Beginning with its first edition and through subsequent editions, Thinking and Deciding has established itself as the required text and important reference work for students and scholars of human cognition and rationality. In this fourth edition, first published in 2007, Jonathan Baron retains the comprehensive attention to the key questions addressed in the previous editions - how should we think? What, if anything, keeps us from thinking that way? How can we improve our thinking and decision making? - and his expanded treatment of topics such as risk, utilitarianism, Baye's theorem, and moral thinking. With the student in mind, the fourth edition emphasises the development of an understanding of the fundamental concepts in judgement and decision making. This book is essential reading for students and scholars in judgement and decision making and related fields, including psychology, economics, law, medicine, and business.

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The Cognitive Reflection Test (CRT) is thought to correlate with measures of utilitarian moral judgment because it measures system-2 correction of an initial intuitive response. And some theories of moral judgment hold that the same thing often happens when people arrive a utilitarian judgments. We find, however, that CRT-type items (using logic as well as arithmetic) can work just as well when they do not have obvious intuitive answers at predicting utilitarian moral judgment, assessed with self-report questionnaires as well as with hypothetical scenarios, and also at predicting a measure of actively openminded thinking (AOT). Moreover, long response times, as well as high accuracy, also predict moral judgment and other outcomes. The CRT might thus be considered a test of reflection-impulsivity (RI). However, RI is only part of AOT, because RI is concerned only with the amount of thinking, not its direction. Tests of AOT also predict utilitarian moral judgments. Individual differences in AOT and moral judgments are both strongly (negatively) associated with belief that morality comes from God and cannot be understood through thought. The correlation of CRT and utilitarian judgment, when found, is thus likely due to the (imperfect) correlation of AOT and CRT. Intuition in these domains is thus not necessarily something that people overcome through additional thinking, but rather what they rely on when they do not think very much.

Subjects are reluctant to vaccinate a (hypothetical) child when the vaccination itself can cause death, even when this is much less likely than death from the disease prevented. This effect is even greater when there is a 'risk group' for death (with its overall probability held constant), even though the test for membership in the risk group is unavailable. This effect cannot be explained in terms of a tendency to assume that the child is in the risk group. A risk group for death from the disease has no effect on reluctance to vaccinate. The reluctance is an example of omission bias (Spranca, Minsk &Baron, in press), an overgeneralization of a distinction between commissions and omissions to a case in which it is irrelevant. Likewise, it would ordinarily be prudent to find out whether a child is in a risk group before acting, but in this case it is impossible, so knowledge of the existence of the risk group is irrelevant. The risk-group effect is consistent with Frisch & Baron's (1988) interpretation of ambiguity. KEY WORDS Omission bias Ambiguity VaccinationThe present study concerns the role of two biases in hypothetical decisions about vaccinations. One bias is the tendency to favor omissions over commissions, especially when either one might cause harm. We show that some people think that it is worse to vaccinate a child when the vaccination can cause harm than not to vaccinate, even though vaccination reduces the risk of harm overall. The other bias is the tendency to withhold action when missing information about probabilities is salientsuch as whether the child is in a risk group susceptible to harm from the vaccineeven though the missing information cannot be obtained. We show that this bias is found even when the overall probability of each outcome is clearly constant across the conditions compared. We take both of these effects to be overgeneralizations of principles or heuristics that are generally useful to situations in which they are not useful.Consider first what we shall call omission bias, the tendency to favor omissions (such as letting someone die) over otherwise equivalent commissions (such as killing someone actively). In most cases, we have good reasons for the distinction between omissions and commissions: omissions may result from ignorance, and commissions usually do not; commissions usually involve more malicious intentions than the corresponding omissions; and commissions usually involve more effort, itself a sign of stronger intentions. In addition, when people know that harmful omissions are socially acceptable, they look out for themselves; this self-help principle is, arguably, sometimes the most efficient way to prevent harm.

Words that conform to spelling-sound correspondence rules (e.g., SWEET) are read aloud more quickly than words that do not conform (e.g., SWORD). This suggests that the rules are used in reading a word, in spite of past learning of the association between the word and its entire pronunciation. This effect is larger in subjects who rely heavily on the rules, as determined by independent tests. In addition, mixing case affects reading of nonconforming words more than of conforming ones, suggesting that word-specific associations use units larger than single letters.Much evidence (e.g., Firth, 1972;Perfetti & Hogaboam, 1975; Rozin & Gleitman, in press) suggests that one of the most difficult steps in learning to read is mastering the use of spelling-to-sound correspondence rules. For example, Firth (1972) found a high correlation between ability to pronounce nonsense words and ability to read text in first and second graders, yet no correlation at all (with intelligence held constant) between ability to learn names of abstract forms and ability to read. Yet the alternative view that words may be recognized as wholes and that the rules are not needed (as suggested by Smith, 1971) is still tenable, if implausible. It seems relevant to this debate to know how fluent readers pronounce words out loud. If the rules are not used, this will suggest that the rules are not necessary for fluent reading, and that their importance for children may be a function of current teaching practices. If the rules are used, this will suggest that ability to use them is a legitimate criterion for success in reading instruction. While reading out loud may be a distinct skill from reading for meaning, it is a valuable skill in itself, particularly in provid-This work was supported by a grant from the National Science Foundation to the first author. Editorial comments about its presentation have been helpful.Requests for reprints should be sent to Jonathan Baron,