I ordered a turkey sandwich and asked for double meat. The guy behind me said, "You shouldn't eat meat." I turned and shrugged.

"Seriously," he said, his voice getting louder. "Meat is bad for you."

"Maybe so," I said. "But I like meat."

Evidently that was not the right response. "A friend turned me on to a vegan diet," he said. "Only fools eat meat. Meat is terrible for you. There's not a single reason to eat meat. The science is irrefutable." Then he paused and moved closer, narrowing his eyes to stare intently into mine.

"It's changed my life," he said.

"I'm not sure all meat is bad," I said. "But that's really cool how being a vegan has worked out for you. How long have you been doing it?"

"This is my second day," he said.

Ah.

The Dunning-Kruger Effect

His certainty provides a perfect example of the Dunning-Kruger effect, a type of cognitive bias described by social psychologists David Dunning and Justin Kruger in which people believe they're smarter and more skilled than they actually are. Combine a lack of self-awareness with low cognitive ability and boom: You overestimate your own intelligence and competence.

As Dunning, a psychology professor at the University of Michigan, says, "if you're incompetent, you can't know you're incompetent. The skills you need to produce the right answer are the very same skills you need to recognize the right answer."

As Bertrand Russell said, "one of the painful things about our time is that those who feel certainty are stupid, and those with any imagination and understanding are filled with doubt and indecision."

Or as my grandfather said, "the dumber you are, the more you think you know."

(On the flip side, people with high ability tend to underestimate how good they are. High-ability individuals tend to underrate their relative competence, and at the same time assume that tasks that are easy for them are just as easy for other people.)

But I shouldn't be too hard on the gentleman who had just adopted a vegan diet. I once spent 20 minutes trying to convince a motorcycle mechanic my bike handled poorly because of issues like spring rate and steering head angle and frame height, only to learn I had unknowingly turned my rear shock's rebound damping to its lowest setting.

Wildly overestimating my knowledge made me a D-K.

We all know people who do the same. They take a position and then proclaim and bluster and pontificate while totally disregarding differing opinions or points of view. They know they're right -- and they want you to know they're right.

Their behavior isn't an indication of intelligence, though. It's the classic sign of a D-K.

Wisdom Is Never Found in Certainty

As Jeff Bezos says, "the smartest people are constantly revising their understanding, reconsidering a problem they thought they'd already solved. They're open to new points of view, new information, new ideas, contradictions, and challenges to their own way of thinking."

That's because wisdom isn't found in certainty. Wisdom is knowing that while you might know a lot, there's also a lot you don't know. Wisdom is trying to find out what is right rather than trying to be right. Wisdom is realizing when you're wrong, and backing down graciously.

Don't be afraid to be wrong. Don't be afraid to admit you don't have all the answers. Don't be afraid to say "I think" instead of "I know."

As my Inc. colleague Jessica Stillman says, "next time you're trying to determine if someone is actually super smart or simply bluffing, don't ask whether they're always right. Instead, ask when was the last time they changed their opinion. If they can't name lots of times they were wrong, they're probably not as smart as they want to appear."