Passion's nailed it. You cap the proper name of the specific class--Psych 101, Advanced Phrenology in Primates, Numismatic Literature--but not shorthand versions of the class name--off to chem!--and not fields of study--Hi, I'm a psychology major--unless the word is a proper noun, such as a language.
Maryn, pretty sure this is one area where she doesn't screw up