(FORTUNE Magazine) – You won't find it in textbooks, and I've never heard a professor explain it to his Econ 101 class. But I'm increasingly convinced that no principle of economics better explains modern American behavior than the Law of Marginal Futility.

I first learned of Marginal Futility in college, not in a classroom but during a visit to the home of a rich classmate. It was a sprawling, hub-and-spoke quasi-palace with eight bedrooms, a couple of Jacuzzis, two kitchens, maid's quarters--the works.

As we pulled in the driveway, my friend mentioned that his parents had just paid an avant-garde decorator $100,000 to redo their house. And the house was indeed avant-garde: Inside it was almost empty. Long, bare hallways stretched off into rooms furnished with an austere table, a single floor lamp, perhaps a chair. There were no rugs or curtains. A plain platform bed stood in the center of each guest room. In the living room a half-dozen beanbag chairs lay dumpily on the floor.

And then it occurred to me precisely how rich my friend's parents were. How rich? So rich that they could pay $100,000 not to furnish their house.

Thus the Law of Marginal Futility. It is, as you'd guess, the opposite of the more widely known concept of marginal utility. The economics dictionary defines marginal utility as "the additional...satisfaction that an individual derives from consuming an additional unit of a good or service."

Marginal Futility, on the other hand, denotes the satisfaction an individual will derive from not consuming an additional unit of a good or service. He will, in fact, pay more to get less. My friend's parents might have bought a lovely ottoman, a fine mantlepiece, or a Louis XVI dining table at an exorbitant cost. Instead they paid far more to strip their house of furniture altogether.

The Law does not merely govern the indulgences of the rich. In America it has become a way of life. Consider our advanced tastes in refreshments. If you take fizzy water, add sugar, caramel coloring, and numerous other ingenious, secret flavorings, you have Coke--sold for, say, 75 cents a can. Now take the fizzy water and...leave it alone. Voila. You've got Perrier--sold at three times the price. Marginal Futility has kicked in. You've paid more for less.

Or beer. Traditional beer can be hearty and delicious, and in proper doses can give you the giggles. Alcohol-free beer tastes worse and will give you only gas. Naturally it's more expensive. For a warmer brew, I can take you to the corner diner, where you can sit comfortably while a waitress brings you a dollar cup of coffee, refills free. Americans today prefer Starbucks: Wait in line, bus your own table, no waitresses, and no refills. And, of course, the coffee at Starbucks costs more.

I myself am not free from the grip of Marginal Futility. Recently I celebrated a birthday with my wife. We went to the mountains. At a local Motel 6, I could have gotten a king-size mattress, TV with free HBO, coffeemaker in the bathroom, clock radio and phone next to the bed--all for $39 a night. I won't mention the box springs wired with Magic Fingers, at three minutes per quarter.

Of course we stayed in a bed-and-breakfast: no TV, no coffeemaker, no HBO, neither radio nor phone nor Magic Fingers. We did get a free breakfast, which we were forced to eat very early in the morning at a table with four people I probably wouldn't have liked even had they not been total strangers. For such discomfort and near-total lack of amenities, you really have to pay through the nose: $150 a night, almost quadruple the rate at the Motel 6. It could have been worse. We could have vacationed with Outward Bound, which asks yuppies to sleep on rocks, get chigger bites, and eat freeze-dried food for the price of a week in a comfortable hotel in Oaxaca.

Nowhere are the paradoxes of Marginal Futility seen more clearly than in contemporary restaurants, where chicken gets really expensive only when it's served without skin, the single tasty part of the whole bird. For our birthday celebration, my wife and I enjoyed a fixed-price dinner. For a fixed-price dinner you pay extra and, in return, forfeit the right to choose what you're going to eat. The food was nouvelle cuisine. The smaller the portions, the bigger the check--the Law reduced to its barest essence.

I'm no economist, so I can't say whether there's a point at which Marginal Futility ceases to operate, even in theory. Mathematicians speak of asymptotes--trend lines that stretch toward infinity without ever quite getting there. I can easily imagine a nouvelle-cuisine dinner with a fixed price of $100,000: a brown M&M and a glass of tap water, with a French waiter to snob you as an extra treat. The federal government offers another possibility. Buying surplus food from farmers would cost the government hundreds of millions of dollars, but paying farmers not to produce food so no one can buy it costs billions. Leave it to the feds to perfect the laws of economics.