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.)
"For someone with $ << 0, the marginal utility of $5 to them is minimal. "
I'm a newbie, which will soon be obvious, but I don't think the utility function is being applied correctly. At each value of U (the worth that a person has at his disposal in goods), we have the utility that can be purchased with U. (So u is negative for U<0 because you get negative things for owing money.)
I understand that if someone is greatly in debt, their utility may not change much if you increase or decrease their debt by some amount. This is why the utility function would be shallow for $ << 0. Thus, I agree that someone with $ << 0 who happens to find $5 on the ground would have little incentive to use $5 to pay off their debt.
However, let's use the function to see what utility they can purchase with their $5...
The fact that they are spending it on gambling or drugs means they are NOT moving from U=(-X) to U=(-X+5) (they're not using the $5 to pay off their debt). They are staying at U=(-X) and spending their $5 as true disposable income -- in other words, exactly as though U=0.
A person with U=0 gets a steep benefit from the spending of $5.
I think there's some confusion here as to what the utility function is defined over. And to be fair, the post itself is somewhat confused in this respect.
The argument that it might be more or less rational to gamble is an entirely different matter to whether it is more or less rational to smoke crack.
The shape of the utility function over money can make it more or less rational to accept particular money gambles: risk aversion is after all a property of the shape of the utility function.
The shape of the utility function over money cannot affect whether s... (read more)