Ad hominem is one of the more common informal fallacies, but I have decided that it is also a significant cognitive bias. A
cognitive bias is a mode of thought that speeds analysis but tends to lead to errors. A famous one is the availability heuristic, the tendency to make judgments on available information rather than seeking fuller information. For example, I might decide to adopt a new diet because I've seen it depicted favorably in television shows and news programs. Those positive depictions are available without further work. However, further study might show that the new diet was much more questionably beneficial, or even harmful.
Another famous one is the confirmation bias, in which I am inclined to approve information that supports what I already believe to be the case (and to discount evidence that suggests I might be wrong). This one is a very powerful bias that affects everyone, no matter how careful a thinker they may be. It is very difficult to correct for it.
The
ad hominem bias, another heuristic bias, which I am proposing works like this: it is easier to understand a known individual's bad qualities than to understand a complex situation in which they are involved. If a bad quality attributed to the known individual can reasonably be inferred to be the cause of a bad but complex situation, the
ad hominem heuristic occurs when you make that easy move.
The
ad hominem cognitive bias is similar to the availability heuristic in that it works on information that is already available, partly in order to avoid the difficulty of understanding the complex situation. However, it is also similar to confirmation bias in that you believe the bad attribution you yourself are making just because you already believe the person is bad. It is more complicated than the other two, however, in that -- because this is your own attribution, and you therefore believe it -- the new, probably false belief gets filed as further evidence of the bad quality of the person you dislike or hate. The
ad hominem heuristic thus contains a feedback loop in which it strengthens itself the more often it occurs; and since it is a cognitive bias and thus likely to lead to error, that means you tend to drift further and further away from what is really true.
In extreme cases, all the problems in your world can end up seeming to be the fault of this one pernicious individual. If only he (or she) could be removed, the improvement would be systemic and massive. This kind of belief can justify many sorts of extreme conclusions, or even violence against the hated individual.
Perhaps it has a name already.