Diminishing
Marginal Utility
The first slice of pizza is wonderful. The fifth? You could probably leave it.
The slices have not changed. Your appetite has. As you eat more in one sitting, each extra slice usually adds less enjoyment than the one before.
Have another slice.
One person. One sitting. Six equal slices.
Watch the extra enjoyment shrink as the total grows.
Added by slice 1+10 points
Total enjoyment so far10 points
Pale bars are slices still to come. The horizontal line marks zero. These invented points make the pattern visible; they are not measurements of happiness.
Slice 1: you were hungry. A big gain of 10 points starts your total at 10.
A smaller addition is still an addition.
After two slices, your total is 17 points. The third adds only 4, but your total still rises to 21. Getting less extra enjoyment does not mean getting less enjoyment overall.
At slice five, the addition is zero. At slice six, feeling overfull makes the addition negative, so the total finally falls.
Utility is an economist’s word for the satisfaction you get from something.
Marginal means the extra amount from one more unit.
Put them together: diminishing marginal utility means each additional unit brings less extra satisfaction, with other relevant conditions held constant.
Same item. A different need left to fill.
The first unit meets your most pressing need. Later units meet needs that are less urgent, or arrive when you have already had enough. Look for that pattern beyond the dinner table.
Clothes
Your first warm coat fills a real gap. A fourth similar coat may spend most of its life in the wardrobe. Ask what the extra one actually adds.
Entertainment
The first episode is a welcome break. Another late-night episode may be less enjoyable as you grow tired. Reassess the next episode on its own.
Bulk offers
A larger pack can cost less per item while giving you more than you want. Compare the extra cost with the value of the extra items, including anything you will save or share.
The limits, and a little math
The compact version: marginal utility = total utility after the next unit − total utility before it. In our example, the third slice adds 21 − 17 = 4 points. A falling positive addition makes total utility grow more slowly; only a negative addition makes it fall.
Not a universal timetable. Your appetite can return tomorrow. A second shoe can be more useful than the first because it completes a pair. Learning to enjoy a new hobby can make later sessions more rewarding. Define the unit and the situation before assuming the pattern applies.
Not a happiness meter. Economists can use utility numbers to represent preferences. The numbers here are a teaching device, not a way to compare one person’s happiness with another’s.
Not diminishing returns in production. An extra cook adding fewer meals in a crowded kitchen concerns output. An extra slice adding less satisfaction concerns consumption.
Value the next slice,
not the memory of the first.
Before choosing more, ask: “Given what I already have, how much will this extra one add?”