On opportunity costs again
I keep on about threshold concepts almost as much as I do about Frank Coffield, and anyone familiar with the idea will be aware of the economists' favourite TC—opportunity cost. It is discussed very well by Robert Frank in his entertaining new book [Frank R H (2008) The Economic Naturalist; why economics explains almost everything London; Virgin Books]. I don't think he has ever heard of TCs, but he acknowledges the significance of the concept in very similar language (pp. 4-7).
Incidentally, the book is also based on an exercise he routinely uses in an introductory economics class; clearly an excellent teaching device!