Why would maximizing expectation on a concave utility function lead to losing your shirt? It seems like any course of action that predictably leads to losing your shirt is self-evidently not maximizing expected concave-utility-function, unless it’s a Pascal mugging type scenario. I don’t think there are credible Pascal muggings in the world of personal finance, and if there are I’d be willing to accept an ad hoc axiom that we limit our theory to more conventional investments.
Now, I’ll admit it’s possible we should have a loss averse utility function, but we can do that without abandoning the mathematical approach—just add a time derivative of wealth, or something.
Why would maximizing expectation on a concave utility function lead to losing your shirt?
Because you’re ignoring risk.
The expectation is a central measure of a distribution. If that’s the only thing you look at, you have no idea about the width of your distribution. How long and thick is that left tail which is curling around preparing to bite you in the ass? Um, you don’t know.
I’m still not sure which line you’re taking on this:
A) Disputing the VNM formulation of rational behavior that a rational agent should maximize expected utility (https://en.wikipedia.org/wiki/Von_Neumann%E2%80%93Morgenstern_utility_theorem), or
B) Disputing that we can write down an approximate utility function accurate enough to sufficiently capture our risk preferences.
VNM doesn’t offer any “formulation of rational behavior”. VNM says that a function with a particular set of properties must exist and relies on assumptions that do not necessarily hold in real life.
I also don’t think that a utility function that can condense the risk preferences into a single scalar is likely to be accurate enough for practical purposes.
When I say “VNM doesn’t offer any formulation of rational behavior” I’m not disagreeing with any particular axiom. It’s like I’m saying that an orange is not an apple and you respond by asking me what kind of apples I dislike.
Which (possibly all) of the VNM axioms do you think are not appropriate as part of a formulation of rational behavior?
I think the Peano natural numbers is a reasonable model for the number of steins I own (with the possible exception that if my steins fill up the universe a successor number of steins might not exist). But I don’t think the Peano axioms are a good model for how much beer I drink. It is not the case that all quantities of beer can be expressed as successors to 0 beer, so beer does not follow the axiom of induction.
I think ZFC axioms are a poor model of impressionist paintings. For example, it is not the case that for every impressionist paintings x and y, there exists an impressionist painting that contains both x and y. Therefore impressionist paintings violate the axiom of pairing.
Which (possibly all) of the VNM axioms do you think are not appropriate as part of a formulation of rational behavior?
I don’t think that rational behaviour as understood on LW (basically, instrumental rationality) has anything to do with VNM axioms. In particular, I do not think that the VNM model is an adequate model of human decision-making once you go beyong toy examples.
Why would maximizing expectation on a concave utility function lead to losing your shirt? It seems like any course of action that predictably leads to losing your shirt is self-evidently not maximizing expected concave-utility-function, unless it’s a Pascal mugging type scenario. I don’t think there are credible Pascal muggings in the world of personal finance, and if there are I’d be willing to accept an ad hoc axiom that we limit our theory to more conventional investments.
Now, I’ll admit it’s possible we should have a loss averse utility function, but we can do that without abandoning the mathematical approach—just add a time derivative of wealth, or something.
Because you’re ignoring risk.
The expectation is a central measure of a distribution. If that’s the only thing you look at, you have no idea about the width of your distribution. How long and thick is that left tail which is curling around preparing to bite you in the ass? Um, you don’t know.
Is that a critique of expected utility maximization in general, or are you saying that concave functions of wealth aren’t risk-averse enough?
Is it an observation that expected utility maximization does not include risk management for free, just because it’s “utility”.
I’m still not sure which line you’re taking on this: A) Disputing the VNM formulation of rational behavior that a rational agent should maximize expected utility (https://en.wikipedia.org/wiki/Von_Neumann%E2%80%93Morgenstern_utility_theorem), or B) Disputing that we can write down an approximate utility function accurate enough to sufficiently capture our risk preferences.
Both.
VNM doesn’t offer any “formulation of rational behavior”. VNM says that a function with a particular set of properties must exist and relies on assumptions that do not necessarily hold in real life.
I also don’t think that a utility function that can condense the risk preferences into a single scalar is likely to be accurate enough for practical purposes.
Can you by chance pin down your disagreement to a particular axiom? You’re modus tollensing where I expected you would modus ponens.
You are looking at the wrong meta level.
When I say “VNM doesn’t offer any formulation of rational behavior” I’m not disagreeing with any particular axiom. It’s like I’m saying that an orange is not an apple and you respond by asking me what kind of apples I dislike.
Which (possibly all) of the VNM axioms do you think are not appropriate as part of a formulation of rational behavior?
I think the Peano natural numbers is a reasonable model for the number of steins I own (with the possible exception that if my steins fill up the universe a successor number of steins might not exist). But I don’t think the Peano axioms are a good model for how much beer I drink. It is not the case that all quantities of beer can be expressed as successors to 0 beer, so beer does not follow the axiom of induction.
I think ZFC axioms are a poor model of impressionist paintings. For example, it is not the case that for every impressionist paintings x and y, there exists an impressionist painting that contains both x and y. Therefore impressionist paintings violate the axiom of pairing.
I don’t think that rational behaviour as understood on LW (basically, instrumental rationality) has anything to do with VNM axioms. In particular, I do not think that the VNM model is an adequate model of human decision-making once you go beyong toy examples.