Social Science Statistics Blog

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21 April 2006

Mass media and the representativeness heuristic

Amy Perfors

Since the days of Kahneman & Tversky, researchers have been finding evidence showing that people do not reason about probabilities as they would if they were "fully rational." For instance, base-rate neglect -- in which people ignore the frequency of different environmental alternatives when making probability judgments about them -- is a common problem. People are also often insensitive to sample size and to the prior probability of various outcomes. (this page offers some examples of what each of these mean).

A common explanation is that these "errors" arise as the result of using certain heuristics that usually serve us well, but lead to this sort of error in certain circumstances. Thus, base-rate neglect arises due to the representativeness heuristic, in which people assume that each case is representative of its class. So, for instance, people watching a taped interview with a prison guard with extreme views will draw conclusions about the entire prison system based on this one interview -- even if they were told in advance that his views were extreme and unusual, and that most guards were quite different. The prison guard was believed to be representative of all guards, and thus the prior information indicating that his views were really quite rare was disregarded.

In many circumstances, a heuristic of this sort is sensible: after all, it's statistically unlikely to meet up with someone or something that is, uh, statistically unlikely -- so it makes sense to usually assume that whatever you interact with is representative of things of that type. The problem is -- and here I'm harking back to a theme I touched on in an earlier post -- that this assumption no longer works in today's media-saturated environment. Things make it into the news precisely because they are unlikely; but even if we know that consciously, it is easy to ignore that information. This may be part of the reason that so many people believe, for instance, that crime is much likelier than it is, that terrorism is an ever-present danger, that most politicians are corrupt, etc. I could go on. The cognitive heuristics that are so useful in the ordinary day-to-day rely on certain assumptions that are no longer even approximately valid when interpreting secondhand, media-driven information. And therein lies a problem.

Posted by Amy Perfors at 6:00 AM