Cass R Sunstein, University of Chicago Law School professor and by some accounts the most prominent legal scholar in the United States, announced this week that he has accepted a senior professorship at the Harvard Law School (HLS).

Sunstein will assume his new position as director of HLS’s Program on Risk Regulation in September.

Sunstein said that he intends to maintain connections to the U of C, and plans to split his time between Chicago and the Boston area.

“I love the University of Chicago, and I’m not completely severing ties with the University. At the minimum, I’ll be teaching a mini-course next quarter,” he said. “I’ll be keeping my apartment in Chicago, and I plan on spending significant time here.”

University of Texas law professor Brian Leiter, who will join the U of C law school faculty later this year, wrote on his popular legal blog earlier this week that Sunstein will teach part-time at the U of C as the Harry Kalven Visiting Professor of Law.

For Sunstein, the departure comes after lengthy negotiations with Harvard and the U of C. He said that the decision was partially fueled by a desire to explore other academic venues after spending nearly three decades at the U of C.

“The [Chicago] Law School is fabulous, but I think it’s reasonable not to spend your whole career at one institution,” he said. “I’ve been here 27 years. The Law School is fabulous here. But I grew up in the Boston area, and I have family there. The Boston area feels like home for me.”

At Harvard, Sunstein will research the legal applications to and ramifications of climate change, natural disasters, and terrorism. “I think the chance to direct that program is really exciting.”

Sunstein is the author or co-author of 15 books on a range of legal topics, including risk regulation, behavioral economics, the nature of rights, and environmental and constitutional law. In his 2006 book, Infotopia: How Many Minds Produce Knowledge, Sunstein points to several positive effects of pooling information from a range of media, Internet, government, and private sources.

While Sunstein said that he considers both the U of C Law School and HSL top caliber institutions, he pointed to the differences between the faculty body and administration of the two institutions.

“Both schools are very different. The University of Chicago Law School is small. Everybody knows everybody else. The faculty is of uniform quality and everyone’s a top scholar. But it’s a small school. Harvard is at least triple its size. I think it’ll be fun to be at a place teeming with people,” he said.

Saul Levmore, dean of the University of Chicago Law School, said he does not expect that Sunstein’s departure for Harvard will significantly affect the quality of a U of C Law School education or the school’s status as a legal powerhouse.

“I think [HSL] is a fine law school and we’re glad Sunstein’s happy,” he said. “Harvard is a great law school. It was a great school before Sunstein. And we’re a wonderful school also—with or without Sunstein.”

Nevertheless, Levmore said that Sunstein’s departure was a difficult one for the U of C Law School administration and faculty, especially since he believed that Sunstein’s decision rested neither on academic nor compensatory rationale.

“Cass Sunstein is a close friend of mine and a wonderful person,” Levmore said. “But I know—I don’t think, I know—that there are a couple of personal reasons for this decision.”

Sunstein had maintained a high-profile romantic relationship with U of C professor Martha Nussbaum, who holds joint positions in the Law School, the philosophy department, and the Divinity School. Nussbaum was similarly courted by Harvard, and the two had previously served together as visiting professors there, but last week she decided to stay at the U of C.

Both declined to comment publicly on whether Sunstein’s move to Harvard was influenced by the relationship, or respond directly to Levmore’s comments.

“Everything about the offer from Harvard was splendid, but all the terms were matched or exceeded by the University of Chicago,” Nussbaum said. “I’m really happy and thrilled by the culture of our Law School. There are no big stars—everyone’s exchanging ideas together.”

For his part, Sunstein emphasized that his decision was not goaded by ideological or administrative rifts within the school.

“No rifts at all,” he said. “Chicago is an extremely amicable, rift-free law school.”

Both Nussbaum and Levmore expressed frustration with the way the departure has played so far in the media, with many outlets characterizing the move as a substantial loss for the Law School.

“I think the publicity that I’ve seen has been rather unbalanced because it suggests that Harvard has scored a great victory, when the real story is that Harvard has made offers to two people, and they got one and Chicago got one,” Nussbaum said. “I think it’s misleading to portray [this] as a great coup for Harvard.”

Levmore said he was disconcerted by the way the Chicago Tribune portrayed Sunstein’s departure in a Wednesday article, for which the U of C declined to comment.

“I’m sort of embarrassed that [the story] said that the University of Chicago couldn’t be reached for comment,” Levmore said. “It looks like we didn’t want to talk, but the truth is that this decision was based on personal reasons and I respect that privacy. The media will find out about them soon enough.”

Still, administrators at Harvard characterized the hire as a major addition to their faculty.

“Cass Sunstein is the preeminent legal scholar of our time—the most wide-ranging, the most prolific, the most cited, and the most influential,” said HLS dean Elena Kagan, in a statement released this week.

Kagan added that the breadth and applicability of Sunstein’s research and publications are “singular and breathtaking.”

“He has a gift for framing and discussing issues in ways that invariably gain traction and make progress. And perhaps best of all, this individual superstar is also the consummate team player—a person whose passion for reasoned intellectual inquiry is contagious and who raises the level of everyone around him. If I could add only one person to the faculty, Cass would be that person, and I am thrilled beyond measure to announce his appointment,” she said.

Sunstein has also written prolifically for several acclaimed legal and political weblogs, including the libertarian “The Volokh Conspiracy” and the personal weblogs of law professors Jack Balkin of Yale and Lawrence Lessig of Stanford. He is a frequent contributer to the bi-monthly liberal political magazine The New Republic.

In a study tabulating total scholarly citations between the years 2000 and 2007, Leiter ranked Sunstein at the top of the legal field. The University of Chicago Law School ranked second in the study for “mean per capita scholarly impact”—the average number of citations for tenured law faculty. The U of C’s mean of 750 was bested only by Yale’s 790. This week Leiter updated the citation rankings to account for Sunstein’s move to Harvard. The new rankings place the University of Chicago fourth—with 550 citations—behind Yale, Stanford, and Harvard.