It may be years in the making, but Harvard has finally joined other colleges and universities that formally prohibit sexual relations between their faculty and undergraduate students.

The university's Faculty of Arts and Sciences made the ban, not because of any abuse or complaints, but the new rule was announced to make it clear that such relationships are not okay, said Harvard history professor Alison Johnson.

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The policy is there to make sure that what the faculty sees when looking at students are not potential romantic partners, but young people to teach.

More colleges, including the University of Connecticut and Yale, have come out with formal written bans on such relationships.

A professor at the University of Cincinnati, Billie Dziech, who has researched relationships between students and professors, said such policies on relationships are evolving. With Harvard's new rule, she said it sends a message that faculty do not have intercourse with students - whether they consented to it - because it is abuse of power.

She added the ban makes it easier for colleges to deal with harassment complaints because they not forced into taking sides or make a ruling themselves.

Harvard clarifies the ban on amorous ties between undergrads and professors is the only outright ban. The university does not ban romantic and sexual relations among faculty members and among students. They also cover contextual bans such as faculty who could not have sexual relations with graduate students if they are supervising them in some way. In turn, graduate students are banned from having sex with undergrads if they are grading them in some way.

 It turned out the ban was not controversial, even surprising undergrads in the university in their early 20s and late teens, who thought such a prohibition was already in place, a professor said.