on grade inflation since 1990

Collegiate Grade Inflation: It's All About Supply and Demand : Mike the Mad Biologist:

So I'll posit that a major factor in grade inflation is the increased competition for graduate school slots. This has led to B's and C's having a much stronger influence on students. At the same time, students have become more willing, when possible, to avoid courses that are graded harder. While it might seem that if everyone is getting high grades, that high grades don't matter. But if you don't have a high GPA, then you stand out as a poor student--there is a rachet effect that pushes grades upwards.

If we want to reduce grade inflation, then we have to stop making grades so critical for acceptance into increasingly selective 'good' graduate programs. Until then, grade inflation it is. The good news, I suppose, is that grades can't get any higher...