The Qualification and Curriculum Authority found that although the emphasis of many syllabuses has changed over the decades, standards are just as high today as in the past.
Researchers examined past candidates' exam scripts and subject syllabuses to compare standards in GCSE, O-level and CSE French, religious education, PE and geography from 1976 to the present day.
In French, the increase in role play and oral work was found to have transformed the syllabus but the study found that the level of performance required to get each grade was equivalent to that in earlier years.
A-level physics, history, government and politics and German were also monitored and found to have consistent standards.
The study began in 1996 when School Curriculum and Assessment Authority, the QCA's predecessor, and the Office for Standards in Education published a study of English, maths and chemistry exam standards.
A QCA spokesman said: "This is the second report in a study which will eventually look at all subjects."
WHAT THE HEADTEACHERS SAY
The Government is due to announce today a broader A-level curriculum which critics say will dumb-down the exam.
Lynn Gadd, of Copthall School, an all-girls' comprehensive in Mill Hill, north London, said: "I do not have a problem with the breadth. Too many students currently face difficult decisions at the end of Year 11 because they are good at a lot of subjects and worry about narrowing their backgrounds.
"But I hope we will not lose some of the essential aspects of sixth-form life which widen and enrich their studies."
Michael Watson, principal of Tendring Technology College, a grant maintained school in Essex, said: "I would welcome these changes. However, it will only work if universities insist on it. Otherwise students will continue to concentrate on getting the best three A-level grades they can. I am concerned about timetabling and staffing but I welcome the attempt to broaden the curriculum and to bridge the gap between A-level and GNVQ."
Michael Booth, head of Benfield School, a mixed comprehensive in Newcastle-upon-Tyne, said: "I see the advantage of broadening A-level studies in principle, but in practice it is going to cause problems.
"We have a small sixth-form of 85 students and currently teach upper and lower sixth together in small groups. It could be a struggle for us to provide the new A-levels.
"We will have to look at creative ways of delivery including working with other schools using distance learning and ICT."
Nick Brown, principal of Oldham Sixth Form College, said:
"Broadening the curriculum is to be welcomed but for real change we needed to have a unitised curriculum to allow students to pass between A-level, GNVQ and BTEC. This was an opportunity for a once-and-for-all change and it is a pity that this has not happened.
"We have more than 2,000 sixth form students and I can envisage staffing and timetabling problems."