Short answer: Goodhart's law says that when a measure becomes a target, it ceases to be a good measure. Economist Charles Goodhart made the point in 1975 about UK monetary policy. The wording everyone quotes is anthropologist Marilyn Strathern's, from 1997. Applied to your own life, it explains why optimising one number, whether weight, steps, income or hours, so often improves the number and nothing else.
I have written about this at work, in predictability is a lagging indicator: turn a team's say/do ratio into a target and you get smaller commitments and padded estimates. The personal version is more dangerous, because nobody audits it. You are the one setting the number, gaming it, and grading yourself.
What does Goodhart's law actually say?
Goodhart's original line is narrower than the slogan: any observed statistical regularity will tend to collapse once pressure is placed upon it for control purposes. A number that reliably tracks something while nobody is pushing on it stops tracking it once people are rewarded for moving it, because they find the cheapest way to move the number, and that is rarely the thing itself.
The same idea turns up under other names: Campbell's law in social science, "teaching to the test" in schools, and reward hacking in AI, where a system scores well on the objective you wrote down and badly on the one you meant.
What are personal examples of Goodhart's law?
| Number you target | Cheapest way to move it | What it was supposed to measure |
|---|---|---|
| Body weight | Water and muscle loss, crash dieting | Body composition and health |
| Daily steps | Pacing at night, wrist-shaking | Being an active person |
| Hours worked | Presence without output | Contribution |
| Income | A job that costs every other area | Security and options |
| Training volume | Junk miles, ignored recovery | Fitness |
| Commits shipped | Smaller, more frequent commits | Software delivered |
The last row is personal. This site charts my weekly commits on the homepage. As a diagnostic it is honest. As a target, the easiest way to double it would be to split every commit in two, and the chart would look better while I shipped exactly the same work.
Why does optimising one area of life backfire?
Goodhart's law has a second effect in personal life that matters more than gaming. The area with the number gets the attention, and the areas without a number get what is left. Career and fitness generate numbers easily. Friendships, a marriage and meaning do not. Optimise the measurable areas and you will drift away from the others, not because you decided to, but because nothing told you they were slipping.
Add the hedonic treadmill, where each gain in one area feels smaller than the last, and you get the familiar pattern: one number keeps rising, it pays less every year, and the areas you did not measure fail without warning.
How do you track your life without gaming it?
- Measure several areas at once. A Wheel of Life forces a number onto the areas that never produce one, so they compete for attention on equal terms.
- Pair every number with a check it cannot fool. Weight with a waist measurement or strength. Hours with output. Income with how many evenings you had free.
- Write the reason next to the number. "7, because..." is much harder to game than a bare 7.
- Score the shape, not the peak. A headline score that rewards your lowest areas cannot be pushed up by maxing your favourite one.
- Rotate the metric when it stops being honest. If you notice you are optimising the number for its own sake, it has already stopped working.
That last principle shapes how Counterweight will score my own areas. A single headline number that a 10 in work could carry would be the purest Goodhart trap I could build for myself.
Sources
- Charles Goodhart, "Problems of Monetary Management: The U.K. Experience", 1975.
- Marilyn Strathern, "'Improving ratings': audit in the British University system", European Review, 1997.
- Donald T. Campbell, "Assessing the impact of planned social change", 1979.
Frequently Asked Questions
What is Goodhart's law in simple terms?
When you start using a number as a target, people (including you) find ways to move the number that do not move the thing it was measuring. The number then stops telling you what you wanted to know.
Who said 'when a measure becomes a target, it ceases to be a good measure'?
That wording is anthropologist Marilyn Strathern's, from 1997, summarising an idea economist Charles Goodhart set out in a 1975 paper on UK monetary policy. Goodhart's own version was more technical: a statistical regularity tends to collapse once pressure is put on it for control purposes.
What are examples of Goodhart's law in everyday life?
Dieting to a scale number by losing water and muscle. Hitting a step target by pacing around the kitchen at 11pm. Working longer hours to feel productive while output stays flat. Chasing a salary number by taking a job that makes the rest of life worse.
How do you avoid Goodhart's law with personal goals?
Track several areas instead of one, pair each number with a second check that cannot be gamed the same way, review the reason behind each number rather than just the number, and change the metric when you notice yourself optimising it for its own sake.
What is the difference between Goodhart's law and Campbell's law?
They describe the same effect from different fields. Campbell's law, from social scientist Donald Campbell in the 1970s, says that the more a quantitative indicator is used for social decision-making, the more it gets corrupted and distorts the process it was meant to monitor.
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