Just to be clear, you know that an exponential utility function (somewhat misleadingly ) doesn’t actually imply that utility is exponential in wealth, right? Bill’s claimed utility function doesn’t exhibit increasing marginal utility, if that’s what you’re intuitively objecting to. It’s 1-exp(-x), not exp(x).
Many people do find the constant absolute risk aversion implied by exponential utility functions unappealing, and prefer isoelastic utility functions that exhibit constant relative risk aversion, but it does have the advantage of tractability, and may be reasonable over some ranges.
Example of the “unappealingness” of constant absolute risk aversion. Say my u-curve were u(x) = 1-exp(-x/400K) over all ranges. What is my value for a 50-50 shot at 10M?
Answer: around $277K. (Note that it is the same for a 50-50 shot at $100M)
Given the choice, I would certainly choose a 50-50 shot at $10M over $277K. This is why over larger ranges, I don’t use an exponential u-curve.
However, it is a good approximation over a range that contains almost all the decisions I have to make. Only for huge decisions to I need to drag out a more complicated u-curve, and they are rare.
Just to be clear, you know that an exponential utility function (somewhat misleadingly ) doesn’t actually imply that utility is exponential in wealth, right? Bill’s claimed utility function doesn’t exhibit increasing marginal utility, if that’s what you’re intuitively objecting to. It’s 1-exp(-x), not exp(x).
Many people do find the constant absolute risk aversion implied by exponential utility functions unappealing, and prefer isoelastic utility functions that exhibit constant relative risk aversion, but it does have the advantage of tractability, and may be reasonable over some ranges.
Example of the “unappealingness” of constant absolute risk aversion. Say my u-curve were u(x) = 1-exp(-x/400K) over all ranges. What is my value for a 50-50 shot at 10M?
Answer: around $277K. (Note that it is the same for a 50-50 shot at $100M)
Given the choice, I would certainly choose a 50-50 shot at $10M over $277K. This is why over larger ranges, I don’t use an exponential u-curve.
However, it is a good approximation over a range that contains almost all the decisions I have to make. Only for huge decisions to I need to drag out a more complicated u-curve, and they are rare.