[chestervonwinch]

I'm inclined to agree about knowledge inflation (in the sense explained in the following paragraphs), but I don't think it has to do with the perceived difficulty of subjects changing, or necessarily with instructors inability to teach at a level of rigor that might pull the average down (by increasing perceived difficulty).

Regarding "knowledge inflation", I'm not sure that we see an inflation in knowledge per se (at least in a way that affects grade inflation in college courses), but I think what we do see is an increase in 1) quantitative methods, and 2) cross-talk between fields. This influences the subject matter people in a variety of fields are expected to know (e.g., the amount of math, comp. sci., stats, etc...) , and thus, I suppose, inflates the expected knowledge in this sense.

Anecdata: I teach calculus 1, and guess how many math majors are enrolled? Zero. It's mostly bio, and other sciencey fields. This affects how I teach (and grade!) calculus. I'm willing to bet other courses -- like intro programming, intro stats, etc... -- experience something similar. Is this an explanatory factor for grade inflation? I don't know. There's obviously a huge number of things to consider.