Fallacy of Composition

“A brick isn’t heavy. A house is made of bricks. Therefore, a house isn’t heavy.”

“Each ingredient is safe on its own, so the mixed supplement is safe.”

You commit the composition fallacy when you assume that what’s true of each part must be true of the whole. Properties don’t always scale; when pieces combine, totals can cross limits (for example, from light to heavy) or interact with each other.

Here are more examples:

“Every player on this team is a star, so it should be unbeatable.” In reality, too many ball-dominant players can hurt teamwork—having players whose skills and playing styles complement each other is important.

“Each investment is ‘low risk,’ so the portfolio is low risk.” If they could all drop in the same kind of downturn, the total risk is high.

To avoid this fallacy, consider:

• Thresholds: When parts accumulate, totals matter. Dose, weight, cost, and noise add up, and crossing a limit (e.g., daily mg, load rating) can change something from safe to unsafe.
• Interactions: When parts affect each other, addition isn’t enough. Drug combinations can create new effects. When success depends on roles (like a team), how parts fit is just as important as individual strength.

The converse is also a fallacy. It's called the fallacy of division. You commit this fallacy when you assume what’s true of the whole must be true of each part. For example: “This cake is delicious, so every ingredient must be delicious,” or “The company is thriving, so every department must be doing great.”

Next fallacy (Nirvana Fallacy)

Back to the Logical Fallacy Handbook

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