In their widely-cited 1983 paper, Extensional Versus Intuitive Reasoning, Tversky and Kahneman discuss the 'conjunction fallacy'. This is an intriguing phenomenon in which adding more detail to a scenario increases its perceived probability, in a way that is unambiguously fallacious.

The conjunction fallacy suggests that when asked about the probability of a scenario like this:

"US forces engage in a fatal confrontation with Chinese forces"

people put a lower figure on it than for a scenario like this:

"A US naval patrol vessel, upholding the Phillippines' claim to the Second Thomas Shoal, attempts to inderdict an approaching Chinese landing craft. The vessels collide and several Chinese sailors are killed."

even though the latter scenario is an extremely specific scenario that forms part of the family of scenarios circumscribed by the former and so must have a smaller probability.

This leaves scenario analysts in a difficult position. Propositional scenarios - like the former of the two above - might be dismissed before being considered appropriately, leading to collective failures of imagination. Narrative scenarios - like the latter of the two above - can seem more vivid to customers and might lead to better decisions about prioritisation of threats, but from a technical point-of-view have a vanishingly small probability of being true.

The best approach might be to see narrative scenarios as a tool, rather than an end-product, for getting decision-makers to identify categories of scenario that are potentially significant and worth investigating. Analysts can then convert these infinitessimally-likely scenarios into propositional scenarios for probabilistic assessment.

More widely, the problem of what to do when faced with a customer who is themselves predictably biased is a difficult one faced by professionals not just in intelligence but across all policy domains.

Thomas Schelling
(photo: T Zadig) "There is a tendency in our planning to confuse the unfamiliar with the improbable. The contingency we have considered seriously looks strange; what looks strange looks improbable; what is improbable need not be considered seriously.”