When students of John Halpin, a chemistry professor at New York University, went online to express what they thought of him through the increasingly popular website Ratemyprofessors.com, he did not take it lying down.

Instead, Professor Halpin accepted an offer to make a video for the site responding to criticism of everything from his sense of humour to his tone of voice. He also addressed the "celebrity status" he has apparently achieved among students, and even dealt with an offer of marriage.

"You get these comments on Ratemyprofessors and you read them and you think, that's fine, that's your opinion, but there are reasons I do the things I do, and people who read these comments won't necessarily understand those things," he explained.

Popularity contests

Independent sites where students can grade their tutors are going from strength to strength in the US. The phenomenon is being propelled by new smartphone apps that make evaluations even easier to post and read for members of a Facebook-savvy generation who rely increasingly on their friends' opinions of everything, from popular culture to which courses to take and where to study.

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The success - and even greater potential - of such sites is evidenced by the fact that Ratemyprofessors has been bought by an arm of MTV.

One of its main rivals, Courserank, which offers students reviews, course workloads and grading histories for individual faculty members, was sold by the Stanford University students who created it to the online textbook rental company Chegg.com.

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Another measure of the sites' impact is that academics are paying more attention to them - and, like Professor Halpin, they are starting to fight back.

So are the universities, which are trying to pre-empt the independent sites by making their own students' evaluations publicly available.

"It's as if they have this mantra: 'If you can't beat 'em, join 'em,'?" said Tina Couch, spokeswoman for Courserank.

Although faculty still largely deride the online review sites, a significant number believe that there is some value to them.

"(The sites) may be motivating teachers and administrators at colleges and universities to do a better job of showing that they are effective at teaching," said Guy Boysen, professor of psychology at the State University of New York at Fredonia, who has studied the various ratings.

Others complain that students who earn low grades tend to mark down their tutors, that the reviews are often contradictory, and that the sites have the potential to encourage academics to cosy up to students in order to earn better ratings.

Other studies have identified further flaws in the evaluations.

Research carried out at Central Michigan University found that professors who are considered physically attractive by students tend to get higher ratings than their less desirable peers.

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And Professor Boysen said his work suggested that students give lecturers a hard time on the sites if they believe they have been rude or have embarrassed them in class.

Bad influence

Carol Simpson Stern, professor of performance studies at Northwestern University and former chair of the American Association of University Professors' Committee on Teaching, Research, and Publication, said that the sites could have a baleful influence on academic standards.

"It is extremely hard for me not to believe that professors are being seduced into constantly adjusting their standards, their grades, in order to win approval on these websites," she said.

Like many universities, Northwestern encourages its students to submit formal evaluations of the academics they encounter during their studies. And like an increasing number of institutions, it now makes those evaluations public.

Professor Stern said that the internal student reviews were supposed to have been coupled with peer evaluations.

"We thought the optimal way to get the trust of the instructor would be a peer-evaluation system balancing student feedback with peer expertise," she explained.

Instead, the student reviews have become dominant, she added.

According to Ms Couch, the number of sites offering students the chance to rank their lecturers is only going to increase, given their popularity.

That is fine, said Professor Halpin: such sites have an important role to play.

His rebuttal video on the Ratemyprofessors site included the following joke meant to counter criticism that he had a "corny" sense of humour: "Sodium and potassium are walking down the street and sodium says: 'I lost an electron.' And potassium says: 'Are you sure?' And sodium says: 'I'm positive.'"

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His more serious take on the evaluations is that "anyone who is really serious about their teaching goes for all the feedback they can get. Not paying attention to them is like not paying enough attention to your own job."