Deciphering the effects of incentive motivation on probabilistic judgments

The ubiquity of over-optimism, overconfidence, wishful thinking and desirability biases suggest that a universal motivational mechanism distorts all decisions and judgments uniformly. Here, we investigate this intuition, by assessing the relative effects of incentives on two types of probabilistic judgments: beliefs about externally generated hypotheses, versus confidence in one’s own decisions. Across four perceptual decision-making experiments, we manipulate the agency of decisions over ambiguous states-of-the-world and the monetary incentives for accurate probabilistic judgments about decision accuracy. Results show that incentives consistently bias participants’ reports, but, contrary to the uniform motivational bias intuition, substantially more for confidence than belief judgments. Modelling probabilistic judgments as an incentive-dependent weighting of evidence that is congruent with a (covert) decision rationalizes this apparent discrepancy. We conclude that gain versus loss prospects modulate how we integrate evidence confirming our endorsed hypothesis, shedding light on the formation of beliefs and confidence, and their interaction with motivational processes.