Since I believe that transparency and collaboration are values that promote student learning, I sometimes provide my students an opportunity at the beginning of the quarter to have a say about grading policies and standards. And I'm always amazed at how enthusiastic students are about curving grades. I'm opposed to curving for various reasons, but what really puzzles me is why students are enthusiastic about curves in the first place.
First off, I'm against the message about learning and achievement sent by a curve: that grades measure students against one another rather than against a defined set of benchmarks for learning. Life is competitive enough without my having to implicitly pit the students against one another. And notice that a curve gives students a rather perverse incentive not to work together on papers and assignments -- something that should be encouraged rather than penalized.
Second, I have an obligation to others that the grades I assign reflect student learning as measured against defined benchmarks. If I give a student an A in, say, a critical thinking course, a future employer has the right to infer that the student is likely to be a good critical thinker. (Yes, I realize that conclusion must be qualified in a hundred different ways, but bear with me.) But if I've used a curve that inference may not be warranted. That would be true if the student got an A by virtue of being in the top 10% of students in the course but the students in the course were extremely low achieving. In that case, the employer could infer that the student is a better critical thinker than most students, but I think the employer is entitled to make a judgment about the student's fitness for a position not just in a relative or comparative sense but in a more absolute sense — does the student have the critical thinking skills necessary for this position, period? And of course, if the employer doesn't know if I've used a curve or not, then any inferences become murkier. So given my obligations to employers, grad schools, etc., I should assign grades that don't carry a risk that students will be inadequate for the tasks or duties that employers, grad schools, etc. ask them to undertake. (And what holds for me, a mere philosopher, holds a fortiori for instructors in other disciplines: I'm pretty sure I don't want future structural engineers or surgeons graded on a curve!)
Third, I suspect that curves contribute to grade inflation, which I gather most of us think is problematic. I have no specific evidence of this, however, and would be interested to know if I'm wrong about that.
But finally, it's hard to understand the student mentality that favors curves. My guess is that students think curves are somehow less risky for their GPA's, but that's only partially true. Yes, curves ensure that some students get A's. But they also ensure that some students get F's and they cap the number of students who can get A's at all. So no matter how hard a student might work or how much she might learn, so long as a predetermined percentage of students works harder or learns more, she can't earn an A. Perhaps students accustomed to A's think that a curve will preserve their high GPA's even in the face of difficult courses. But I've found enthusiasm for curves at all levels of preparation and ability. Yet it's hard to see how curves are inherently better for students from the standpoint of their self-interest.
All this being said, I'm not averse to adjusting my grading standards on a given assignment, which could be called a mild form of curving. Sometimes an assignment proves more difficult than I anticipated, a result that might well be due to my own unrealistic expectations or my own failures as a teacher. Still, in my estimation, the benefits of curves pale next to their clear pedagogical and professional shortcomings.