Downturns Are Not Existential Risks
Epidemics, wars, and quakes are distributed with long tails, so that most of the expected harm from such events are in the largest possible events. Most who are expected to die in epidemics die in events that kill much of the population. These long tail disasters plausibly embody existential risk – a risk to the existence of civilization and humanity.
A new paper on economic downturns suggests that such events do not have long tails, and so are not existential concerns:
In the rare-disasters setting, a key determinant of the equity premium is the size distribution of macroeconomic disasters, gauged by proportionate declines in per capita consumption or GDP. The long-term national-accounts data for up to 36 countries provide a large sample of disaster events of magnitude 10% or more. For this sample, a power-law density provides a good fit to the distribution of the ratio of normal to disaster consumption or GDP. The key parameter of the size distribution is the upper-tail exponent, alpha, estimated to be near 5, with a 95% confidence interval between 3-1/2 and 7. …
We work with the transformed disaster size z ≡ 1/(1-b), which is the ratio of normal to disaster consumption or GDP. … We start with a familiar (single) power law, which specifies the [probability] density function as f(z) = Az-α.
For a long tail, alpha needs to be two or less.
As the 1918 flu coincided with World War 1, it would be nearly impossible to separate the economic effects of the two of them.
Looks like a nice paper, and fits in with some papers I have read that claim stock market price fluctuations have alpha's around 3.
However, plenty of natural disasters (and human wars) have mortality alphas between 1 and 2, the domain where the average is undefined and very big disasters are fairly common. There the important question is whether there exist cut-offs (likely for landslides and flooding due to geography, but not guaranteed for epidemics or wars).
Together, this seems to suggest that while endogenous downturns and crashes occur, they are relatively unlikely to crash the market completely. But it might be exogenous disturbances that we should watch out for, both in terms of mortality and in their market impact.
Still, the 1918 flu did not seem to have had a very noticeable economic effect - if extremes like that doesn't show up in the data, maybe the economy is much more resilient than we normally tend to think?