What rational reason would anyone have for expecting Jessica Alba to know which sunscreen is safest for baby? Why would we look to Gwyneth Paltrow to assess the medical benefits of bee venom, even if she did agree to be stung 80 times?

Blame it on Peter Bergman.

In 1986, the American actor, who played Dr. Cliff Warner on All My Children, did an ad for Vick’s cough syrup, uttering the 10 words that changed everything: I’m not a doctor but I play one on TV.  Despite the obvious leap in logic, the campaign stands as the most successful in the brand’s history.

Today, even as Bergman’s line is played for laughs and influencers threaten to overtake celebrities in the turf war for consumer trust, our belief in the oxymoron of ‘celebrity authority’ dies hard.

Why is that?

Neil Patrick Harris

A few years back, a research team at McMaster University analyzed celebrity influence on society and concluded that, yes, the human species is hard-wired to trust celebs.  Beyond wanting to emulate the people we admire, we seek mental consistency: to ignore or question our favourite celebrity’s advice would conflict with our adoration. And of course there’s the Bergman-esque Halo Effect: celebrities possess a cloak of credibility that extends well beyond their area of expertise.

But what about luminaries like Gwyneth who take pains to educate themselves on the health products and practices they endorse?

Gwenyth Paltrow

The best they (and we) can hope for is ‘chauffer knowledge’.

The term, coined by insightful author Rolf Dobelli, comes from a story about a Nobel-winning physicist who traveled across Germany giving a speech on quantum mechanics. His driver, who had heard the talk so many times he memorized it, offered to swap places with his boss. He’d give the speech. And the physicist could relax in the front row, dressed like a chauffer.  The plan went off without a hitch – until a physics professor stood up with a question. The driver recoiled: “Never would I have thought that someone from such an advanced city as Munich would ask such a simple question. My chauffer will answer it.”

How can you tell the difference between chauffer knowledge and the real thing? Dobelli thinks he knows: “True experts recognize the limits of what they know. If they find themselves outside their circle of competence, they keep quiet or simply say, I don’t know. From chauffeurs, we hear every line except this.”

Jenny McCarthy

The Co’s Katherine Gougeon explores social and cultural details that have an outsized ripple effect. Follow her on Twitter @kgougeon