The Weak Evidence Effect

What is the Weak Evidence Effect?

Should you use more evidence or less? The weak evidence effect suggests that if you’ve got some strong arguments, it is best to leave out any weak evidence, even if it seems to be supportive evidence overall. Recent research by Brown University researchers shows that you’ll achieve stronger effect if you use only strong reasons.

Consider the two passages below. Researchers asked people to read one of the passages and predict how likely is the adoption of hybrid cars?

Widespread use of hybrid and electric cars could reduce worldwide carbon emissions.
How likely is it that at least one-fifth of the U.S. car fleet will be hybrid or electric in 2025?

Widespread use of hybrid and electric cars could reduce worldwide carbon emissions.
One bill that has passed the Senate provides a $250 tax credit for purchasing a hybrid or electric car.
How likely is it that at least one-fifth of the U.S. car fleet will be hybrid or electric in 2025?”

The only difference between the two passages is in the middle sentence of the second passage. When people read this sentence in isolation, they considered it positive evidence for higher adoption of hybrid cars. Yet, the people who read the first passage predicted the higher adoption of hybrid cars.

So why did the first passage sway more than the second one? The middle sentence in the second passage is the weak evidence: although in isolation it is positive evidence, it is weak in comparison to the other one.

The supportive, but weak evidence reduces the overall strength of your convincing reasons. As the study authors noted, “give people a weak reason and they’ll focus too much on it. Give people no evidence and they’ll supply their own probably more convincing reason to believe that the outcome is likely.” The authors call this phenomenon “the weak evidence effect”. The authors tested this effect in different contexts and it always worked.

In marketing, for example, this effect effect might explain why adding more features to a product might decrease its sales. It can also explain why, for example, trial lawyers fare worse when they supplement strong evidence with weak evidence.

In a competitive setting, as when your campaign is opposed by someone else, you should not only leave out your weak arguments, but also focus the audience’s attention on your opponent’s weak arguments.

So in persuasion at least, less is really more. And Justice Joseph Story was right:

Strike but a few blows, but strike them to the hear.

Source

The original study was published in the journal Cognition. You can find it here.