An apparently unusual event of very high impact.
Particularly one which - before it happened - was believed in error to be highly improbable, or even impossible.
The use of the term in finance derives from the widespread historical (and wrong) belief in the Northern hemisphere that black swans did not exist. This wrong belief was held in the period before the common occurrence of black swans in the Southern hemisphere had been reported in the North.
The concept was popularised in a 2007 book by Nassim Nicholas Taleb - "The Black Swan".
Taleb summarises the problem in risk management as "the confusion of absence of evidence of Black Swans (or something else) for evidence of absence of Black Swans (or something else)".
This means that the existence of financial "black swans" tends to lead to systematic under-assessment and understatement of financial risk.
Turning black swans white
- Taleb points out that black swan events depend on the observer, and the information and analysis obtained and applied by him or her.
- Being slaughtered shortly before Christmas is a black swan surprise for a turkey; especially following 1,000 days of consistent - apparently predictable - feeding and friendliness from humans.
- The slaughter of the turkey is not a black swan event for the human butcher.
- Turkeys need to gather more information and to analyse it.
- How not to be a sucker - A Black Swan is relative to knowledge - The Black Swan, 2010 pp40-44.
Robustness not fragility
The key message from Taleb's work is about seeking robustness and avoiding fragility.
- "You have to avoid debt because debt makes the system more fragile. You have to increase redundancies in some spaces.
- You have to avoid optimization. That is quite critical for someone who is doing finance to understand because it goes counter to everything you learn in portfolio theory....
- I have always been very sceptical of any form of optimization. In the black swan world, optimization isn't possible.
- The best you can achieve is a reduction in fragility and greater robustness.
- You may have heuristics, but not an optimization rule.
- I hope the message will finally get across because I haven't succeeded yet.
- People talk about black swans but they don't talk about robustness, which is the real lesson of the black swans."
- Living with Black Swans - Nicholas Nassim Taleb.