Students should get more university entry points for taking tougher A-level subjects, researchers suggested this week.
Academics at Durham University analysed data from nearly 1 million school pupils and found that it was much more difficult to earn top grades in some subjects than in others.
On average, the researchers concluded, subjects such as physics, chemistry and biology at A level are a whole grade harder than drama, sociology and media studies; they are also three quarters of a grade harder than English, religious education and business studies.
Robert Coe, deputy director of Durham's Curriculum, Evaluation and Management Centre, said: "If universities and employers treat all grades as equivalent, they will select the wrong applicants. A student with a grade C in biology will generally be more able than one with a B in sociology, for example."
He added: "With the Universities and Colleges Admissions Service points we have at the moment, students get the same number of points for each grade. One way we could address this is by changing it so you get more points for an A grade in one subject than you do in another."
This week, the National Council for Educational Excellence was due to present to the Prime Minister a report that is expected to suggest, among other things, that every secondary school should have a senior teacher who can steer bright pupils away from "soft" subjects if they want to get into a top university.
Earlier this year, the Qualifications and Curriculum Authority found that some exams may be harder than others but concluded that immediate action was not needed. The Russell Group of highly selective research-intensive universities has said some subjects are less good preparation for a university degree, and the University of Cambridge has published a list of subjects it considers fall into this category.