A recent study has examined the prevalence of grade inflation at US universities over the last 100 years or so, and has found some identifiable patterns. The chart below shows the increase in grades between various types of schools in the primary colors, with the grey representing (unnamed individual schools).
What is clear is that there was a huge increase in grade in crease in the 60’s and then a steady increase over the last 30 years of so. From the study
The rise in grades in the 1960s correlates with the social upheavals of the Vietnam War. It was followed by a decade
period of static to falling grades. The cause of the renewal of grade inflation, which began in the 1980s and has yet to
end, is subject to debate, but it is difficult to ascribe this rise in grades to increases in student achievement. Students’ entrance test scores have not increased (College Board, 2007), students are increasingly disengaged from their studies (Saenz et al., 2007), and the literacy of graduates has declined (Kutner et al., 2006). A likely influence is the emergence of the now common practice of requiring student-based evaluations of college teachers. Whatever the cause, colleges and universities are on average grading easier than ever before.
Further science and engineering students are graded more harshly than their fellow students in liberal arts degrees.