A psychiatrist blogging under the nomme de plume of 'Scott Alexander' has a fantastic recent blog post about the psychology of motivated reasoning, specifically as it applies to emotionally charged debates about politics, religion, and so forth.
His main thesis is that most such debates are not really about meaningfully answerable questions, but are instead competitions in which each side tries to associate its own position with key words that have positive emotional associations and the opposing position with key words that have negative emotional associations. The purpose of such debates is not to prove that a given thesis is true or false, but rather to create pro or con attitudes in the audience toward a given cause or concept, through the strategic use of emotionally-laden verbal associations. In other words, the purpose of such debates is to secure loyalty to a cause, not to secure belief in a proposition.
From the blog post:
This sort of conflation between a cause and its supporters really only makes sense in the emotivist model of arguing. I mean, this shouldn’t even get dignified with the name ad hominem fallacy. Ad hominem fallacy is “McCain had sex with a goat, therefore whatever he says about taxes is invalid.” At least it’s still the same guy. This is something the philosophy textbooks can’t bring themselves to believe really exists, even as a fallacy.
But if there’s a General Factor Of McCain, then anything bad remotely connected to the guy – goat sex, lying campaigners, whatever – reflects on everything else about him.
This is the same pattern we see in Israel and Palestine. How many times have you seen a news story like this one: “Israeli speaker hounded off college campus by pro-Palestinian partisans throwing fruit. Look at the intellectual bankruptcy of the pro-Palestinian cause!”
It’s clearly intended as an argument for something other than just not throwing fruit at people. The causation seems to go something like “These particular partisans are violating the usual norms of civil discussion, therefore they are bad, therefore something associated with Palestine is bad, therefore your General Factor of Pro-Israeliness should become more strongly positive, therefore it’s okay for Israel to bomb Gaza.” Not usually said in those exact words, but the thread can be traced.