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How We Know What Isn't So The Fallibility of Human Reason in Everyday Life Thomas Gilovich lffil THE FREE PRESS A Division of Macmillan, Inc. NEW YORK Maxwell Macmillan Canada TORONTO Maxwell Macmillan International NEW YORK OXFORD SINGAPORE SYDNEY Copyright © 1991 by Thomas Gilovich All rights reserved. No part of this book may be reproduced or trans mitted in any form or by any means, electronic or mechanical, includ ing photocopying, recording, or by any information storage and retrieval system, without permission in writing from the Publisher. The Free Press A Division of Macmillan, Inc. 866 Third Avenue, New York, N. Y. 10022 Maxwell Macmillan Canada, Inc. 1200 Eglinton Avenue East Suite 200 Don Mills, Ontario M3C 3N1 Macmillan, Inc. is part of the Maxwell Communication Group of Companies. First Free Press Paperback Edition 1993 Printed in the United States of America printing number 3 4 5 6 7 8 9 10 Library of Congress Cataloging-in-Publication Data Gilovich, Thomas. How we know what isn't so: the fallibility of human reason in everyday life / Thomas Gilovich. p. cm. Includes bibliographical references and index. ISBN 0-02-911706-2 1. Reasoning (Psychology) 2. Judgment. 3. Evidence. 4. Error. 5. Critical thinking. 6. Fallacies (Logic) I. Title. BF442.G55 1991 153.4'3-dc20 90-26727 CIP To Karen and Ilana Contents Acknowledgments 1. Introduction Vll 1 PART ONE Cognitive Determinants of Questionable Beliefs 2. Something Out of Nothing: The Misperception and Misinterpretation of Random Data 3. Too Much from Too Little: The Misinterpretation of Incomplete and Unrepresentative Data 4. Seeing What We Expect to See: The Biased Evaluation of Ambiguous and Inconsistent Data 9 29 49 PART TWO Motivational and Social Determinants of Questionable Beliefs Seeing What We Want to See: Motivational Determinants of Belief 6. Believing What We are Told: The Biasing Effects of Secondhand Information 7. The Imagined Agreement of Others: Exaggerated Impressions of Social Support 5. 75 88 112 [vi] Contents PART THREE Examples of Questionable and Erroneous Beliefs 8. 9. 10. Belief in Ineffective "Alternative" Health Practices Belief in the Effectiveness of Questionable Interpersonal Strategies Belief in ESP 125 146 156 PART FOUR Where Do We Go from Here? Challenging Dubious Beliefs: The Role of Social Science Notes Index 11. 185 195 214 Acknowledgments Four people made unusually significant contrihutions to this work and deserve special thanks. Lee Ross commented on drafts of many of the chapters and provided a number of his uniquely illuminating insights on the phenomena at hand. Beyond that, I would like to thank Lee simply for being Lee-for being the most interesting "intuitive psychologist" I know, and for making the discussion of people and their commerce through everyday life so enjoyable. Karen Dashiff Gilovich read every word of this book and at times seemed to have something to say about nearly every one. She was in many respects my most challenging critic, but, as always, she delivered her critiques in the most loving, disarming, and helpful ways. I owe Dennis Regan and Daryl Bern a great debt for the helpful feedback they provided on earlier drafts and for their en couragement throughout the project. Various chapters were improved by the comments of numerous people, and I would like to express my sincere thanks to all: Robert Frank, Mark Frank, David Hamilton, Robert Johnston, David Myers, James Pennebaker, Barbara Strupp, Richard Thaler, and Elaine Wethington. To protect them from blame for any wrong-headed ideas presented in this