Originally Posted by teaser
Also, people playing the home game, whether it's McDougall's starch solution or Atkins, are already sort of invested and probably have some reason to believe that the diet could work for them, or they wouldn't be trying it in the first place, that sort of investment might be lacking when people are randomly assigned to a diet group, believing something will actually work if you apply it properly is major motivation for compliance.
I think the knowledge of what kind of food you personally tolerate more readily is an even bigger factor than simply believing something will actually work, based on the theory and research behind the diet.
I know some people who simply can not tolerate even a moderately high fat diet. For some, it makes their stomach hurt (testing for gall bladder problems are negative, so they just avoid more than minimal fats). For others, they end up with the runs. Still others simply can't stand the taste of fatty foods, or the texture of more than absolutely minimal dietary fats.
Others (like many of us on ALC) will end up eating everything they can get their hands on if dietary fat is severely restricted, protein minimized, with starches or sugars the mainstay of their diet, no matter how much fiber is involved. Raise the blood sugar with all those carbs, triggering a massive insulin response, and the raging appetite is never appeased on carbs.
These are extreme examples of course, but I pity anyone who was randomly picked in those studies to do a diet which they would find so physically difficult that it would be impossible for them to stick to it.
Not that having reason to believe a diet will work, because you find the theory behind it to be more believable (CI/CO, vs satiation of dietary fats and minimizing carbs to control appetite) doesn't have something to do with it too, especially for people who haven't dieted before, or grew up eating one way, gained weight when they got away from that way of eating, and are looking for a a diet they think they can stick to.