People often test hypotheses about two variables (X and Y), each with two levels (e.g., Xl and X2). When testing "IfXl, then YI,~observing the conjunction ofXl and Y1 is overwhelmingly perceived as more supportive than observing the conjunction ofX2 and Y2, although both observations support the hypothesis. Normatively, the X2&Y2 observation provides stronger support than the XI&YI 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 XI&YI observation under certain conditions. Furthermore, participants' default strategy ofjudgingXI&YI observations more informative might be generally adaptive because hypotheses usually regard rare events.A fundamental issue in the study of human inference is what, psychologically speaking, constitutes confirmatory evidence for a hypothesis. In 1945, philosopher Carl Hempel noted the following paradox regarding confirmatory evidence. Assume that the hypothesis of interest is "All ravens are black." This statement can be rewritten as "If something is a raven, then it is black," or RavenB lack. Clearly, observing a black raven would count as confirming evidence. Similarly, if the hypothesis were "If something is not black, then it is not a raven," or -Black~-Raven, observing a nonblack nonraven (e.g., a white shoe or a yellow pencil) would clearly be confirming evidence. Because these two hypotheses are logically equivalent (one is the contrapositive ofthe other), any evidence that confirms one must confirm the other. It follows, then, that observing a nonblack nonraven confirms Raven~Black. Thus, one could apparently confirm the hypothesis about the color of ravens by sitting in one's office and never even observing a raven. Most people find this highly counterintuitive; hence, the paradox.Other philosophers have pointed out that the paradox can be resolved if one conceives of confirmation as a matter of degree rather than all-or-none. Although black ravens and nonblack nonravens both confirm RavenB lack, they do not do so equally strongly. From a Bayesian perspective, confirming evidence supports a hypothesis to the extent that it is rare, or surprising. Because nonblack things and nonravens are both common, observing a nonblack nonraven would not be unusual and would therefore confirm the hypothesis only negligibly. In contrast, because few things are black and few things are ravens, observing a black raven would be surprising and would constitute stronger confirmation (Alexander, 1958;Good, 1960;Horwich, 1982;Hosiasson-Lindenbaum, 1940; Howson & Urbach, 1989,pp. 88-91;Mackie, 1963).1 The paradox appears to stem from our inability to distinguish intuitively between nonconfirmatory and minutely confirmatory evidence: The nonblack nonraven appears completely u...