I just wanted to weigh in on your comment here because I think this is a huge misconception. The paleo / primal diet is really not a "diet" per se. A diet is a short term solution to a long term problem. Eating paleo is much more a healthy lifestyle choice rather than a diet. And, as far as it being short term goes, evolutionary biology has established humans ate this way for well over 2 million years. Prior to the Argricultural Revolution 10,000 years ago, you had no choice. So, actually the current grain based diet a la the USDA food pyramid is the short term diet which does not work.
I think it's been established (and it's definitely common sense) that elaborate diets such as the so-called Paleo diet, work well in the short term because, with their intense focus on the minutia of diet, they make people very much aware of eating. When you're paying close attention to eating, you're less likely to overeat, more likely to eat regularly, etc. That change alone encourages a tendency towards healthier weight and better health overall. Unfortunately, elaborate diets tend not to work long term because most people can't follow them for very long (although there are exceptions -- my boss is one).