As a previous poster has said, the absurdity heuristic works very well indeed - if something seems absurd to me, I need a lot of evidence before I'll believe it. As Hume said:
"no testimony is sufficient to establish a miracle, unless the testimony be of such a kind, that its falsehood would be more miraculous than the fact which it endeavors to establish."
If someone claims that a talking snake is the reason for every bad thing that anyone has ever done, they're going to have to provide some evidence that this is the case. If they claim that people are related to monkeys (which seems inherently less absurd to me, but I'm probably biased by the fact that it's true), then they're also going to have to provide some evidence, and whatever the extraordinary claim, the evidence is going to have to be enough to shift my belief from "that's absurd" to "oh, I guess that's true then".
On the other hand, having written that I guess it's more likely that the Absurdity Heuristic is more specific, and is the tendency to stick our fingers in our ears, say "that's absurd, la la la", and refuse to listen to any evidence to the contrary. I suppose this is a heuristic that people might use, and might be useful (in ruling out hypotheses which aren't worth spending time falsifying), but as you say, does have its dangers. It's not clear how to avoid the danger of ruling out an hypothesis which is absurd but true while getting the benefits of ruling out hypotheses which are simply absurd, but the heuristic still has its uses (perhaps some threshold of absurdity, some "suspension of absurdity" for certain types of proposition?)
I think that my bias towards our being related to monkeys is due to the meanings I invest in "monkey" and "human" as not being greatly dissimilar.
On the other hand, if I had already accepted the existence and human-exclusiveness of a soul, and/or a supernatural account of the world's origin that afforded special primacy to humans as distinct from animals, then clearly I would think relations that crossed these distinct boundaries of type were too absurd to consider.
Also, another limitation on the heuristic might be, as you suggest, we...
I particularly remember one scene from Bill Maher's "Religulous". I can't find the exact quote, but I will try to sum up his argument as best I remember.
I have read of the absurdity heuristic. I know that it is not carte blanche to go around rejecting beliefs that seem silly. But I was still sympathetic to the talking snake argument. After all...a talking snake?
I changed my mind in a Cairo cafe, talking to a young Muslim woman. I let it slip during the conversation that I was an atheist, and she seemed genuinely curious why. You've all probably been in such a situation, and you probably know how hard it is to choose just one reason, but I'd been reading about Biblical contradictions at the time and I mentioned the myriad errors and atrocities and contradictions in all the Holy Books.
Her response? "Oh, thank goodness it's that. I was afraid you were one of those crazies who believed that monkeys transformed into humans."
I admitted that um, well, maybe I sorta kinda might in fact believe that.
It is hard for me to describe exactly the look of shock on her face, but I have no doubt that her horror was genuine. I may have been the first flesh-and-blood evolutionist she ever met. "But..." she looked at me as if I was an idiot. "Monkeys don't change into humans. What on Earth makes you think monkeys can change into humans?"
I admitted that the whole process was rather complicated. I suggested that it wasn't exactly a Optimus Prime-style transformation so much as a gradual change over eons and eons. I recommended a few books on evolution that might explain it better than I could.
She said that she respected me as a person but that quite frankly I could save my breath because there was no way any book could possibly convince her that monkeys have human babies or whatever sort of balderdash I was preaching. She accused me and other evolution believers of being too willing to accept absurdities, motivated by our atheism and our fear of the self-esteem hit we'd take by accepting Allah was greater than ourselves.
It is not clear to me that this woman did anything differently than Bill Maher. Both heard statements that sounded so crazy as to not even merit further argument. Both recognized that there was a large group of people who found these statements plausible and had written extensive literature justifying them. Both decided that the statements were so absurd as to not merit examining that literature more closely. Both came up with reasons why they could discount the large number of believers because those believers must be biased.
I post this as a cautionary tale as we discuss the logic or illogic of theism. I propose taking from it the following lessons:
- The absurdity heuristic doesn't work very well.
- Even on things that sound really, really absurd.
- If a large number of intelligent people believe something, it deserves your attention. After you've studied it on its own terms, then you have a right to reject it. You could still be wrong, though.
- Even if you can think of a good reason why people might be biased towards the silly idea, thus explaining it away, your good reason may still be false.
- If someone cannot explain why something is not stupid to you over twenty minutes at a cafe, that doesn't mean it's stupid. It just means it's complicated, or they're not very good at explaining things.
- There is no royal road.
(special note to those prone to fundamental attribution errors: I do not accept theism. I think theism is wrong. I think it can be demonstrated to be wrong on logical grounds. I think the nonexistence of talking snakes is evidence against theism and can be worked into a general argument against theism. I just don't think it's as easy as saying "talking snakes are silly, therefore theism is false." And I find it embarrassing when atheists say things like that, and then get called on it by intelligent religious people.)