The Most Common Cognitive Bias

A simple three-number puzzle exposes a common cognitive bias: people latch onto an early guess and then keep generating answers that confirm it, even when the goal is to discover the real rule. Given the sequence 2, 4, 8, the challenge is to infer the hidden rule by proposing other three-number sets and receiving only “yes” or “no.” Many participants quickly settle on an obvious pattern—most often “multiply by 2”—and then keep testing variations that preserve that assumption. The feedback system does not reward their creativity; it rewards their willingness to be wrong.

As the game continues, “yes” responses appear for many sequences that look like they fit the multiplication idea—3, 6, 12; 5, 10, 20; 100, 200, 400—yet those are not the actual rule. Even when the rule is not “multiply by 2,” the early hypothesis keeps pulling attention toward familiar-looking answers. The turning point comes when the puzzle master deliberately offers sequences that should feel “wrong” under common assumptions—like 2, 4, 7—and watches whether participants can abandon their first model. Eventually, the correct rule emerges: the only requirement is that the numbers are in increasing order. That means many seemingly unrelated sets—1, 2, 3; 7, 8, 9; 8, 16, 39; 1, 7, 13—receive “yes,” while a set like 10, 9, 8 receives “no.” The participants realize they were “on the right track” only in the sense that they were generating plausible answers, not in the sense that they were identifying the true constraint.

The discussion then broadens from the puzzle to how people reason in real life. The creator ties the behavior to Nassim Taleb’s “The Black Swan,” using the metaphor that unknown or unexpected cases can break the comfort of a tidy theory. In the old belief that all swans were white, each white swan reinforced confidence—until black swans appeared. The same pattern shows up in everyday thinking: once people form a rule early, they tend to search for supporting examples and ignore disconfirming ones.

The practical takeaway is methodological rather than philosophical. Instead of asking only questions that are likely to yield “yes,” the scientific method aims to disprove. Setting out to find “no” is more informative than collecting confirmations, because failure to disprove a claim is what gradually increases confidence. Applied broadly, the advice is to treat any belief as something to stress-test: try hard to break it. Only then can people avoid self-deception and move closer to what is actually true about reality.