The transmission of coronavirus (SARS-COV-2) is, like many infectious diseases, exponential. An exponential process is one that doubles once a particular time period, *t, *has elapsed. For instance, if *t* = 1 week, then if I have 100 people infected today, then 200 people will be infected in 1 week, 400 in 2 weeks, 800 in 3 weeks, 1600 in 4 weeks, etc. The time period *t *is known as the “doubling time”. A study from the early phase of the Chinese outbreak estimated a doubling time of around 6 days.

It’s not quite as simple as a pure exponential in reality, especially when lots of people get infected, but it’s a good model of the early stages.

The problem is that the early stages of an exponential also don’t seem that bad. 100, 200, 400, 800 cases… that all sounds manageable. But exponentials, by definition, run away with you. That same process, after 16 weeks, gets to ~6.5 million!

Humans are remarkably bad at intuiting such exponential growth. This was allegedly used to great effect by the inventor of chess in ancient India. He asked his king for some modest compensation for inventing the game: 1 grain of rice on the first square of the chessboard, 2 grains on the second square, 4 grains on the third square… and so on, all the way out to the 64^{th} square. The king laughed at what seemed like such a small reward, and agreed – until his advisers figured out that the exponential growth led to a total of 18,446,744,073,709,551,615 grains of rice (the sum of all the exponents up to 2^64), an impossible number for the kingdom to provide.

Not only are we bad at forecasting exponential growth, *we also don’t know that we’re bad*. In other words, we have poor self-awareness about our forecasting abilities. People in one experiment were asked to forecast an exponential process, and were paid based on the accuracy of their answer. Most people were unwilling to pay even a small amount for the correct answer *even when they were wrong. *In other words, they were overconfident in their forecasts.

In the case of coronavirus, this is potentially devastating when, as in London today, things seem under control, and people may not understand why urgent advice is being given to stay home. If we’re unable to forecast what the situation is going to look like in a week or two weeks’ time, then we are also unlikely to take individual action to slow the spread of the virus today. This is especially dangerous given that the median incubation time of coronavirus before symptoms appear is ~5 days. This is exactly the time window when taking steps to curb exponential growth is critical. Most of the transmission in China appears to be by people who didn’t know they had the virus.

These trends also imply that, once this crisis is over, psychologists should look for ways of improving people’s self-awareness about these kinds of forecasts. We may not be able to change our ability to imagine the curve. But if we can encourage people to know when they *don’t *know what will happen, it might make them less likely to rely on intuition, and increase their willingness to listen to advice.

By practising physical distancing, avoiding pubs, avoiding going to the gym, basically avoiding going out, we can increase the doubling time, and slow the exponential growth. This prevents everyone getting sick at once, giving healthcare systems the capacity to respond, and science the time to find treatments. *We need to be doing this even if things look ok today.* Even in the last few squares of the chess board, things look manageable. Until they are not.