When refuting manipulative arguments or debunking false beliefs, it is best to avoid using weakly favorable reasons.
As an illustration, consider two passages used in a 2011 study titled “When Good Evidence Goes Bad: The Weak Evidence Effect in Judgment and Decision-Making.” Participants read one of these passages:
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 the middle extra sentence in the second passage. Read in isolation, it is a favorable piece of evidence suggesting that there will be a more widespread adoption of hybrid cars. Yet participants who read the first passage predicted higher adoption than those who read the second.
So why did the first passage have stronger effect than the second one? The middle sentence in the second passage is positive evidence but it is quite weak on the whole.
Such supportive but weak evidence reduces overall persuasiveness. As the study authors note, “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 researchers tested this effect across different contexts and consistently found the same result. For example, participants were asked whether Republicans would retake the House in 2010. Some participants were shown weak but positive evidence, for example, that a Republican candidate in a close race received an endorsement. Again, the participants who were shown such positive but weak evidence were less likely to gamble with real money that Republicans will retake the House than those participants who were not shown such weak favorable evidence.
The simple takeaway is that if we want to more effectively counter misinformation or false beliefs, it’s better to use fewer but stronger arguments rather than adding weak supportive reasons which will undermine persuasiveness — “strike but a few blows, but strike them to the heart.”
References
Philip M. Fernbach, Adam Darlow, and Steven A. Sloman. “When Good Evidence Goes Bad: The Weak Evidence Effect in Judgment and Decision-Making.” Cognition, Volume 119, 459-467 (2011). https://doi.org/10.1016/j.cognition.2011.01.013