A Homo gullibilis Interactive

The Four-Card Task

The science of irrationality

The Challenge

Four cards are laid out in front of you. Each card has a letter on one side and a number on the other.

You can see one face of each card. Your task: determine which card(s) you must flip to test whether the following rule is being violated.

"If a card has a D on one side, then it has a 3 on the other side."
The Rule to Test
The Cards

Click the cards you need to flip to test the rule.

D
Card 1
F
Card 2
3
Card 3
7
Card 4
0 cards selected
The Correct Answer
D and 7

Only these two cards can reveal a rule violation.

You Selected
—
Result
—

Why Most People Get This Wrong

Less than 10%
of people answer this correctly

Card by Card
D
Must flip. The rule says D must have a 3 on the back. If you find anything other than 3, the rule is broken.
Flip
F
Don't flip. The rule says nothing about F. It doesn't matter what's on the other side—F can have any number.
Skip
3
Don't flip. This is the trap. Finding a D on the back would confirm the rule, but finding F wouldn't violate it. The rule doesn't say "only D can have 3."
Skip
7
Must flip. If there's a D on the back, the rule is violated—a D would have a 7, not a 3. This is the card most people miss.
Flip
The Deeper Point

This task reveals a fundamental flaw in human reasoning: we instinctively seek confirmation rather than falsification.

Most people pick D (correct) and 3 (incorrect)—because they're looking for evidence that supports the rule. But logically, you can only test a rule by looking for violations.

The 7 card is the key insight: to truly test "If D, then 3," you must check whether any D exists without a 3. The 7 card is the only place where a hidden D would violate the rule.

This same logical blindspot shows up everywhere: in how we evaluate political claims, how we assess risks, and how we decide what evidence matters. We're wired to confirm, not to question.

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