I've been thinking about this post for a while, and have sort of written it a few times over in my head...the concept here is not a cognitive bias in the same sense as the others discussed before, but for me the idea of ad hominem arguments fall under the same broad category of "things people do without thinking them through".
In brief, an ad hominem argument doesn't argue for or against an idea but for or against a given proponent of an idea. So, for example, you want to argue against the Bush administration's tax cuts and do so by claiming that Bush is self-interested because his family is wealthy. Or you argue against the Obama admin's plan for health care reform by pointing out that Obama's father is Kenyan (one thing about ad hominem attacks is that they don't necessarily have to make sense to be effective.)
This tactic in arguing a point must be innate; it seems like kids just learning to play together resort to this sort of thing in the sandbox. And there is a sort of human logic to the ad hominem...say if there is something obvious that we don't like about somebody, it's easy to point that flaw out rather than consider the merits of their argument:
The news in the last few weeks has been replete with stories about Julian Assange and Wikileaks...one of the themes that has developed over the last week has been about calls for Assange's arrest (and more troubling, calls for his assassination (which is a more literal sort of ad hominem attack than I mean to address here). Some of the rhetoric around whether he ought to be arrested or not, or the general legality of what he is doing has devolved into attacks on his character (as of this writing, Mr. Assange has been arrested in London on charges of rape...a charge completely unrelated to his part in airing sensitive and secret political communications.
This blog will not be the first to question the timing of the rape charges and subsequent arrest (I do wish to clarify that I have no opinion on the legitimacy of the rape charges - if he's guilty of that crime he needs to be held to account) but I do wish to make the point that that quality of Assange's moral character or his alignment with the law is not relevant in the discussion of whether or not people have the right to know the intent and content of their government's actions.
So, for the sake of argument, let's assume that attacks on Julian Assange's character are classic ad hominem tactics, and are meant to distract from the core ideas in the debate. Who is served by that? If there are legitimate and rationale reasons to oppose the "leaks", let us focus on those in the debate.
The contra is also true; if Assange was the Mother Theresa of this early millenium, the positive attributes of his character ought not factor in the consideration of Wikileaks.
Maybe all the thinking on this concept wasn't quite enough to bang out a coherent post...but in the rough-draft-is-final-draft typing tonight it has occurred to be that the reflex to use the ad hominem is a bias - it is a reflection of one's basic bias against another, and allowing that bias to preclude a fair and honest hearing on the merits of an idea.