Which economic ideas are hard to popularize?

Ryan, a loyal MR reader, asks:

1. What are the most important economic ideas that are not popularized, i.e. not accessible to laypeople in books and articles by credible authors? …Are there any theories that have gained traction over competing theories based primarily on their ability to be more easily conveyed to a layperson audience as opposed to their providing a better solution to a particular problem?

As for non-popularized theories, I have a few nominations. First, the sensitivity of many economic results to assumptions about Bertrand, Cournot-Nash, and other solution concepts is not easily popularized.  Second (until the Cowen-Tabarrok macro text), the Solow growth model was not easily popularized.  The difference between a "once-and-for-all" change and a "change in the rate of growth" is not well understood, probably not at any level, yet it is important.  Tax incidence theory is not easily popularized, although an incorrect version of it — "they'll pass it all along to consumers" — circulates.

Most behavioral economics can be easily explained in popular terms and that partially accounts for its broad influence.  Most people are also capable of grasping a crude version of Keynesian economics, albeit without the subtleties of Keynes ("we should spend more" resonates).  The insights of supply-side economics and monetarism have been popularized without much difficulty.

Most of all, it is hard to popularize "maybe" claims, agnoticism, uncertainties, confidence intervals, and contingencies.  The marketing process encourages excess certainty.

In terms of good but hard to popularize economic theories, what else can you think of?