How do you catch a liar when there's no evidence?
A king, medieval priests, and a rock band each faced a claim they couldn't check. None of them asked a better question. Each designed a test that would cost the liar and the truth-teller differently, then watched who flinched.
Ancient Israel · 1 Kings 3
King Solomon and the two mothers
Two women who share a house each gave birth days apart. One baby died in the night. Both now claim the living child, and no one else saw either birth.
The test
Solomon calls for a sword and orders the child cut in two, half for each woman.
Solomon gives the child to the woman who tried to give him up.
Medieval Europe · 9th–13th centuries
Trial by ordeal
An accused person with no witnesses could be sent to "God's judgment": carry a red-hot iron or plunge a hand into boiling water. If the wound healed cleanly, God had declared them innocent.
The test
Let the accused choose: undergo the ordeal, or confess and settle.
In 308 cases recorded at Várad, Hungary (1208–1235), 62.5% of those who went through the hot-iron ordeal passed. Priests knew who had chosen it, and could quietly make sure the iron wasn't that hot.
U.S. concert tours · 1980s
Van Halen's brown M&Ms
Van Halen's stage show was heavy, high-voltage, and full of pyrotechnics. Every venue promised it had read and followed the band's long technical contract. There was no way to check every line.
The test
Bury one trivial line in the contract: M&Ms backstage, "absolutely no brown ones."
Finding a brown M&M meant the crew rechecked the whole setup. At one Colorado arena that skipped the specs, the stage sank into the floor, causing about $80,000 in damage.
A question can be answered with a lie. A well-designed test can't be faked for free. Each test works because the truth-teller and the liar would react differently to the same situation: a real mother can't bear to see her child killed, an innocent person who believes in divine judgment has nothing to fear from it, and a venue that actually read the contract has nothing to hide.
The same idea runs through modern life: a warranty signals a maker who trusts its product, a security deposit filters out tenants who plan to skip out, and a spot check on one small detail tells you whether someone paid attention to the big ones.
What small, checkable detail would show whether someone got the big thing right?
What makes a test like this work
It has to cost the liar more than the truth-teller. Asking both women to describe the baby wouldn't work, because they had shared a house for days. Asking both to pay for the baby wouldn't work either; it would only reveal who had more money. The sword worked because the price was different for each of them.
People have to believe in it. The ordeal only sorted the guilty from the innocent because nearly everyone believed God would reveal the truth. After the Fourth Lateran Council barred priests from taking part in 1215, the practice faded, and so did the trick.
And the reason for the test can look absurd from outside. For years the brown M&M rule was retold as proof of rock-star excess. It was actually a safety check.
Explore more ways incentives reveal the truth
See Split or Steal for a game where a player's real incentives show up in a single choice, and The Monty Hall Problem for how the host's choice carries information.
Related exhibits
Sources: 1 Kings 3:16–28 (the judgment of Solomon); Peter T. Leeson, "Ordeals," Journal of Law and Economics 55, no. 3 (2012), for the Várad register (308 cases, 1208–1235: 100 ended before the ordeal, 130 passed, 78 failed) and how priests could influence outcomes; Snopes, "Brown Out," and Wikipedia, "Van Halen test," for the 1982 tour rider ("M&M's (WARNING: ABSOLUTELY NO BROWN ONES)") and David Lee Roth's account of the Pueblo, Colorado, arena damage.