Some of the more high-profile and ubiquitous academics include the president of Stanford, John L. Hennessy, who sits on three boards, including that of Google. (Google also has the president of Princeton, Shirley M. Tilghman, on its board.) The chancellor of the University of California, San Diego, Marye Anne Fox, is on three boards.

Image Shirley Ann Jackson, a physicist and president of Rensselaer Polytechnic Institute. Credit... James Estrin/The New York Times

And then there is Dr. Jackson of Rensselaer, the highest-paid private college president and one of the most prominent black university leaders. In addition to FedEx, she sits on the boards of four other companies, including I.B.M. and Marathon Oil, and she recently stepped down from the board of NYSE Euronext.

THE attractions are clear for the president: lucrative extra pay and useful networking, among other reasons. For a dozen hours or so each month for each board served, in addition to preparation time, and their wise advice, they can receive hundreds of thousands of dollars a year.

Ruth J. Simmons, the president of Brown University and the first African-American woman to lead an Ivy League university, sat on the Goldman Sachs board until she stepped down this year. In 2009, she earned $323,539 from her Goldman directorship, including stock grants and options, as calculated by Goldman, and left the board with stock worth at the time around $4.3 million. This is in addition to her salary from Brown, $576,000 this year.

Dr. Jackson earned $1.38 million from her directorships, comprising both cash and stock. That’s in addition to $1.6 million from her day job, including bonuses and other benefits.

Beyond personal financial benefits, the interchange of ideas between campus and corporation can allow for a fruitful cross-fertilization. For example, Dr. Hennessy sits on the boards of Cisco Systems and Atheros Communications as well as Google. As an electrical engineer and a pioneer in computer architecture, he is well placed to bring industry expertise to the boards he serves.

William G. Bowen was president of Princeton University for 16 years and served on two boards, including Merck’s. It was an experience, he says, that was invaluable in helping him build up Princeton’s then-fledgling life sciences activities.