The lottery came up in a recent comment, with the claim that the expected return is negative - and the implicit conclusion that it's irrational to play the lottery. So I will explain why this is not the case.
It's convenient to reason using units of equivalent value. Dollars, for instance. A utility function u(U) maps some bag of goods U (which might be dollars) into a value or ranking. In general, u(kn) / u(n) < k. This is because a utility function is (typically) defined in terms of marginal utility. The marginal utility to you of your first dollar is much greater than the marginal utility to you of your 1,000,000th dollar. It increases the possible actions available to you much more than your 1,000,000th dollar does.
Utility functions are sigmoidal. A serviceable utility function over one dimension might be u(U) = k * ([1 / (1 + e-U)] - .5). It's steep around U=0, and shallow for U >> 0 and U << 0.
Sounds like I'm making a dry, academic mathematical point, doesn't it? But it's not academic. It's crucial. Because neglecting this point leads us to make elementary errors such as asserting that it isn't rational to play the lottery or become addicted to crack cocaine.
For someone with $ << 0, the marginal utility of $5 to them is minimal. They're probably never going to get out of debt; someone has a lien on their income and it's going to be taken from them anyway; and if they're $5 richer it might mean they'll lose $4 in government benefits. It can be perfectly reasonable, in terms of expected utility, for them to play the lottery.
Not in terms of expected dollars. Dollars are the input to the utility function.
Rationally, you might expect that u(U) = 0 for all U < 0. Because you can always kill yourself. Once your life is so bad that you'd like to kill yourself, it could make perfect sense to play the lottery, if you thought that winning it would help. Or to take crack cocaine, if it gives you a few short intervals over the next year that are worth living.
Why is this important?
Because we look at poor folks playing the lottery, and taking crack cocaine, and we laugh at them and say, Those fools don't deserve our help if they're going to make such stupid decisions.
When in reality, some of them may be making <EDITED> much more rational decisions than we think. </EDITED>
If that doesn't give you a chill, you don't understand.
(I changed the penultimate line in response to numerous comments indicating that the commenters reserve the word "rational" for the unobtainable goal of perfect utility maximization. I note that such a definition defines itself into being irrational, since it is almost certainly not the best possible definition.)
IAWY right up to the penultimate sentence. Humans continuously modify their utility functions to maintain a steady level of happiness. A change in your utility function's input--like winning the lottery, or suffering a permanent injury--has only a temporary effect. The day you collect your winnings, you're super-happy; a year later, you're no happier than you were when you bought the ticket. If you're considering picking up a crack habit, you had better realize that in a year your baseline happiness will be no higher than it is now, despite all the things you'll sacrifice trying to be happy.
Supplying yourself with cocaine and money isn't an effective way to achieve a goal of happiness, just like supplying a country with foreign aid isn't an effective way to improve quality of life there. The rational thing to do is to grab the levers of your hedonic treadmill and set it where you want it to be. But it's risky to monkey around with such things--which is why I'm interested in That Which Must Not Be Named. I have no personal stake in Eliezer's mission, but his methodical approach to studying utility functions suggests what parts of your utility function you can safely alter.
That does introduce another level of complication. Utility functions assume a static model. They are not happiness functions. We talk about maximizing utility all the time on LW, when really we want to maximize happiness.
Maximizing your happiness is a higher level of rationality than maximizing your utility. I think it's still okay to sometimes define "rational" as maximizing expected utility.
(I don't think foreign aid has anything to do with the delta-nature of happiness, btw.)