Conjunction Fallacy

Shrink Definition

The conjunction fallacy is when we judge a specific, detailed scenario as more likely than the broader one that contains it. A vivid story feels more probable than a plain category, even though adding conditions can only make something rarer. We're swayed by how representative the story feels, not by the math. Probability doesn't reward detail, it punishes it.

Plain language

We think a detailed story is more likely than the simple version it fits inside.

Shrink Insight

The more a scenario fits our mental picture, the more we overrate its odds.

Why it matters

It shows up in forecasting, clinical judgment, and risk, where a specific narrative can crowd out the base rate. Naming it helps you notice when a story feels right but the numbers say otherwise.

Common misunderstanding

People assume more detail means more accuracy. Detail can raise plausibility while lowering probability at the same time.

Shrink Perspective

A believable story and a likely one are different things.

Shrink Reflection

Where am I trusting a detailed story over the simpler odds?

Shrink Step

When a detailed prediction feels convincing, compare it to the simpler version it sits inside.

Shrink Minute

Take a worry with lots of specifics and ask if the broad version is actually more likely.

Shrink Takeaway

Every added detail narrows the odds, even when it widens the story.

Medical boundary

Evidence summary

Grounded in the classic work on the representativeness heuristic, with robust replications, though the effect size varies with how questions are framed.