How can we understand the public’s distrust of AstraZeneca’s vaccine? On the one hand, one in 700 French people has died from Covid-19 in the last year. On the other hand, one case of thrombosis per 100,000 eliminated. Calculating the probabilities will not be enough. A smartphone application, called Risk Navigator, assesses the risks involved in common activities. The unit of measurement is the “micromort”: a probability of 1 in 1 million of dying. Thus, 1,000 km in a car costs 3 micromorts. But humans almost never perceive risks in terms of chi#ers or micromorts. Fortunately, we are not calculating machines. Our behavior is often irrational, and that’s good. vaccinations. The balance seems clear: the risk of thrombosis is 140 times lower than that of Covid-19. And yet, mistrust has set in and will be difficult to eliminate. Calculating probabilities won’t be enough. A smartphone application called Risk Navigator assesses the risks
in common activities. The unit of measurement is the “micromort”: a probability of 1 in 1 million of dying. Thus, 1,000 km in a car costs 3 micromorts. But humans almost never perceive risks in terms of chi#ers or micromorts. Fortunately, we are not calculating machines. Our behavior is often irrational, and that’s good.
The debate is not new. Smallpox inoculation – the voluntary transmission of an attenuated form of the disease – dates back to the 18th century in Europe. An inoculated child had a 1 in 200 “chance” of dying within a month, but if he or she survived, he or she would not be contaminated for life, at a time when 1/8 of the population died of smallpox. How can we compare these fractions 1/200 and 1/8? Are they of the same nature? Is it legitimate to risk someone’s death to protect them from a disease they might never catch? The Swiss mathematician Daniel Bernoulli published a remarkable work in 1766 in which he compared two populations, depending on whether they used inoculation or not. Using the statistical data at his disposal, he showed that, while 1/200 of the children died quickly when everyone was inoculated, life expectancy increased by three years. He concluded that it was necessary to inoculate.
The discussion that followed was fascinating in this century of the Enlightenment where the value of human life was being questioned. The mathematician D’Alembert was convinced of the advantages of inoculation, but he thought that these “are not of a nature to be appreciated mathematically.” He opposed many arguments, such as the fact that one cannot compare an immediate death with another in an indeterminate future.
For the past few decades, psychologists have been studying how we perceive risk. They have described and measured a large number of systematic biases. For example, we accept risks that are much greater when we choose them (such as driving a car) than when we cannot (such as a nuclear accident). Similarly, we minimize risks if they threaten us only in the indefinite future (like smoking). And we exaggerate a risk that is widely reported in the media (like thrombosis). These biases are universal and we cannot get rid of them with mathematics courses. They are part of human nature. Even experts are subject to them as soon as they leave their field of expertise. On the other hand, the good news is that these biases are now well understood by psychologists and can be explained to the public, something that schools and the media unfortunately do very little about. It’s not about making calculations but about understanding our behavior and controlling our risk-taking. We make most decisions instinctively, but when things get serious, we must learn to think and analyze our irrational reactions. Listen to the doctors and mathematicians, of course, but also to the psychologists. You can accept your uncontrolled fear of spiders, but for the risks that really threaten you, take the time to educate yourself and think before you make a decision!