'100% of anything looks good': the appeal of one hundred percent.

People overweight certainty, even when certainty is only an illusion. A vaccine that was described as 100% effective against 70% of disease targets was preferred to one described as 70% effective against 100% of disease targets (Studies 1 and 2). The appeal of 100% extends beyond the probability attribute. In Study 2, participants preferred both of the vaccines above to normatively equivalent vaccines that were less than 100% effective toward fewer than 100% of targets. In Study 3, participants preferred a 100% discount on a cup of coffee every 10 days to other more frequent, but lower amount, discounts. This preference evaporated, however, when savings were framed as points rather than as percentage discounts. We propose that people view 100% as a salient reference point and overweight it in those domains where it cannot be exceeded (e.g., probability, discount); the overweighting is weaker in domains where 100% can be exceeded (e.g., target range, points).