Diminishing marginal utility: why the same $1,000 changes one life and barely touches another

Personal Money 5 min 80 sources

Each extra dollar is worth less to you than the one before, because you spend the first dollars on what matters most. That single curve explains why insurance is rational and why risk aversion exists.

Key takeaways

Each extra dollar is worth less to you than the last, because you spend your first dollars on what matters most and later ones on things that matter less.
That single curve is why the same $1,000 can change a struggling person's life and barely register for a rich one.
It also explains why insurance is rational and why most people avoid fair gambles: you are trading dollars, but what counts is the satisfaction those dollars carry.

Hand $1,000 to someone who is one missed paycheck from eviction, and you may change the shape of their month. Hand the same $1,000 to a millionaire, and they might not notice it left the account. Same money, wildly different effect. The reason is one of the oldest ideas in economics, and once you see it, a lot of money decisions stop looking mysterious.

The one dollar is not worth what the last one was

Economists use the word “utility” to mean satisfaction or usefulness [20]. Marginal utility is the extra satisfaction you get from one more unit of something [20]. The law of diminishing marginal utility says that each additional unit gives you less than the one before [50].

Food is the easy example. You are hungry and buy a slice of pizza at $2 [50]. The first slice is bliss. The second is good. By the fifth, you are full, and it does almost nothing for you [50]. The slices are identical; your satisfaction from each one shrinks. Buy a vacuum cleaner for $100 and you gladly pay it; a second identical vacuum is worth maybe $20 to you, because you had no need for it [50].

Money works the same way [35]. You spend your first dollars on the things you need most: rent, food, heat. Later dollars go to things that matter less, because the urgent needs are already covered [35]. The economist Alfred Marshall put it plainly in 1890: “The additional benefit which a person derives from a given increase of his stock of a thing diminishes with every increase in the stock that he already has” [35].

The same money, a different life

This is why $1,000 is not one fixed amount of good. For a person choosing between rent and food, that $1,000 buys the most valuable things money can buy [35]. For someone whose needs are all met, the same sum buys a slightly nicer version of something they already have. The dollars are equal; the utility they deliver is not.

The evidence on money and happiness follows the same shape, though the details are debated. A famous 2010 study by Daniel Kahneman and Angus Deaton found day-to-day emotional wellbeing rose with income only up to about $75,000 a year, then flattened [56]. In 2021, the researcher Matt Killingsworth found nearly the opposite: happiness kept climbing well past $75,000, with no clear ceiling [56]. When the two camps later worked together, the honest picture was that more money still helps for most people [56]. But notice the pattern even where money keeps helping: Killingsworth’s later work, sampling 33,269 US adults, found the happiness gap between wealthy and middle-income people was wider than the gap between middle- and low-income people [48]. The extra dollar does something; it just tends to do less the more you already have [19].

Why insurance is rational, not a bad bet

Here is where the curve earns its keep. On average, insurance is a losing bet: the premiums you pay are designed to be more than the claims you get back, because the insurer pools everyone’s risk and takes a margin to cover costs and profit [69]. So why is buying it sensible?

Because you are not trading dollars for dollars. You are trading utility for utility. A small, certain premium costs you dollars from the comfortable, low-value end of your money, where each dollar is worth little. The loss it protects against, say your house burning down, would take dollars from the desperate, high-value end, where each one is worth enormously more [1]. Losing $300,000 you cannot replace does not hurt a thousand times as much as losing $300; because of the curve, it hurts far more than that. You pay a small, painless certain cost to avoid a rare, catastrophic one whose lost utility would dwarf its dollar size [3]. That trade is rational precisely because money’s value is not flat.

Risk aversion is just the shape of the curve

The same idea explains why most people dislike gambles even when the odds are fair. This was worked out three centuries ago. Mathematician Daniel Bernoulli was puzzled by a game, the St. Petersburg paradox, whose average payout was infinite, yet no one would pay more than a few dollars to play it [1]. His answer: people judge a gamble by its expected utility, not its expected dollars [1]. Because each extra dollar is worth less, a coin-flip that could double your money or halve it is a bad deal in utility terms, even though it is even money in dollar terms. The gain adds a little happiness; the loss subtracts a lot [3].

That reluctance has a name, risk aversion, but it is not a personality flaw or timidity [1]. It is simply the mathematics of a curve that bends: because the dollars you could lose are worth more to you than the dollars you could win, a fair bet is a bad bet. Expected utility, the probability-weighted value of outcomes measured in satisfaction rather than cash, is the tool economists use to capture this [3].

Where it bends the other way

The curve is not a law of nature for every person in every moment. Someone with almost nothing may take a wild gamble precisely because their situation can barely get worse, so the downside costs them little utility. A lottery ticket sells hope cheaply to people for whom the small certain loss is trivial and the tiny chance of transformation is not. And “utility” is not the same as visible spending; a dollar saved for security can carry more satisfaction than a dollar spent, depending on the person [35]. The shape is a strong tendency, not a script.

What it does give you is a lens. It explains why the same raise thrills you early in a career and barely registers later, why the rich can be casual with sums that would rescue someone else, and why paying a premium to protect what you cannot afford to lose is not fear, but arithmetic.

02 · Lesson · why it matters

Why the same dollar is worth more to some people than to you

Each dollar you get is worth a little less than the last - and that quiet curve shapes much of your money life.

Start with the fifth slice of pizza

You are hungry and buy a slice. It is wonderful. The second is good. By the fifth, you are stuffed, and it barely registers. The slices never changed. What changed is you: each one arrived to a fuller stomach, so each did a little less.

Money behaves the same way. The word economists use is utility, meaning satisfaction or usefulness. Each extra dollar carries a little less of it than the dollar before. Not because dollars shrink, but because of where they land.

Your first dollars do the heavy lifting

Picture your money arriving in order of what it does. The first dollars cover rent, food, heat, the bus to work. Those are the highest-value things money can buy, so those dollars are worth an enormous amount to you.

The next dollars cover things that matter, but matter less. Then things that are merely nice. Then a slightly better version of something you already own. By the time you reach the dollars at the far end, the urgent work is long done. Each one still helps. It just helps less.

That is the whole idea. The value of a dollar is not printed on it. It depends on how many you already have and what is left to buy.

Why the rich can seem careless

Hand a struggling person $1,000 and you might change the shape of their month. Hand the same $1,000 to someone wealthy and it may vanish into an account they rarely check. The bill is identical. Its effect is not.

This is worth sitting with, because it is easy to read as a character flaw. The rich person seems careless with money; the struggling person seems to sweat every small sum. Neither is about character. They are standing at different points on the same curve. Down where needs are unmet, every dollar is loud. Up where they are all met, dollars go quiet. Each person is responding sensibly to the value a dollar actually has for them.

Insurance is that curve, priced

Now the curve pays for itself. On average, insurance is a losing bet by design. The company collects more in premiums than it pays out, because it pools everyone’s risk and keeps a margin. So why is buying it clever rather than foolish?

Because you are not trading dollars for dollars. You are trading the value behind them. The premium takes a few dollars from the comfortable end of your money, where each is worth little. The disaster it guards against, a house fire, a wrecked car, a sudden death, would take dollars from the desperate end, where each is worth a fortune to you.

Losing money you cannot replace does not hurt in proportion to the number. Because of the curve, it hurts far more. So you hand over a small, painless, certain cost to escape a rare, unbearable one. That is not fear. It is arithmetic on a curve that bends.

Why a fair coin flip is still a bad deal

The same shape explains something people feel but rarely name: most of us dislike gambles even when the odds are honest.

Offer someone an even bet, double your savings or lose half on a coin flip, and most refuse. In dollars it looks fair. In value it is lopsided. The dollars you might lose are worth more to you than the dollars you might win. Losing pulls you down the steep part of the curve, while winning only inches you along the flat part. The gain adds a little; the loss subtracts a lot.

That reluctance has a stiff name, risk aversion, and it sounds like timidity. It is not. It is just what a bending curve does. A three-hundred-year-old puzzle made the point: a game with an infinite average payout that no sane person would pay much to enter. The resolution was that people weigh satisfaction, not raw cash, and satisfaction runs out faster than money piles up.

What the curve hides from every seat

Here is the part that asks for some humility. You can only feel the curve from where you are standing on it.

From a comfortable seat, another person’s caution over a small sum can look like fuss, and their willingness to gamble on a lottery ticket can look reckless. From a stretched seat, someone shrugging off a lost $1,000 can look obscene. Each of us reads the others through the value our own dollars carry, and that value is not shared. The same amount of money is quietly doing different amounts of work in every life around you.

There is a wider shape underneath, too. In a place where money is held very unevenly, the same dollar does wildly different amounts of good depending on whose hand it lands in. That is not a fact of nature; it is the arrangement we happen to live inside, and the curve is what makes it matter. You do not have to draw a conclusion from that. It is enough to notice that when money moves, its worth changes with the person, and that no single seat, including yours, can feel the whole of it.

03 · Lab · your turn

Where You Stand On The Curve

Rehearse the insurance decision from different points on the money curve, and feel why the same loss is worth more to some people than others.

04 · Hope · carry this

The same curve that makes a dollar worth little to the comfortable makes it worth a great deal to someone with less. That is the quiet reason a small kindness can do far more good than it ever costs the person giving it.