Lani Guinier, the law professor who caused a furore in Washington when President Clinton nominated her for the job of assistant attorney general for civil rights, is creating new controversy where she teaches - at the University of Pennsylvania.

This time it is her fellow professors at the Ivy League law school who are furious, not the Washington chattering classes. And sex, or gender, is the issue rather than race.

Professor Guinier, who has a black father and a Jewish mother, has written an article claiming that law schools, specifically the University of Pennsylvania law school, creates a "hostile learning environment" for women students.

She and her four co-authors have found that men in the law school outperform women, although there is no reason why they should because the women had equivalent grades on entry. Men were three times as likely as women to be in the top 10 per cent by the end of the first year, and twice as likely by graduation, but there was no difference in the sexes's performance levels when they set out.

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The reason for this gender gap, says Guinier, is the pervasive male culture of elite law schools. It damages women students' confidence and their ability to compete.

Support for this thesis comes from a study carried out by the Law School Admission Council which looked at 6,000 students of both sexes at 90 law schools. It found a similar pattern of women falling short by one grade in one of eight courses in the first year.

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This may sound small, but experts say it is enough to result in large disadvantages for women in a fiercely competitive profession. And it explains what has been observed anecdotally - that there are relatively few women graduating from law school with the highest grades.

The University of Pennsylvania is taking the findings seriously and looking at whether differences in teaching style affect the performance of women. Some professors use the Socratic method, for example, more than others. In this system students are called upon by professors, usually without warning, and asked to apply legal principles to problems. Some students find this verbal cut and thrust intimidating. In her article, Guinier says that the Socratic method encourages a sort of "ritualised combat" among students. The fight comes more naturally to men, she says.

Guinier, who was at Yale law school with Bill and Hillary Clinton during the 1970s, remembers how inhibited she felt in class.

The controversy over gender at the University of Pennsylvania is being played out with a good deal more civility than the furore over Professor Guinier's views of voting rights.

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President Clinton dropped his former friend, who had become labelled "Quota Queen", when it became clear how much opposition she was encountering.