# Does vNM rationality depend on the good chosen?

The von Neumann-Morgenstern theorem states that, assuming a person's preferences under risk satisfy certain rationality axioms, then there exists a utility function u, the von Neumann utility function, such that the person will tend to maximize the expected value of u. For this reason, the hypothesis that people satisfy the von Neumann-Morgenstern rationality axioms is known as expected utility theory. Now in my question here, I asked about a confusion I had concerning the Ellsberg paradox, one of the major challenges to expected utility theory. But as I thought more about it, it seems to me that the fundamental issue was more general than the Ellsberg paradox.

Consider some good X. Let $L_1$ be the lottery that gives you a guaranteed 2 units of X, and let $L_2$ be the lottery that gives you a 50% chance of 1 unit of X, and a 50% chance of 3 units of X. Then my question is, is it always consistent with the von Neumann-Morgenstern axioms to prefer lottery $L_1$ to lottery $L_2$? Or are there some goods X such that it's inconsistent with the vNM axioms to prefer $L_1$ to $L_2$?

Well, suppose that good X is dollars. Then if u is the vNM utility function, then the expected utility of $L_1$ is equal to $u(2)$, and the expected utility of $L_2$ is equal to $.5u(1) + .5u(3)$. And it's certainly not inconsistent with the vNM axioms for $u(2)$ to be greater than $.5u(1) + .5u(3)$; that just means the person has diminishing marginal utility/risk aversion.

But now let's choose a different good X. Consider a raffle where four raffle tickets are placed in a hat, a single ticket is pulled out, and whoever the ticket belongs to wins a 100 dollars. Let good X be raffle tickets in this raffle. Then the probability of winning a 100 dollars is equal to .25 times the number of tickets you own. So under this scenario, the expected value of $L_1$ is $.5u(100)$, and the expected value of $L_2$ is $(.5)(.25)u(100)+(.5)(.75)u(100) = .5u(100)$. Thus the expected value of the two lotteries are equal, so it is irrational to prefer $L_1$ to $L_2$.

So what is going on here? How is preferring 2 units of X to a 50% chance of 1 unit of X and a 50% chance of 3 units of X vNM-rational for one good X but not vNM-rational for another? Just to add to the absurdity, what if someone were making a decision between lottery $L_1$ and $L_2$ where good X is dollars, and then was planning to use the money he made to buy raffle tickets? Then wouldn't the choice between $L_1$ and $L_2$ where X is dollars reduce to the choice between $L_1$ and $L_2$ where X is raffle tickets?

Any help would be greatly appreciated.

• The Ellsberg paradox has to do with subjective expected utility, where people can have any beliefs about the likelihood of some event, consistent with basic probability theory. Von Neumann and Morgenstern only treated the case of objective probabilities. – Michael Greinecker Jun 13 '15 at 11:46