When college students are tested for knowledge of basic economics six months after taking a traditional Economics 101 class, they fail to perform significantly better than students who have never taken an economics course, according to a survey in the American Economic Review (2002).

So why does Cornell professor of economics Robert Frank keep hearing from alumni who vividly recall his introductory economics course and still report using the economic principles he laid forth in his class?

The answer can be found in Frank's latest book, "The Economic Naturalist" (Basic Books), due out this spring. Frank's collection of intriguing questions are illustrated by numerous original drawings by New Yorker cartoonist Mick Stevens.

Half of the royalties from the book, the cover of which reveals little of the treasures inside, will go to Cornell's John S. Knight Institute for Writing in the Disciplines.

Frank supplements many of his own examples with those collected from the best of the best student essays from courses he has taught for two decades. Over those years he has doggedly posed what he calls "the economic naturalist question." The only thing that has changed is the word length of the assignments (down from 20 pages to 500 words, and if a student can provide a good example in those few paragraphs, all the better). Frank has found the assignment useful at every student level.

"It's proven to be a powerful learning tool," said Frank. "And personally I think providing a chance to learn how to raise an interesting question and answer it clearly in writing is one of the best things we can do for our students."

The economic naturalist assignment asks the student to use one or more economic principles discussed in the course to "pose and answer an interesting question about some pattern of events or behavior that you personally have observed."

For instance: Why do drive-up cash machines have Braille dots? (That was posed by former student Bill Tjoa.) Or Jennifer Dulski's query: Why do "brides spend so much money -- often thousands of dollars -- on wedding dresses they will never wear again, while grooms rent cheap tuxedos even though they will have many future occasions that call for one?" (See below.)

Culling through thousands of essays, Frank selected the finest examples from a range of questions the students posed. He then rewrote the essays and added data, keeping one consistent voice throughout. He credits the students whose questions -- and even some prose -- he used. At the end of the book, he includes two student essays "that were so charmingly well written I felt I had to use the originals," he said.

It was the Knight Institute that approached Frank in 1983 to teach a course in writing across the disciplines. He was given a teaching assistant, and classes generally drew about 40 students.

Frank describes his decision to donate half the royalties to the institute as an expression of gratitude "for his students' contribution and for the fact that the book would not exist" except for his participation in the Knight program.

"This is such a good book," he said, not boastfully, but with a seasoned author's confidence. "It's not because I improved upon the material, but because I had such fabulous ideas to work with in the first place."

Frank is the Henrietta Johnson Louis Professor of Management at the Johnson School. His previous books include "The Winner Take All Society" (1995) and "Luxury Fever" (1999). His column, "Economic Scene," appears in The New York Times every fourth Thursday.

Keypads and wedding gowns: It's all economics

The following student essay questions and answers were written for an assignment in professor of economics Robert Frank's class. The essays were written by former students Bill Tjoa, MBA '99, and Jennifer Dulski, AB '93, MBA '99, respectively, and address the cost-benefit principle, an essential economic concept.

Q: Why do keypad buttons on drive-up cash machines have Braille dots?

A: ATM producers have to make keypads with Braille dots for their walk-up machines anyway, so it is cheaper to make all machines the same way. The alternative would be to hold two separate inventories and make sure each machine went to the right destination. If the Braille dots caused trouble for sighted users, the extra expense might be justified. But they do not.

Q: Why do brides spend so much money on wedding dresses they will never wear again while grooms rent cheap tuxedos even though they will have future occasions that call for one?

A: Most brides want to make a fashion statement, so rental companies would have to carry perhaps 40 or 50 distinctive dresses in each size. Because each garment would be rented only infrequently, the company would have to charge more than the garment's purchase price just to cover its costs. But because grooms are willing to settle for a standard style, a rental inventory of only two or three tuxedos in each size is sufficient, enabling companies to charge only a fraction of the suit's purchase price.