One guess, pointed out in the original comments, might be that there is reason to prefer certainty when making deals with untrustworthy agents. For instance, if I promise you a certain $24,000, and then you don't get it, you know for sure that I lied, as does everyone else who was aware of the deal, which is pretty bad for me. If I promise you a 33/34 chance of $27,000 then if you don't get it I can always claim you were just unlucky, giving me at least plausible deniability. Thus there is significant reason for you to prefer the first, since the more I have to lose by betraying you the less likely I am to do it. The same argument does not carry in the case of 33% versus 34%.
I suspect that with infinite computational power on all sides this effect would vanish, and failing to deliver on any deal would decrease my trustworthiness by a certain amount depending on the plausibility of other explanations. However, humans don't have infinite computational power, so we tend to just save time by labelling people as "trustworthy" or "untrustworthy" meaning, creating the incentive to bias towards absolute promises rather than probabilistic promises.
Of course, this is all quite complicated, it's just one thought that springs to mind. It may be better just to favour the null hypothesis of "evolution is stupid, the human brain is a massive kludge that doesn't normally operate on anything resembling expected utility, massive mistakes are to be expected".
The vertebrate retina is a kludge, but we don't have a percentage of the population with octopus-style retinas, so there's no selectable variance to favor the genes that that produce octopus-type retinas. Similarly, we can't evolve a proper set of long back bones because there's no variance in the human population to use to select against our ludicrous stacked vertebrae arrangement.
But the degree to which people favor certainty does vary, and accordingly it is vulnerable to selection pressure. There accordingly must be a why as to the continued existence of certainty bias.
Today's post, Zut Allais! was originally published on 20 January 2008. A summary (taken from the LW wiki):
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