Hanoi Paid for Rat Tails, and What That Teaches an Alarm App

Hanoi’s rat catchers were paid by the tail. The scheme, run by the French colonial government in 1902, made the tail the receipt: bring one in, get paid, and the city counts one rat gone. Before long officials were finding rats with no tails, because catchers had worked out that a live rat with its tail cut off was worth more than a dead one. It could go back to the sewer and breed.

An alarm app faces the same question as the city did: what, exactly, does it reward or punish? Any reward or penalty attaches to a proxy, and people deliver the proxy by the cheapest route available. This teardown goes through five common wake-up checks, what each proves, and where DontSnooze stands.

What did Hanoi pay for?

The historian Michael Vann reconstructed the episode from colonial records in “Of Rats, Rice, and Race: The Great Hanoi Rat Massacre, an Episode in French Colonial History” (French Colonial History, 2003). The authorities first hired Vietnamese rat-catching crews, then opened a bounty to the public of about one cent per rat, accepting the tail as proof because a tail was easier to collect and count than a carcass.

Vann’s records also show people responding the way the incentive invited. Some catchers snipped tails and released the animals. Some residents, by the reports Vann found, raised rats for the payment. Every tail was real — each payout followed the rules. The goal, fewer rats, moved the other way.

Did the cobra story happen?

You may have heard the other version: a British governor in Delhi pays a bounty on cobras, people breed cobras, the governor cancels the bounty, the breeders release their stock, and Delhi ends up with more cobras than before. Horst Siebert named it the “cobra effect” in a 2001 book. Vann has called the Delhi story probably apocryphal, since nobody has produced documents for it. It may be true. It may also be a tale that sounded right enough to survive without a source. Hanoi has documents behind it, so it is the better case to build an argument on.

What does each wake-up check prove?

Every alarm that asks you to do something is paying for a proxy. The useful comparison is how cheap each proxy is to satisfy while half asleep.

Check	What it proves	Cheapest way to satisfy it
Tap to dismiss	A finger moved	Never open your eyes
Solve a puzzle or shake the phone	Motor activity for 20 seconds	Do it lying down, then sleep
Scan a code in another room	You walked to the room	Walk, scan, return to bed
Record a short video after the alarm	Your face was on camera at that minute	Record it from the pillow, then lie back down
Photo sent to friends if you fail	Nothing, until you fail	Not applicable; it is a penalty

The pattern is the Hanoi one. The check is easy to count, so it gets counted, and the thing it stands in for (being awake and staying that way) goes unmeasured. A longer hurdle raises the price of the proxy, but a person determined to sleep is unusually good at working out the cheapest route.

Hanoi and wake-up checks: three follow-ups

What is the general rule behind the Hanoi story?

It is usually stated as Goodhart’s law. The economist Charles Goodhart made the original point about monetary policy in 1975, and the anthropologist Marilyn Strathern gave it the wording most people quote in 1997: when a measure becomes a target, it ceases to be a good measure.

Is there an older statement of the same problem?

Donald Campbell put a version in a 1976 paper, now called Campbell’s law: the more a quantitative social indicator is used for social decision-making, the more subject it is to corruption pressures and the more apt it is to distort the processes it was meant to monitor.

What would a harder-to-game wake-up check look like?

One that measures the goal at two points instead of one, such as a check at the alarm and a second at a fixed later time, or a first task that can only be done standing. No check is unbeatable, but each extra point raises the cost of the shortcut.

Where does DontSnooze stand?

DontSnooze sits in the bottom two rows. Its check is a video your friends see, and the penalty for skipping it is something they see too. Two things differ from the tap-and-puzzle family. A person who knows you looks at the proxy, so a bleary face and a bedroom wall have an audience. And the consequence for failing is a penalty, which means there is no payout to farm the way Hanoi’s catchers farmed tails.

The limit is the one the table shows. The video proves a moment, and DontSnooze does not check what happens after it is sent. Someone who records the video and goes back to bed has satisfied the check. A friend watching makes that far less appealing than a code scan does, but on a bad morning it can still happen.

So the recommendation comes with a condition. Use a check that has a witness, because that is the cheapest way to raise the price of the shortcut, and then give the minutes after the video a job. Fill a glass of water. Open the curtains. Put the first task somewhere you can only reach on your feet. Choose it the night before, because the morning version of you is the one the proxy was built to fool. I quote no tail or rat counts here; Vann’s paper is the place for those. Nor do I know of published figures on how many people go back to bed after a video check, so that claim rests on how the check works, not on a measurement.

If you are curious about the cases where someone does cheat, five tricks people try on proof photos covers them. Hanoi is about the cases where nobody forges anything and the number still goes wrong. For why a penalty and a payout behave so differently, what airlines learned about charging passengers makes a useful comparison.

The wider pattern of metrics bending the thing they measure has its own entry in Goodhart’s law and the cobra effect.