Here we investigated whether, given equivalent supporting evidence, we judge self-selected hypotheses differently from those selected by an external source. On each trial of a probabilistic reasoning task requiring no retrieval from memory, participants rated the probability of a focal hypothesis, relative to two alternatives. The focal hypothesis was either selected by the participant or by a computer. In four experiments, self-selected focal hypotheses were judged to be more probable than externally selected ones, despite equivalent supporting evidence. This self-selection bias was independent of level of difficulty in selecting the focal hypothesis (cognitive effort) and of whether evidence was gradually accumulated or all presented instantaneously. These results suggest that the cognitive operations involved in selecting a hypothesis lead to assignment of higher probability to that hypothesis, and that this effect is independent of hypothesis selection difficulty and of the rate of evidence accumulation.
