While it’s plausible that utility functions are sigmoidal, it’s not obviously true, and it’s certainly not true of many of the utility functions generally used in the literature.
Moreover, even if experienced-utility (e.g. emotional state) functions are sigmoidal, that doesn’t imply that decision-utility functions are, except in the special case that individuals are risk-neutral with respect to experienced utility. More generally than that, a consistent decision-utility function can be any positive monotonic transform of an experienced utility function.
EDIT: I should have added that the implication of that last point is that you can rationalize a lot of behavior just by assuming a particular level of risk preference. You can’t rationalize literally anything (consistency is still a constraint), but you can rationalize a lot. All of this makes it especially important to argue explicitly for the particular form of happiness/utility function you’re relying on.
(EDITED again to hopefully overcome ambiguities in the way different people are using the terms happiness and utility.)
FWIW, Charles Karelis makes this argument extensively in his book The Persistence of Poverty.
While it’s plausible that utility functions are sigmoidal, it’s not obviously true, and it’s certainly not true of many of the utility functions generally used in the literature.
Moreover, even if experienced-utility (e.g. emotional state) functions are sigmoidal, that doesn’t imply that decision-utility functions are, except in the special case that individuals are risk-neutral with respect to experienced utility. More generally than that, a consistent decision-utility function can be any positive monotonic transform of an experienced utility function.
EDIT: I should have added that the implication of that last point is that you can rationalize a lot of behavior just by assuming a particular level of risk preference. You can’t rationalize literally anything (consistency is still a constraint), but you can rationalize a lot. All of this makes it especially important to argue explicitly for the particular form of happiness/utility function you’re relying on.
(EDITED again to hopefully overcome ambiguities in the way different people are using the terms happiness and utility.)