These many cognitive biases we all have—confirmation bias, fundamental attribution bias, and so forth—were once our friends. They helped our species reach the top of the food chain by allowing us to make snap judgments that, at the time, were right often enough to justify how often they were wrong. But “at the time” was before they built the Harris County Criminal Justice Center. Before we started living together in cities, even. Probably before we started growing our food instead of pursuing it.
In the modern world the balance is different—we are much less often exposed to things that will eat us if we take the time to apply reason and logic instead of making a correct snap judgment. We have more access to information, and more time to reflect on it before making most decisions.
We have a popular name for these cognitive biases. We call them, collectively, “common sense.” Cognitive biases are—common sense is—what we rely on when reason and logic fail us.
While we might not say it out loud, it is common sense that people who do bad things are bad people (fundamental attribution bias); common sense confirms our beliefs about the world (confirmation bias); common sense makes the bits of information that we remember more important to making correct decisions than are those we don’t (availability heuristic).
Common sense is a feeling, more akin to emotion than to rational thought. Common sense is what we use when reasoning is too much trouble. Common sense is ineffable: “How do I know that? Well, it’s just common sense, innit?”
The operations we unconsciously perform on information to create our cognitive map of the world—generalization, deletion, and distortion—likewise were once our friends. A human brain could not possibly retain all of the information delivered to it by the senses, so we deleted most of it. We couldn’t afford to learn experientially whether every large felid is hostile, so we generalize. (The case for the adaptive value of distortion is less obvious; perhaps we distort our picture of the world so that we can better live with the things over which we have no control.)
In fact these operations are still our friends. We are even less able to retain all of the information we receive; we still need to live with things that we cannot control; and nobody has time to learn how to open each new door she comes to.
Cognitive biases too are still our friends. Common sense can get us through most of life better than the lack of both common sense and rational thought. Most people are not great at rational thought anyway, but fortunately for them, common sense is enough for most of our business.
And sometimes you shouldn’t argue with the ineffable: When your “spidey sense” tingles, and tells you that you are approaching a dangerous situation, listen to it.
You’ll hear prosecutors asking jurors in criminal cases to rely on their common sense. In Texas, at least, the jury instructions do not mention “common sense.” ((In federal court and other jurisdictions, reasonable doubt is defined in terms of “reason and common sense,” which might more honestly be described as “reason and emotion.”)) What the prosecutors are asking is for the jurors to substitute this feeling for the written instructions that they will be given on the law.
In criminal court, however, common sense won’t cut it.
Why not? An answer soon.