Foreseeable unforeseeables

Cambridge

EVENTS like the Covid-19 pandemic, the US housing-market crash of 2007-2009, and the terrorist attacks of September 11, 2001, are often called "black swans". The term is meant to suggest that no one could have seen them coming. But, in fact, these episodes each involved known unknowns, rather than what former US Secretary of Defence Donald Rumsfeld famously called "unknown unknowns".

After all, in each case, knowledgeable analysts were aware not only that such a thing could happen, but also that it was likely to happen eventually. Although the precise nature and timing of these events were not predictable with high probability, the severity of the consequences were. Had policymakers considered the risks and taken more preventive steps in advance, they might have averted or mitigated disaster.