The move from the Qualifications and Curriculum Authority comes after controversial claims this summer that numbers in the two subjects are booming as students try to boost their university chances.
However, the authority this week denied the inquiry had been prompted by the controversy. It was, it said, part of a larger, ongoing project comparing students' experiences of different subjects.
When the A-level results were announced in August, John Dunford, general secretary of the Secondary Heads Association, said students were shunning subjects such as sciences and languages and choosing psychology and media studies. He said that students were being guided by teachers who knew that some subjects were easier than others and would therefore give them a better chance of getting into university.
Dr Dunford described the move as a hidden scandal.
Now the QCA is advertising for teachers to take part in a pair of inter-subject comparability studies. One part of the project will assess the relative difficulty of A-levels in biology, psychology and sociology.
A separate study will compare the difficulty of history, English literature and media studies. The research will start in January next year.
A QCA spokeswoman said the work was one element of research which began last year, using statistics and teachers' views to analyse whether certain subjects were harder than others.
The QCA is seeking professionals with experience of teaching more than one subject, who would be asked their opinions on relative difficulty.
The research began with a look at the relative difficulty of humanities subjects, followed by comparable work with sciences earlier this year.
This work will feed into a larger project on standards being headed by Barry McGaw, deputy director of education at the Organisation for Economic and Social Development. No results have yet been published.
The investigation will provide much-needed new research into the consistency of standards across A-level subjects. Dr Dunford's original claims date back to a study carried out in the mid-1990s.
He said: "I'm pleased that the QCA is taking this matter seriously, and that it is looking at this issue from a qualitative viewpoint. It's vitally important that no subjects are perceived to be easier than others."