This comment provides more confirmation for a view I've held for a long time, and which was particularly reinforced by some of the reactions to (the first version of) my Amanda Knox post.
People have trouble distinguishing appropriately among degrees of improbability. This generalizes both underconfidence and overconfidence, and is part of what I regard as a cluster of related errors, including underestimating the size of hypothesis space and failing to judge the strength of evidence properly. (These problems are the reason that judicial systems can't trust people to decide cases without all kinds of artificial-seeming procedures and rules about what kind of evidence is "allowed".)
The reality is that given all the numerous events and decisions we experience on a daily basis and throughout our lives, something with a 10% chance of happening or being true is something that we need to take quite seriously indeed. 10% is, easily, planning-level probability; it should attract a significant amount of our attention. By the same token, something which isn't worth seriously planning on shouldn't be getting more than single digits of probability-percentage, if that.
There is a vast, huge spectrum of degrees of improbability below 1% (never mind 10% or 30%) that careful thinking can allow us to distinguish, even if our evolved intuitions don't. Consider for instance the following ten propositions:
(1) The Republicans will win control of both houses of Congress in the 2010 elections.
(2) It will snow in Los Angeles this winter.
(3) There will be a draft in the U.S. by 2020.
(4) I will be dead in a month.
(5) Amanda Knox (or Raffaele Sollecito) was involved in Meredith Kercher's death.
(6) A U.S. state will make a serious attempt to secede by 2020.
(7) The Copenhagen interpretation of quantum mechanics, as opposed to the many-worlds interpretation, is correct.
(8) A marble statue has waved or will wave at someone due to quantum tunneling.
(9) Jesus of Nazareth rose from the dead.
(10) Christianity is true.
I listed these in (approximately) order of improbability, from most probable to least probable. Now, all of them would be described in ordinary conversation as "extremely improbable". But there are enormous differences in the degrees of improbability among them, and moreover, we have the ability to distinguish these degrees, to a significant extent.
The 10%-30% range is for propositions like (1) ; the 1%-10% range for things like (2) (the last time it snowed in LA was in the 1960s). Around 1% is about right for (3). Propositions (4), (5), and (6) occupy something like the interval from 0.01% to 1% (I find it hard to discriminate in this range, and in particular to judge these three against each other). Propositions (8), (9), and (10), however, are in a completely different category of improbability: double-digit negative exponents, if you're being conservative. We could argue about (7), but it probably belongs somewhere in between (4)-(6) and (8)-(10); maybe around 10^(-10), if you account for post-QM theories somehow turning Copenhagen into something more mundane than it seems now.
So the point is, we, right here, have the tools to make estimates that are a lot more meaningful than "probably yes" or "probably no". I remember reading that we tend to be overconfident on hard things and underconfident on easy things; I think we can afford to be a little more bold on the no-brainers.
Propositions (4), (5), and (6) occupy something like the interval from 0.01% to 1% (I find it hard to discriminate in this range, and in particular to judge these three against each other).
I'd guess you could estimate (4) to within an order of magnitude or better from an actuarial table.
I would like to propose this as a thread for people to write in their predictions for the next year and the next decade, when practical with probabilities attached. I'll probably make some in the comments.