A recent meta-analysis of global psychology literature by Henrich et al. asks the important, obvious, and rather disturbing question, How reasonable is the assumption made by many psychological studies that we can understand human nature based on experimental data derived largely from undergraduate research volunteers, nearly all from "Western, Educated, Industrialized, Rich, Democratic (W.E.I.R.D.) societies"? The Weirdest People in the World?
The authors look through the psychological literature and find that this dominant group of psychology subjects is a consistent outlier when compared with much of the rest of the human race, and essentially question the generality of vast swaths of the field of experimental psychology.
So it's an interesting & controversial read (and from the title you can tell already the authors intend to amuse as well as edify), and as a former fMRI research tech responsible for subject recruitment, I can tell you that while we screened subjects to prevent obvious outliers (on meds, history of mental illness), there is no screening against run-of-the mill head cases. If there were, we would have had much smaller 'n's. Furthermore, Henrich et al would argue that from the perspective of most of the human race, even the most average western undergrad is a head-case.
(ps. thanks to Steve for the tip)