Economists’ and engineers’ minds are very similar, as indicated by (among other things) the fact that many economist jokes are recycled engineer jokes, and vice versa. It’s no coincidence that my dad is an engineer. But one major difference between them is their notions of efficiency. Engineers’ notion of efficiency is typically driven by the physical or technological relationship between inputs and outputs. Economists’ notion of efficiency, on the other hand, is typically driven by subjective human valuations.
Here’s a nice illustration: What does it mean for the human body to be “a more efficient machine”? That’s a phrase you’ll sometimes hear about the alleged effect of dieting and exercise on the human body. To an engineer, a more efficient human body would presumably be one the generates the greatest amount of energy (or work) from a given amount of caloric intake. But I suspect that is not the kind of efficiency people want out of their bodies, at least in the modern age of cheap food. What most people (or Americans, at any rate) really want is a body that burns up lots of calories without really doing much with them. We want to maximize the amount of tasty food we can ingest without having to exert energy to lose the calories. And we don’t even want the calories to be stored; that just means fat. Nope, what we want is to waste calories.
The point is that our bodies could be inefficient from an engineering perspective and yet economically efficient. Economic efficiency is measured in terms of preference satisfaction, and a body that “wastes” lots of calories actually makes many of us happier.
ADDENDUM: Just to be clear, I realize that engineering efficiency could be “tweaked” to make it isomorphic to economic efficiency. It’s just a matter of redefining what the relevant inputs and outputs are. My point is about how notions of efficiency are typically deployed by the two professions. Specifically, economists usually place much greater emphasis on the subjective aspect of the problem.