The psychological side of Hempel's paradox of confirmation.

People often test hypotheses about two variables (X and Y), each with two levels (e.g., X1 and X2). When testing "If X1, then Y1," observing the conjunction of X1 and Y1 is overwhelmingly perceived as more supportive than observing the conjunction of X2 and Y2, although both observations support the hypothesis. Normatively, the X2&Y2 observation provides stronger support than the X1&Y1 observation if the former is rarer. Because participants in laboratory settings typically test hypotheses they are unfamiliar with, previous research has not examined whether participants are sensitive to the rarity of observations. The experiment reported here showed that participants were sensitive to rarity, even judging a rare X2&Y2 observation more supportive than a common X1&Y1 observation under certain conditions. Furthermore, participants' default strategy of judging X1&Y1 observations more informative might be generally adaptive because hypotheses usually regard rare events.