Our decision making is bad enough when the disease is hypothetical. But when the disease is real — when we see actual death tolls climbing daily, as we do with the coronavirus — another factor besides our sensitivity to losses comes into play: fear.

The brain states we call emotions exist for one reason: to help us decide what to do next. They reflect our mind’s predictions for what’s likely to happen in the world and therefore serve as an efficient way to prepare us for it. But when the emotions we feel aren’t correctly calibrated for the threat or when we’re making judgments in domains where we have little knowledge or relevant information, our feelings become more likely to lead us astray.

Let me give you an example. In several experiments, my colleagues and I led people to feel sad or angry by having them read a magazine article that described either the impact of a natural disaster on a small town or the details of vehement anti-American protests abroad. Next, we asked them to estimate the frequencies of events that, if they occurred, would typically make people feel sad (for example, the number of people who will have to euthanize a beloved pet this year) or angry (for example, the number of people who will be intentionally sold a “lemon” by a dishonest car dealer this year) — estimates for which people wouldn’t already hold a knowledgeable answer.

Time and again, we found that when the emotion people felt matched the emotional overtones of a future event, their predictions for that event’s frequency increased. For instance, people who felt angry expected many more people to get swindled by a car dealer than did those who felt sad, even though the anger they felt had nothing to do with cars. Likewise, those who felt sad expected more people to have to euthanize their pets.

Fear works in a similar way. Using a nationally representative sample in the months following Sept. 11, 2001, the decision scientist Jennifer Lerner showed that feeling fear led people to believe that certain anxiety-provoking possibilities (for example, a terrorist strike) were more likely to occur.