  Human understanding of randomness and variation is shaped by a number of
cognitive biases. Here we relate a lesser-known cognitive bias, the "outcome
orientation", to medical questions and describe the harm that the outcome
orientation can do to medical research and practice. People who reason
according to an outcome orientation interpret probability as a subjective
degree of belief that is constrained to consider events one at a time, in a way
that is incompatible with Bayesian reasoning. Instead of accepting that
uncertainty is inevitable, and generalizing from the frequency of similar
events, the outcome orientation prefers one-off causal narratives. In medicine,
the outcome orientation therefore erodes support for randomized controlled
trials in favor of reductionist approaches. The rhetoric of personalized
medicine resonates with, and can promote, the outcome orientation, by
emphasizing how the measurable attributes of individual patients, rather than
chance or unknowable factors, causally produce each particular patient's
outcome.
