Psychology and Crime News has this great post about the famous Stanford Prison Experiment. Briefly, the question posed is whether those who would volunteer to participate in a prison study are somehow unique, and thus, would bias the results. The answer is yes:
Volunteers for the prison study scored significantly higher on measures of the abuse-related dispositions of aggressiveness, authoritarianism, Machiavellianism, narcissism, and social dominance and lower on empathy and altruism, two qualities inversely related to aggressive abuse. Although implications for the SPE remain a matter of conjecture, an interpretation in terms of person-situation interactionism rather than a strict situationist account is indicated by these findings. Implications for interpreting the abusiveness of American military guards at Abu Ghraib Prison also are discussed.
The Situationists would have us believe that most behavior is determined by the situation a person is in and not their character or personality. The end product of such thinking is that most of us would engage in behavior X given the right circumstances. Yet, that position must overcome mountains of empirical research on personality and genetics. Revisiting the Stanford Prison Experiment shows us just how important sample characteristics are and why many social psychology studies suffer because they rely so heavily upon psychology undergraduate students.