Drug trials for mood disorders (etc) often show a really strong placebo effect, and I've heard some speculation that this happens for a similar reason. People

Drug trials for mood disorders (etc) often show a really strong placebo effect, and I've heard some speculation that this happens for a similar reason. People running the trial really want you to complete the study. The payments are structured to encourage this, but the staff also try to be warm and friendly to encourage you to come back, and this (along with getting out of the house and checking something off a to-do list), helps lift people's mood.