[willsun]

I think this is a fair point, and the final section of the post attempts to address this. For what it's worth, most studies do not selectively draw from a highly-motivated cohort - participants are rarely the most severely obese, and they also usually aren't people who are actively looking for clinical trials or free weight loss programs. Money is also controlled for, since the interventions are almost always offered for free, and any time or work involved with the study will be compensated as well. From my perspective, if anything, the motivational factor may actually be _too_ well-controlled in many of these studies, so the observed result does not translate properly to real-world settings.