Persuasion Tip: The Dilution Effect

More is more, right? Not in persuasion

When you pile argument after argument onto your case, you don’t strengthen it. You water it down. Your best point gets dragged toward the average of all your points. And suddenly your killer argument doesn’t feel so killer anymore.

This is the Dilution Effect. And it might be quietly undermining your pitches, presentations and proposals right now.

The science: your brain averages, it doesn’t add

The Dilution Effect was first identified by social psychologists Nisbett, Zukier and Lemley in 1981.

It was brought into the persuasion spotlight by Niro Sivanathan, professor of organizational behavior at London Business School, and confirmed again in a 2024 study by Obermaier and Koch, published in Scientific Reports.

The core finding: when people evaluate a set of arguments, they don’t add them up. They average them out. So a strong argument paired with a weak one doesn’t score 10 + 4. It scores somewhere around 7.

Your weakest argument isn’t just unhelpful. It’s actively pulling your strongest one down.

How to use it in practice

Before your next pitch or presentation, ask yourself: which of my arguments are genuinely strong? Which are just filler? You might even ask your favorite AI assistant.

Then cut the filler. Ruthlessly.

Not recommended:
“We should go with this supplier because they’re cheaper, faster, have a nicer website, are local, and our colleague once had a good experience with them.”

Recommended:
“We should go with this supplier because they deliver 30% faster and that directly solves our bottleneck.”

One strong argument. Clean. Memorable. Undiluted.

The magic number: it’s 3 again

Research suggests three arguments is close to optimal for most audiences. Enough to feel substantive. Not so many that the weak ones drag the strong ones down.

If you can’t find three genuinely strong arguments, one is better than five average ones.

Summary: quality beats quantity

In persuasion, more arguments are not more persuasive. Build your case around your strongest points only. Cut the rest. Your audience’s brain will average everything you give them, so give them only your best.

Less really is more. Just make sure what’s left is strong.