Students tend to believe that researchers overemphasize internal validity (although students are more accepting of these tradeoffs in fields such as sensation/perception than in fields such as social/personality). Students often point to the extensive use of college students and animals as participants in psychological research. Therefore, prior to the debate, you will want to emphasize the extent to which physical sciences value internal validity as well as the value and difficulty of establishing internal validity. In addition, you may want to have students read the following articles.
Anderson, C. A., Lindsay, J. J., & Bushman, B. J. (1999). Research in the
psychological laboratory: Truth or triviality? Current Directions in
Psychological Science, 8, 3-9.
Banaji, M. R. & Crowder, R. (1989). The bankruptcy of everyday memory.
American Psychologist, 44, 1185-1193.
Berkowitz, L. & Donnerstein, E. (1982). External validity is more than skin deep.
American Psychologist, 37, 245-257.
Mook, D. G. (1983). In defense of external invalidity.
Stanovich, K. E. (1990). How to think straight about psychology. Glenview, IL: