Our good pals Von Neumann and Morgenstern lend us four axioms against which to adjudge blackboard rationality:
- Completeness: preferences can be weakly rank-ordered. I either prefer apples to oranges, oranges to apples, or I'm indifferent between the two. I cannot both prefer apples to oranges and oranges to apples. I think of the completeness condition as the anti-schizophrenic axiom.
- Transitivity: preference rank order preservation. If I prefer apples to oranges and oranges to bananas, I prefer apples to bananas. No rock-paper-scissors when it comes to preferences. This one is where it gets tricky when we include extra dimensions.
- Continuity: there's some exchange rate at which I'll swap apples for bananas. Yes, I like one apple more than one banana, but all else equal, I'll give up a slice of apple for a bunch of bananas. And then it'll get awkward because I'm always tempted to call a bunch of bananas a "group" of bananas, much in the same way I'll call a bunch of grapes a "cluster" of grapes just for pure contrary cussedness.
- Independence: irrelevant choices shouldn't affect my rank ordering. If I prefer one apple to one banana, I should prefer one apple and one orange to one banana and one orange. This one is the most obviously difficult to sustain in an ecological setting. Suppose I prefer milk to apples. Add oranges to each. With apples and oranges, I can make fruit salad (yum!). With milk and oranges, all I can make is sadness and disappointment. Independence is violated (sort of).
These are nice axioms to use when we create those cute utility models with which to derive demand curves. The problem is, as with the GDP stuff I keep grousing about, is when our model shifts from being a tool to describe the world to being Procrustes' bed upon which to rack and hew mankind. Nutritional do-goodniks wage bright campaigns against sellers of chow balanced upon the thin reed of probable regret. No raw milk, no imported cheese, no big-ass sodas, no bundling of toys with fast food, no trans-fats, no bacon-wrapped frankfurters, no this, no that. You can't control your appetite, so it's up to us to control it for you. Don't worry, you'll thank us later.
This is the world of ex post regret. Would you like to super-size that?
Funny thing though, there's a lot to the argument. Ignore the shallow reasoning based on fiscal externalities and assume that we've got a world undistorted by farm bills and "health" policy (the scare quotes are there because we subsidize medicine and insurance and then pretend that we're subsidizing health). Even in a world where all costs are borne by the consumer, discounting is such that many folks will overeat. Gluttons will regret all the face-stuffing. Gorging yourself isn't euvoluntary.
There's a now-ridiculed old-timey virtue called temperance which unfortunately got all muddled up in the anti-alcohol campaigns of the late 19th and early 20th centuries. Temperance and a fair dose of table manners may have helped keep appetites in check. As with most other virtues, the path that leadeth there cannot be trod by political elites on the behalf of constituents. Forced virtue is no virtue at all. Duh. So if I prefer more food to less food today and I prefer being svelte to being obese tomorrow, I've got a challenge that confounds my Max-U rationalist. Maybe it's a fine time to recall that this problem is far from intractable. It's just that the particular traction happens to be out of the hands of politics but rather impaled on the forks of diners.
So yes, people are getting fatter. The solution isn't more and thicker bans. People are too hungry and too clever for that. At a minimum, it's directionally euvoluntary to stop doing harm.