The Dilution Effect

When More Information Weakens Your Argument

Perceived Persuasiveness 85
0 — Weak100 — Compelling
Strong argument — every point is diagnostic.

Evidence Presented

What is the Dilution Effect?

Research by Nisbett, Zukier, and Lemley (1981) showed that when people evaluate evidence, adding non-diagnostic (irrelevant or weak) information alongside strong evidence reduces the overall perceived strength of the case. Evaluators unconsciously average all the evidence rather than summing it.

Lesson for communicators: Including every piece of information you can find does not make your argument stronger. Curate ruthlessly — keep only the points that directly support your claim.