The findings, published yesterday by the Qualifications and Curriculum Authority (QCA), confound traditionalists' views that exams are getting easier, though they show that candidates' performance in a few subjects has declined.
Today's exams are not easier, just different, the report says: they put more emphasis on understanding and less on learning facts. Some GCSE exams are actually harder than their Seventies equivalents.
However, the investigation suggests that in some A-level subjects, standards vary between exam boards.
Experts compared syllabuses, question papers and marking schemes, and scored exam scripts and examiners' reports for 1976 and 1996. At GCSE, they looked at geography, physical education, religious education and French, and at A-level at government and politics, history, physics and German. At A-level, the conclusions were checked by new independent exam standards tribunals.
Three years ago, a similar report looked at English, maths and chemistry, and found that standards had largely been retained, though candidates were doing less well in some maths topics such as algebra.
At GCSE, the report says geography and religious studies exams are harder than in 1976, while the demands of physical education and French are about the same.
But the standard of writing in French has declined, with candidates paying too little attention to grammatical accuracy. Performance in religious studies was worse: candidates understood the subject better but did not know enough facts.
At A-level, exams remain as demanding as ever, says the report, but it is easier to pass German than it was 20 years ago. In both history and physics, the report questions whether standards are comparable across different exam boards.
For history, the report concludes: "Differences in grading standards between the awarding bodies in any one year were more marked than changes across time... it raises questions about the comparability of grades in any one year."
In physics, the exam board with the most demanding examination was also slightly more severe in its grading. It was easier to get a grade A from the exam board that also had the easiest exam.
Both the Government and the authority are trying to iron out differences between the exam boards, which in England have recently been reduced from five to three.
The Joint Council for General Qualifications, which represents exam boards, pointed out that it was bound by a code of practice designed to ensure that grading standards were constant across both subjects and boards.
Changes to A-level grading standards will be made as part of a reform package announced last week by the Government.
Sir William Stubbs, the QCA's chairman, said: "This work on consistency helps us ensure that future standards are maintained. We shall be keeping a close watch through the period of change."