When Uncertainty Increases Persuasiveness

Com­mon wis­dom would sug­gest that the more cer­tain a per­son is on a sub­ject, the more per­suas­ive and cred­ible we per­ceive them to be. How­ever a study look­ing look­ing at how cer­tainty affects per­suas­ive­ness and per­ceived cred­ib­il­ity found that the oppos­ite is true:

Experts are more per­suas­ive when they seem tent­at­ive about their con­clu­sions […] but the oppos­ite is true of novices, who grow more per­suas­ive with increas­ing cer­tainty.

This res­ult held across the three exper­i­ments described in the paper (pdf, doi), but it’s worth not­ing that this only applies in situ­ations where there is no object­ive truth – such as in con­sumer situ­ations (the exper­i­ments used res­taur­ant reviews, and I ima­gine product reviews would give sim­il­ar res­ults):

Earli­er research […] had made the case that express­ing cer­tainty gen­er­ally increases people’s per­suas­ive power, because it boosts their per­ceived cred­ib­il­ity. [How­ever] those stud­ies con­cerned top­ics such as wit­nesses testi­fy­ing in court or stock mar­ket advisers giv­ing stock recom­mend­a­tions where there is an object­ive truth or cor­rect answer. In those instances […] people might rely on a person’s cer­tainty as an indic­at­or of his or her cred­ib­il­ity. “In more sub­ject­ive domains like con­sumer con­texts, though, […] express­ing cer­tainty appears to have a more dynam­ic effect, giv­ing a mes­sage more or less impact depend­ing on who is express­ing it.”

via Mar­gin­al Revolu­tion / NYT