Published here October 2003.

Abstract | Introduction | Reason 1 | Status Quo Bias
Sunk Cost Bias | Supporting Evidence Bias | Framing Bias
Estimating and Forecasting Biases | Garbage In, Garbage Out  | PART II

Supporting Evidence Bias

Supporting evidence bias is our tendency to want to confirm what we already suspect and look for facts that support it. We avoid asking tough questions and discount new information that might challenge our preconception. Suppose, for example, you are considering an investment to automate some business function. Your first inclination is to call an acquaintance who did it and got really good results. What response do you expect other than, "It's the right choice"?

Despite our inclination to look for supporting evidence, it is usually much more informative to seek out contradictory evidence. Confirming evidence often fails to discriminate among possibilities well. To illustrate, in one study students were given the initial sequence of numbers 2, 4, 6 and told to determine the rule that generated the numbers. To check hypotheses, they could choose a possible next number and ask whether that number was consistent with the rule. Most students asked whether a next number "8" would be consistent with the rule. When told it was, they expressed confidence that the rule was "the numbers increase by 2." Actually, the rule was "any increasing sequence."

Supporting evidence bias often strongly influences the way we listen to others. It causes us to pay too much attention to supporting evidence and too little to conflicting evidence. Psychologists believe the bias derives from two fundamental tendencies. The first is our nature to subconsciously decide what we want to do before figuring out why we want to do it. The second is our inclination to be more engaged by things we like than by things we dislike.

Some advice for avoiding supporting evidence bias:

  • Check to see whether you are examining all the evidence. Avoid the inclination to accept confirming evidence without question.
  • Get in the habit of looking for counter arguments. In meetings, consider appointing someone to serve as devil's advocate, to argue against the prevailing point of view.
  • Be honest with yourself and your motives. Are you really gathering information to help you make a smart choice, or are you just looking to confirm what you already believe.
  • Don't surround yourself with "yes men."
Sunk Cost Bias   Sunk Cost Bias

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