, others have claimed we might choose to gamble, aiming to maximize utility (utilitarianism). Health systems such as the NHS are based on utilitarianism. Our research suggests that under the veil of ignorance, people will have different intuitions depending on the kind of uncertainty they attend to. When it comes to vaccinations, for instance, highlighting the meagre chances that not being vaccinated will lead to infecting another person will increase people’s risk taking in favour of their self-interest. But if people attend to the possibility that they might harm a baby, a very ill person or a pregnant woman, they will become risk-averse, forgoing self-interest to minimize the chance that they will harm such vulnerable people. These findings suggest that when we communicate the necessity to vaccinate, we need to make sure that we do not only talk about the uncertainty that surrounds the outcomes of, for instance, measles outbreaks but also how such outbreaks might cause disabilities and death to the most vulnerable amongst us.

Rawls’ Theory of Justice certainly captures one important aspect of human psychology: concern for the plight of the worst off. By harnessing this concern, we might nudge people to higher vaccination rates and hopefully again towards herd immunity to protect the most vulnerable in society.


This opinion piece reflects the views of the author, and does not necessarily reflect the position of the Oxford Martin School or the University of Oxford. Any errors or omissions are those of the author.