Some GCSEs and A-levels are harder than others, England's exams watchdog says.
Teachers have long thought some subjects are harder than others
The Qualifications and Curriculum Authority report said history GCSE was harder than geography, and chemistry was harder than physics and biology.
Overall the QCA said subjects were broadly in line, and no immediate action was needed to even things out.
There was also little evidence that so-called "soft" A-levels were less demanding than more established counterparts, it added.
The Qualifications and Curriculum Authority compared a cluster of subjects with each other but did not produce an overall ranking of all secondary school exams.
It analysed candidates' work and exam papers in 10 subjects overall, producing four separate reports.
But in the detail of the reports, experts found potentially significant differences in the way subjects were assessed.
The most significant of these was between history and geography.
At both GCSE and AS (the first year of A-level) history was judged to be more demanding than geography.
The report also found GCSE geography candidates showed less "evidence of attainment" than history candidates.
A separate comparison of sciences found chemistry was the most demanding of physics, biology and double science at GCSE.
At A-level, biology candidates were found to perform less well than chemistry candidates, with physics candidates falling somewhere in between.
A third study comparing biology with sociology and psychology found that biology demanded the greatest breadth of knowledge.
This was compounded by the fact that across biology AS - there was no choice of questions.
But biology did not require the same demands of psychology and sociology candidates in terms of interpretation or evaluation.
There was also a concern in sociology that too much credit could be given for common sense responses.
The report said: "In the case of psychology and biology, the question papers made use of complex concepts. Candidates could neither infer nor guess answers."
It continued: "This was not clearly the case in sociology."
A fourth study comparing English literature, history and media studies A-levels found all English literature units too high at AS.
Some texts were judged to be particularly inaccessible and the questions were tough too.
Meanwhile, some of the materials used for media studies were thought to be potentially very enjoyable, and so "motivating for candidates".
But the report found little difference between the subjects at grade A at AS.
At grade E, however, media studies candidates were considered to be less impressive than those doing English, with history candidates falling somewhere in between.
Schools minister Jim Knight said: "I am pleased that it confirms that there is little evidence that A-levels sometimes described as 'soft' are any less demanding than others."
QCA chief Ken Boston said the report showed the level of demand is broadly comparable across the subjects considered.
A much broader piece of work comparing subjects' difficulty would be take forward by the new regulator, Ofqual, he said.
Martin Ward, deputy general secretary of the Association of School and College Leaders, said school and college leaders knew that exams in all subjects were not equal.
He added: "Relatively small differences in the nature of the questions asked can have a big effect on the difficulty of the examination for the student."
Association of Teachers and Lecturers head Dr Mary Bousted said: "The comparison of exam standards is helpful to try to end the annual free for all over what are 'hard' or 'soft' subjects.
"But the focus on exams misses a vital aspect of education - the curriculum itself."