Sat, 04 Mar 2006
I pulled some sleight-of-hand in the previous paragraph. I said the game was a good deal "because" the expected winnings were positive. But that's not sufficient. If it were, the following game would also be a good deal: You bet one million dollars on the throw of a die. If the die comes up 6, you get your million back plus 25 million more. Otherwise, you lose your million.
For some people, the second game is a good deal. For most people, including me, it's obviously a very bad idea. To get a million dollars, I'd at least have to mortgage everything I owned. Then I'd be under a crushing debt for the rest of my life, with 83% likelihood. But the expected return of the two games is the same; this shows that a good expected return is not a sufficient condition for a good investment.
The difference, of course, is that the second game is much riskier than the first.
I think most people understand this, but nevertheless you still hear them say a lot of dumb stuff about risk. For example, many people like to say that the lottery is a stupidity tax on people who don't understand basic arithmetic, and that nobody would play the lottery unless they were very stupid, because it's trivial to see that the expected return is very poor.
I used to meet people at parties who said this. I would point out that by this reasoning, fire insurance is also a stupidity tax on people who don't understand basic arithmetic, because it's clear that the expected return on fire insurance is negative. I did get argument from folks from time to time, but it's really not arguable. If fire insurance didn't have an expected negative return for the customer, the insurance company would go out of business. In fact, the insurance company employs a whole department full of mathematicians whose job it is to make sure that the value of the premium you pay exceeds the expected cost of the benefits that the company will pay. So there are only three choices here:
Once again the issue is not so much the expected return as it is risk. You pay the insurance premiums in order to mitigate the risk of a fire. One big fire could wipe you out completely. So you insure your house against fire so that you can't be completely wiped out. In return, you pay small, predictable sums of money regularly.
Another way to look at this is to consider the idea of a utility function. This is just a fancy term for the observation that the usefulness of money is not a linear function of the face value of the money. Once you have a million dollars, the utility of another hundred is much lower than it is to someone who only has ten thousand.
When you calculate expected returns, you need to calculate the expected increase of the utility, not the expected return of the nominal face value of the money. Consider this thought experiment: you may bet one cent on a game that will pay you ten thousand dollars if you win, which it will do one time in two million. Do you play? Well, maybe you do, because if you lose, so what? It's only one cent, and you will never miss it. The utility of one cent is essentially zero. The utility of ten thousand dollars, on the other hand, is very high, much higher than two million times zero. But if you like this game, you're open to the same charge of not understanding simple arithmetic as the lottery people are, because the expected return is very low, about the same as the lottery. The game is the same as the lottery, only the cost and the payoff are each a hundred times smaller.
In the fire insurance scenario, I am betting a small amount of money, with comparatively low utility, against a very large amount with much higher utility. One can view the lottery as analogous. If I buy a lottery ticket for $1, it's not because I misunderstand arithmetic. It's because the utility of $1 is low for me. I could blow $.85 on a candy bar tomorrow at lunch without thinking about it much. But the utility of winning millions is very high. With ten million dollars, I could pay off my mortgage, quit my job, and spend the rest of my life travelling around and writing articles. The value of even a hundred-millionth chance of this happening might well be higher than the value of gobbling one more candy bar that my body didn't need anyway.
Here's an exercise I've been doing lately, trying to estimate the value I ascribe to my own life. I am afraid that this is a trite subject, If so, I apologize. But if not, try it yourself, and you might discover something interesting. Suppose you have the option to play Russian Roulette, in return for which you will receive a fee of x. The gun has one million chambers, one of which holds a bullet. If you get the bullet, you die. Otherwise you collect the fee. What is the minimum value for x that will induce you to play? Would you play if x were one million dollars? I would. It's an almost sure million, and a million is a huge amount of money to me. And I probably take bigger than million-to-one risks every time I cross the street, so why not? So one might say that this demonstrates that my own estimate of the value of my own life is less than 1012 dollars.
Would I play for a thousand dollars? No, probably not. But where's the cutoff? Ten thousand is a maybe, a hundred thousand is a probably. (I rather suspect that the cutoff is on the same order of magnitude as the mortgage on my house. This thought threatens to open a whole can of disturbing philosophical worms.)
Now let's up the risk. I've already agreed to bet my life on a million-to-one chance in return for a million dollars. The expected-value theory says that I should also be willing to bet it on a thousand-to-one chance for a billion dollars. Am I? No way. The utility of a billion dollars is much less than a thousand times the utility of a million, for me. For Donald Trump, it might be different.
As a final exercise in thinking about risk, consider this: Folks at NASA estimate that your chance of being killed by a meteorite are on the order of 1 in 25,000. It's not because you're likely to be hit in the head. Nobody in recorded history has been killed by a meteor. It's because really big meteors do come by every so often, and when (not if, but when) one hits the earth, it'll kill just about everyone.
[ Addendum 20060425: There is a followup article to this one. ]
[ Addendum 20160208: There reports today that a man in Tamil Nadu has been killed by a meteor. ]