The federal judge selected to mediate the Microsoft case is a highly regarded iconoclast who has tackled topics as varied as the economics of sex, antitrust and the legal implications of the Monica Lewinsky scandal.

In legal circles, Richard A. Posner is known as a hyper-prolific author with a mind that is powerful and unpredictable enough to have offended liberals and conservatives alike. His new assignment yesterday was hailed by academics, many of whom consider the Chicago jurist to be one of the brightest scholars in the country and believe he has a fighting chance of prodding both the software giant and the Department of Justice to rethink their positions and settle the case.

"He is the Oliver Wendell Holmes of our day," said Jeffrey Rosen, a professor at George Washington University Law School. "He's the quintessential pragmatist with the keenest analytical mind. It will be exhilarating to watch him think this through."

A 60-year-old father of two sons, Posner writes his own judicial opinions and cranks out books at an astonishing one-a-year pace. His day job on the 7th U.S. Circuit Court of Appeals keeps him busy while the sun is up; he works on his books and essays during the evening, followed by midnight strolls with his wife near his Hyde Park town house. He is a senior lecturer at the University of Chicago School of Law, and a sometimes cantankerous presence on the bench. The Chicago Council of Lawyers complained in an evaluation that Posner and one of his colleagues on the bench are given to "demeaning criticisms of well-intentioned counsel who are doing the best they can."

While other judges effectively quarantine themselves from public debate, Posner runs headlong into it. In fact, many academics believe that Posner would be a lock for the Supreme Court had he kept some of his more controversial musings to himself. Among the most noteworthy: that mothers who put their children up for adoption should be allowed to sell their kids to the highest bidder.

Most recently, he authored "An Affair of State," a caustic and cool-eyed appraisal of the Monica Lewinsky scandal. In it, Posner dismissed the more zealous of President Clinton's critics, but bluntly concluded that Clinton's tortuous evasions amounted to perjury. Posner defends most of independent counsel Kenneth Starr's tactics, reprimanding him chiefly for the amount of salacious detail included in the report to Congress.

Much of Posner's career has been dedicated to explaining the intersection of law and economics. One theme of his writing is that both laws and human behavior are largely explainable as the result of economic calculations. For instance, in "Sex and Reason," Posner argues that people are strongly influenced by cost and benefit analysis when they choose sex partners. The lower the search costs--the less trouble it takes to actually find someone to have sex with--the more appealing that someone is as a partner, he maintains. That explains why there would be a more-than-average amount of homosexuality at, say, a secluded all-boy prep school, or why people practice abstinence during an AIDS epidemic, he says.

Posner has brought this unsentimental perspective to the world of antitrust. As a member of the "Chicago school," he's known for having an affection for free markets as well as skepticism about the chances that regulators could tinker with competition without inhibiting it. While other scholars worry about the size of a particular merger--equating bigness with badness--Posner is less interested in market shares and more interested in whether a deal will actually harm consumers by leading to higher prices.

"The general attitude of the Chicago school is to let the market alone and we'll all come out better," said Albert Foer of the American Antitrust Institute. "It focuses on consumer welfare and maximizing efficiency in society so there are more products to go around."

But as Posner's scholarship proves, pigeonholing his views on any topic is tricky and deciding whether Microsoft or Justice has just recruited an ally is nearly impossible. In his seminal 1976 book "Antitrust Law," Posner expresses doubts that any company would have enough market power long enough to actually engage in predatory pricing. In addition, he worries that if enforcers jump on too many dominant firms for discounting products, the result will simply be higher prices.

Unlike some of his more ideological fellow travelers, however, Posner is not wholly averse to intervening when a company appears to cross the line. In Hospital Corp. of America v. FTC, for instance, he sided with regulators who blocked a merger on the grounds that it would unacceptably increase prices.

"It would be really hazardous to predict what Posner will do," said Cass Sunstein, a colleague at the University of Chicago. "To say that he would favor one side or the other would be neglecting his independence of mind and his scrupulousness as a judge. This is going to be a close encounter of the fourth kind with facts."

Staff researcher Richard Drezen contributed to this report.

IN PROFILE

Judge Richard A. Posner was appointed mediator to help bring about an out-of-court settlement between the government and Microsoft Corp.

Born: Jan. 11, 1939, in New York.

Education: Bachelor's degree from Yale University, summa cum laude, 1959; law degree from Harvard University, 1962, where he was president of the Harvard Law Review.

Career highlights: Was a law clerk to Supreme Court Justice William J. Brennan Jr.; general counsel to President Lyndon B. Johnson's Task Force on Communications Policy; associate professor at Stanford University Law School and professor at University of Chicago Law School; currently chief judge, 7th U.S. Circuit Court of Appeals (Chicago), appointed by President Ronald Reagan in 1981.

Personal: Married to Charlene Ruth Horn in 1962; two children.

Other: Author of 30 books and more than 200 articles, mainly on the application of economic analysis to law in a variety of fields.

SOURCES: The American Bench, Judicial Staff Directory

CAPTION: Judge Richard A. Posner is known for his powerful intellect and unpredictability.