Expert at: person who often makes and implements decisions, with good (measurable) outcomes far exceeding an occasional bad one.

Expert on: person who knows all (or most) the published/reported data on a particular topic.

These categories are not mutually exclusive. If you’re seeking medical help, you’ll aim at the Expert at.

“A prosperous and successful plumber is an expert at plumbing. Someone who is a good source for accurate information on plumbing is an expert on plumbing.”

They are not mutually exclusive.

“More generally, an expert at a topic is someone who has gained the most attention, praise, income, and so on via their association with the topic. But this may not be the best expert on that topic.”

Of course not.

“He may have succeeded by not giving the most accurate information, but by telling people what they want or expect to hear, or by entertaining them.”

But an expert on a topic might succeed for the same reasons? Because they are funny or good looking or…

“We often rely on the heuristic of looking to an expert at a topic, when what we want is an expert on a topic.”

As noted above, they are not mutually exclusive and in instances where they diverge, we might in fact want *both*.

“In fact, most of the people we see being labeled as “experts” are primarily experts at topics. For example, TV talking heads discussing topic X are usually people who have made a successful career in X. We may see a general talk about war, or a CEO talk about business.”

Really?! I’ve noticed just the opposite. Frequently the people labeled experts on TV are experts on the topic, not experts at the topic.

“But it a rare field where the best way to succeed is to always be completely honest with everyone about everything.”

Hard to refute that statement considering how absolute it is. I don’t believe I know of a living human that is completely honest with everyone about everything. In the context of this discussion I’d be concerned not only with honesty on the part of an expert, but equal with cognitive bias.

“We could greatly benefit from better ways to determine who is really an expert on a topic. Prediction markets are one possibility.”

Finally something we can agree on.

Expert at religion: A theologian.
Expert on religion: An atheist.

Expert on animal behavior: An ethologist.
Expert at animal behavior: A cat.

For example in the fields Innovation and Entrepreneurship, experts on predominate. And this limits effective policy making.

We need a balance of advice from both experts at and experts on – both have their merits.

- Expert on: Knows all about it.
- Expert at: Can do it really well.
- Expert in: Has a lot of success while doing it.

Let’s take another subject I know even more about. I’m an owner of a print shop. I know a lot about the capabilities of presses, and about the chemistry and mechanics involved in making them work. I can often make correct decisions about the effect of an unusual adjustment that skilled operators cannot. In some ways I’m more of an expert *on* presses than they are. But if I tried to actually run the press tomorrow, I’d be lucky to produce 20% of the work that my best operators do. They not only have some physical skills that are superior to mine, they have practice at doing the work every single day, and have developed a rhythm. Lying or marketing is not a significant part of their success (except to the extent that they might get me to believe that some PITA job is actually impossible), but there’s still a pretty big gulf between their expertise at, and my expertise on.

Coaching sports would be another good example. Plenty of the best coaches never played their sport at the highest levels, because they simply did not have the physical gifts required.

In any case I don’t think this is purely a distinction of successful vs. skilled, although that may be one dimension. I think it’s also about two different skills, one which involves immediate performance and is easier to measure, and one which involves pure knowledge and is harder to measure.