The bias toward false positives is probably especially strong in criminal cases. The archetypal criminal offense is such that it unambiguously happened (not quite like the Willingham case), and in the ancestral human environment there were far fewer people around who could have done it. That makes the priors for everyone higher, which means that for whatever level of probability you’re asking for it takes less additional evidence to get there. That a person is acting strangely might well be enough—especially since you’d have enough familiarity with that person to establish a valid baseline, which doesn’t and can’t happen in any modern trial system.
Now add in the effects of other cognitive biases: we tend to magnify the importance of evidence against people we don’t like and excessively discount evidence against people we do. That’s strictly noise when dealing with modern criminal defendants, but ancestral humans actually knew the people in question, and had better reason for liking or disliking them. That might count as weak evidence by itself, and a perfect Bayesian would count it while also giving due consideration to the other evidence. But these weren’t just suspects, but your personal allies or rivals. Misweighing evidence could be a convenient way of strengthening your position in the tribe, and having a cognitive bias let you do that in all good conscience. We can’t just turn that off when we’re dealing with strangers, especially when the media creates a bogus familiarity.
The bias toward false positives is probably especially strong in criminal cases. The archetypal criminal offense is such that it unambiguously happened (not quite like the Willingham case), and in the ancestral human environment there were far fewer people around who could have done it. That makes the priors for everyone higher, which means that for whatever level of probability you’re asking for it takes less additional evidence to get there. That a person is acting strangely might well be enough—especially since you’d have enough familiarity with that person to establish a valid baseline, which doesn’t and can’t happen in any modern trial system.
Now add in the effects of other cognitive biases: we tend to magnify the importance of evidence against people we don’t like and excessively discount evidence against people we do. That’s strictly noise when dealing with modern criminal defendants, but ancestral humans actually knew the people in question, and had better reason for liking or disliking them. That might count as weak evidence by itself, and a perfect Bayesian would count it while also giving due consideration to the other evidence. But these weren’t just suspects, but your personal allies or rivals. Misweighing evidence could be a convenient way of strengthening your position in the tribe, and having a cognitive bias let you do that in all good conscience. We can’t just turn that off when we’re dealing with strangers, especially when the media creates a bogus familiarity.