Is there an “unhealthy volunteer effect” in mental health outpatient research?

Zivin, Kara; Pfeiffer, P N; Szymanski, B; Dhawan, N; DiFranco, D; Valenstein, M. (2010). Is there an "unhealthy volunteer effect" in mental health outpatient research?. Psychiatry Research. 176(2-3), 224-228.