From http://www.stanford.edu/~krollag/org_site/encyclop/theory_xy.html
Human relation theorists emphasize the impact of individual characteristics like race, sex, class, cultural background on organizational and group behavior. Douglas McGregor's book on "The Human Side of Enterprise" ISBN: 0070450986 distiguished between Theory X (classical systems theory) and Theory Y (human relations theory).
The names are purportely meaningless, like LeftWing an RightWing in politics, so that they can be meant to be anything, as is convenient when future needs make the change necessary?
Relation to Extreme Programming
ExtremeProgramming takes the TheoryYy approach. If you mistrust your people's commitment and motivation, you're certainly not going to trust them with a process like ExtremeProgramming. XP definitely assumes that its practitioners are dedicated professionals who really care about doing good work.
Contributors: RobHarwood, MikeSmith