I’m pleased to say that while this trap is seductive, it’s usually easy to guard against if you watch out for it.
What is this trap? It’s simply that just because someone is an expert in one field, it doesn’t follow that they’re an expert in another (even related) field. For example, I’m an astrophysicist by training. I study Saturn’s rings. While I can’t claim my pronouncements on the topic are authoritative, I’d hope you might listen carefully to my thoughts. But as you get farther away from that topic, my expertise wanes. If I explained the geology of Venus, you might want to take it with a grain of salt. If I gave you medical advice, you’d be well-advised to take it with a heaping spoonful of salt. Now, there are other areas that I have at least minor expertise in, but you can’t know what they are from just my professional degree. For example, I think I do know more than average about migraines, but that has nothing to do with my degree so I’d entirely support you asking me about that before taking what tell you as valid.
Does this really happen? Yes! I just finished reading Merchants of Doubt (which I recommend to everyone, everywhere). One common theme in the book was scientists talking about issues from the medical dangers of cigarette smoke to the environmental dangers of ozone depletion and global warming. What’s interesting about these scientists is that nearly all of them were (and are) speaking well outside their expertise area. Usually, it was physicists (solid state of nuclear) talking about medical effects of smoke or the effects of CO2 on climate. While it’s possible that they did, in fact, do the work needed to become experts in those areas, it’s by no means assured. (I know of no reason to think that they did and, in fact, their pronouncements on the issues suggest that they didn’t. Or that they were willfully misleading, which is even more depressing.) And yet they were (and still are) taken very seriously by congress, the White House, and by the press. People see “scientist” and assume that our fields are interchangeable. They’re not.
Similarly, an engineer is not qualified to diagnose your sore arm. Even a dentist, a medical professional, is probably well outside of her qualifications. (In fact, a lot of MDs are probably unqualified to give a solid diagnosis. A skin specialist, for example, is probably out of his depth with joint problems and you’d be better off talking to either your GP/family medicine/internalist or a specialist in joints.)
So what’s the solution? Check the qualifications of the speaker and think carefully about whether or not they match up with the claims being made. The internet has made this a lot easier, although generally if someone is touting their qualifications, they tell you what you need to know when they do. And if you’re not sure what an impressive-sounding qualification means, check. Not everyone practicing a medical field is well-verses in biochemistry, for example, so not everyone with a degree in a medical field should necessarily be trust if they claim that they have a new theory about some aspect of biochemistry. (They might be right, of course, but don’t rely on it! Check!)