Paleo is mainly about what not to eat. To say someone can't succeed on paleo means they can't survive (or do well) without neolithic foods. Essentially, you would have to demonstrate someone's health is dependent on dairy, grains, legumes, or sugar.

I have never seen evidence suggesting people do better on wheat vs no wheat, milk vs no milk, or soda vs no soda. Quite the opposite.

If on the other hand, someone wants to claim that a particular approach to paleo failed (low fat, low carb, raw, lots of nuts, high in nightshades, etc.) then they need to modify the diet to suit their needs. Not blame the concept of eating an ancestral diet.

I very much like the idea behind this question. Attempting to falsify belief and recognizing bias is important.

The main problem is that there haven't been many studies (at least that I know of) which have tested a "paleo" diet. To top it off, "Paleo" is more of a philosophy than a specific diet, more of a food first principle than something like Atkins or Ornish, so you'd have a tough time defining things well enough that some paleo group wouldn't say, "Well yeah but that's not how I eat".

Also, "paleo" often converges with many other food philosophies when you get down to basic implementation, so you end up with labeling issues. Am I eating real food or am I eating paleo? What's the difference?

So I guess you'd probably have a hard time finding a group of people for whom paleo would fail, since at the end of the day paleo boils down to "eat real food, minus grains". If you managed to falsify the idea that "Eat real food, minus grains" is healthy, it would be a revolution in human nutrition with ramifications well beyond the paleosphere.

The implication is that, if one is regularly eating Neolithic foods, one is not thriving. Yet there are thousands of athletes around the world setting records eating wheat, dairy, sugar etc. Further, millions of people around the world seem to subsist and even (depending on one's definition0 thrive on diets that include, or are even based on, wheat, rice, dairy etc. While the experience of many Paleos is that eliminating these foods can also eliminate the problems they cause for those who suffer from gluten-related issues (or dairy allergies etc) seems sound, it strikes me as a stretch to apply this logic to everyone in the world, particularly those with no health issues.

The original post suggested that the onus is on the non-Paleos to prove the superiority of a diet that includes neolithic foods. I disagree, the onus is clearly on the Paleo diet to prove its utility to those who do not have any manifest diet-related health issues with dairy, grains and legumes (even those issues they are unaware of). I say this as someone who has completed a 30 day induction and had no change in health, BP, blood work, recovery, etc. I later reintroduced milk with no problem and from time to time I eat grains and some sugar. I'm basically 80% compliant still, but I see no need (socially or health-wise) to go 100%.

I think that people are most likely to keep up with paleo (and be more strict about their personal definition of it) if they had health problems that clear up with paleo, and come back with trigger foods. Knowing in theory what is optimal for you is psychologically different from knowing through X Food = Major Pain. I recently came across a traditional foods/paleo forum where I least expected to find it. I was pleasantly surprised until I realized that all the posts were titled things like "I really want to stay Paleo, but I'm addicted to grains!" Or "Maybe I'll go WAPF instead so I can have grains!" Perusing these, it quickly became clear that they hadn't experienced major health benefits from paleo. Maybe a slight increase in energy and clearing of mental fog, but not "my knee doesn't ache anymore, but it swells up so badly I can barely walk when I accidentally stray" like for me.

So, maybe paleo "survivors" were sicker in general? A study would be fascinating.

From anecdotal reports, the main problems seeme to be:

1 Induction flu and/or withdrawal symptoms. Many people need a period of adaptation.
2 Some people needing more carb than others-gotta work out ideal carb intake.
3 Allergies to one or some of the paleo foods
4 Some people choosing a list of foods that do not fully meet nutritional needs (magnesium, etc)
5 Paleo eating not solving every problem the person has by itself.

These issues are only 'failures' if the problem is not solved and only the first two issues are more paleo specific. The others can happen on any loosely organized diet. Part of the problem is that paleo is not a set prescribed diet plan. There is actually a huge variation in the way some paleo people eat compared to others. Mix that with epigenetics and it's not a surprise that results can vary greatly as well. In some ways, several of the tenants of paleo are even at crosshairs with eachother. You can't really have a diet that is not really a diet but just a list of thousands of choices, and then expect a similar and controlled response by everyone on that 'diet.' Paleo eating is really less of a diet and more of a theory that many of us think is correct. This theory shapes our eating and lifestyle plan, but each of us interpret it differently and our eating plans are actually very individual from another. There is no just one paleo 'diet' plan.

I think the major reason why people fail is psychosocial. You have to be able go against the flow. That's not easy!

I'm lucky, my wife and I work at home. We can rather easily control our dietary environment. We're both physical therapists, so we don't sit still all day long. We control our own agenda (more or less).

Other reasons could be finding 'your' paleo approach. I think Melissa McEwen talks about a 'faileo-diet'. You should check her blog for this post. Eating low fat paleo could not be your optimal diet. Although I'm convinced that in principle and for most, a paleo diet is easy, for some the basic paleo concept could need a little tweeking and adjusting. These little things could make a big difference in result.

A failure of the paleo diet is actually a victory for neolithic food addictions. I just can't fathom someone feeling better or being healthier eating these "food" aberrations.