What Is a Black Swan?
Europeans who believed "all swans are white" were shocked when they saw a black swan in Australia. A single one overturned a long-held belief. What do we call such an event in investing?
The three conditions of a black swan
The black swan is a concept popularized by the statistician and investor Nassim Taleb. He defined a black swan by three characteristics.
(1) Unpredictable: hard to know in advance from past data or existing knowledge. (2) Enormous impact: once it happens, the influence is tremendously large. (3) Retrospective explanation: after it passes, people plausibly explain it, saying "I actually knew it would happen."
The third is especially cruel. Only after an event erupts do we attach 'causes' and delude ourselves that it was predictable (hindsight bias). But in that moment, no one knew.
Why do we miss black swans?
The human brain and most statistical models are familiar with the 'normal distribution.' They handle common events near the average well, but they set the probability of extreme events lower than reality.
Taleb pointed out that "many trading desks looked only at the middle of the bell curve, mistook it for a normal distribution, and collapsed." What actually lacks data is the extremes (the tails), and because there's no data, people mistakenly think 'it almost never happens.'
We also tend to believe only what we can see and what can be explained by a story. A catastrophe we've never experienced is hard to even imagine, so we grow lax in preparing for it.
An investing attitude for the age of black swans
A black swan is by definition an event you 'cannot know in advance,' so trying to guess 'what the next black swan will be' is futile. Instead, you can change your attitude.
- Humility: Make 'my prediction can be wrong, and something I can't imagine can happen' the default. - Survival first: Avoid bets you can't afford, so a single extreme event doesn't wipe out your entire wealth. - Accept drawdowns: A long-term investor accepts that they will someday experience a large drawdown, and keeps the capacity to endure it.
A black swan isn't only in the bad direction (a crash). There are also unexpected windfalls (positive black swans). However, what tends to break us in investing is usually the left tail, i.e., the loss side, so defense comes first.
It's a misunderstanding to use the black swan as an excuse to think 'since it's unpredictable anyway, no preparation matters.' Taleb's point is actually the opposite: instead of predicting, build 'a structure that doesn't collapse.'
よくある質問
Q. Was the COVID-19 pandemic a black swan?
There's debate. Taleb himself viewed COVID not as a true black swan but as closer to a 'white swan (a foreseen risk),' arguing that 'a pandemic is a sufficiently foreseeable risk that experts had long warned about.' People simply didn't prepare. In this way, interpretations differ event by event over 'whether it was predictable.'
Q. Should I just buy a product that hedges against black swans?
This article does not recommend any specific product. 'Tail-risk hedging' strategies (e.g., buying deep out-of-the-money options) exist, but in normal times the cost keeps draining, and if the event doesn't come, losses accumulate. For individual investors, 'a bearable weight' and 'a plan to endure drawdowns' are more realistic preparation than complex products.
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