My first reaction to reading this letter was a question: Is the premise true? Are economists unlikely to enter electoral politics? I am not so sure. There are some successful politicians who started life as economists--Paul Douglas, Phil Gramm and Dick Armey, for example.
Dear Professor Mankiw,
I am a Freshman at the Indiana University Kelley School of Business. I'm currently taking an introductory microeconomics course and reading from your textbook. Above all this course has taught me that economists are extremely skilled at examining real world issues and probably even better at clearly communicating their findings. I'm curious to know why I don't see more professional economists entering the world of politics as electable candidates.
Moreover, economists are relatively rare in the overall population, so you would expect them to be rare in any subpopulation, such as politicians. Let's put some numbers to this. Only about 1000 econ PhDs are awarded each year, and only about half these are U.S. citizens (source). There are about 4 million Americans born each year. As a rough approximation, therefore, only 1 in 8000 Americans would likely qualify to be called a professional economist by training. There are 535 members of Congress. If economists had an average rate of entering politics, the probability that Congress at any moment would include an economist would be about 7 percent. I think that we economists do better than that: otherwise, the event of having two in Congress at the same time (Gramm and Armey) would be very unlikely.
My second thought is that if I were a young person planning a political career, I would not start by becoming a professional economist. An undergraduate degree in economics is ideal, as it teaches a lot about the fundamentals of public policy, but an aspiring politician would find it unnecessarily costly to spend the 5 or 6 additional years typically necessary for a PhD. A law degree is much easier. It is also more diversifying as an educational experience once one has studied econ in college for four years. The same is true of an MBA or a master's in public policy. The specialized technical and research skills taught in econ grad school are not needed, or even particularly helpful, for a successful political career.