There are many different ways in which we could discount the future. The problem with almost all of them -- including the "hyperbolic" discounting Ainslie describes -- is not (necessarily) the mere fact that they discount, nor that they discount too much, but that they discount inconsistently: given times t1,t2,t3,t4, the relative importance of times t3 and t4 as seen from t1 is not the same as their relative importance as seen from t4. Or, to put it differently: if I apply t2-as-seen-from-t1 discounting together with t3-as-seen-from-t2 discounting, I don't get the same as if I apply t3-as-seen-from-t1 discounting.
It is possible to discount the future consistently, but there's basically only one degree of freedom when you choose how to do so. If you give events a time t in the future weight proportional to (constant)^t then that's consistent. It doesn't open you up to the bug ciphergoth describes, where your judgement now is that times t1 and t2 are almost equally important, whereas when t1 comes along you regard it as much more important than t2. (If you don't discount at all, that's the special case where the constant is 1.)
Ooo, no, actually you have more degrees of freedom than that: the most general scheme is that you choose a function F(t) and weight things according to that function. (Important note: one function, and its argument is absolute time, not time difference.) But the exponential case is the only possibility if you want your discounting function to be invariant if your whole life is shifted in time. (Which you might not -- if, e.g., there are external events that make a big difference.)
Anyway, the point is: it's not discounting "more than expected" that's the issue, it's having a pattern of discounting that's not internally consistent.
Okay, I see how my comment was off-target. To explain the pattern described would require something more along the lines of "People know that the state of things (external or internal) can change quickly, yet over the long term tend to regress to the mean. Therefore they privilege the present over the immediate future, but regard two points in the far future as the same, having no way to distinguish between them." But that's both speculative and fairly empty of content.
Akrasia is the tendency to act against your own long-term interests, and is a problem doubtless only too familiar to us all. In his book "Breakdown of Will", psychologist George C Ainslie sets out a theory of how akrasia arises and why we do the things we do to fight it. His extraordinary proposal takes insights given us by economics into how conflict is resolved and extends them to conflicts of different agencies within a single person, an approach he terms "picoeconomics". The foundation is a curious discovery from experiments on animals and people: the phenomenon of hyperbolic discounting.
We all instinctively assign a lower weight to a reward further in the future than one close at hand; this is "discounting the future". We don't just account for a slightly lower probability of recieving a more distant award, we value it at inherently less for being further away. It's been an active debate on overcomingbias.com whether such discounting can be rational at all. However, even if we allow that discounting can be rational, the way that we and other animals do it has a structure which is inherently irrational: the weighting we give to a future event is, roughly, inversely proportional to how far away it is. This is hyperbolic discounting, and it is an empirically very well confirmed result.
I say "inherently irrational" because it is inconsistent over time: the relative cost of a day's wait is considered differently whether that day's wait is near or far. Looking at a day a month from now, I'd sooner feel awake and alive in the morning than stay up all night reading comments on lesswrong.com. But when that evening comes, it's likely my preferences will reverse; the distance to the morning will be relatively greater, and so my happiness then will be discounted more strongly compared to my present enjoyment, and another groggy morning will await me. To my horror, my future self has different interests to my present self, as surely as if I knew the day a murder pill would be forced upon me.
If I knew that a murder pill really would be forced upon me on a certain date, after which I would want nothing more than to kill as many people as possible as gruesomly as possible, I could not sit idly by waiting for that day to come; I would want to do something now to prevent future carnage, because it is not what the me of today desires. I might attempt to frame myself for a crime, hoping that in prison my ability to go on a killing spree would be contained. And this is exactly the behavour we see in people fighting akrasia: consider the alcoholic who moves to a town in which alcohol is not sold, anticipating a change in desires and deliberately constraining their own future self. Ainslie describes this as "a relationship of limited warfare among successive selves".
And it is this warfare which Ainslie analyses with the tools of behavioural economics. His analysis accounts for the importance of making resolutions in defeating akrasia, and the reasons why a resolution is easier to keep when it represents a "bright clear line" that we cannot fool ourselves into thinking we haven't crossed when we have. It also discusses the dangers of willpower, and the ways in which our intertemporal bargaining can leave us acting against both our short-term and our long-term interests.
I can't really do more than scratch the surface on how this analysis works in this short article; you can read more about the analysis and the book on Ainslie's website, picoeconomics.org. I have the impression that defeating akrasia is the number one priority for many lesswrong.com readers, and this work is the first I've read that really sets out a mechanism that underlies the strange battles that go on between our shorter and longer term interests.