I’m old enough to remember that when I was a child going on a diet meant something like “eat fewer calories and get some exercise.” The proliferation in the number and approaches of diets since the 1980s has likely resulted from a combination of scientific advancement, fundamental scientific limitations, the internet’s ability to reach people at a low cost, and countless developments across politics and culture. If that seems like a mouthful, consider just how many diets you’ve heard about in the last 20 years.
First are the diets that where something on the back label is “the enemy,” at best a necessary evil to be counted and limited at almost all costs. I can remember that enemy being cholesterol, then fat, then carbs, often with contradictory prescriptions for what people should eat. Then there are the diets that try to trigger some specific bodily process–to name just a few, the keto diet, the glycemic index diet, and most recently intermittent fasting.
We also have what I’ll call the “identity diets.” Last Friday was World Vegan Day, which was like throwing water on the grease fire of social media exchanges between vegans and carnivores. I don’t have a horse in that race, but considering these vituperative all-meat/no-meat exchanges are never far from the top of my Twitter feed, plenty of people I know do.
I call these identity diets because there’s generally some worldview over and above physiology that the diet is meant to affirm. Identity dieters run the gamut from believing meat is destroying the world to believing vegans are destroying society.
The thing is, adherents to all the examples above and dozens more come armed with both theory and data to explain why they’re right and everyone else is wrong. What’s going on here?
Curse of Complexity
I’m perpetually amazed at how many fellow economists I’ve heard use these contradictory results about nutrition as somehow impugning the practice of science in general, as though telling us the best thing to eat in a couple of sentences is something the field is “supposed to” be able to do.
Next time someone learns you’re an economist and asks how your field didn’t see the 2008 crisis coming, or how there could still be poor people in the world, consider that a payment against the principal of your karmic debt.
Like economics, the science of nutrition is complex–there’s human physiology, individual differences, and a host of psychological, cultural, economic, and even logistical factors that go into decisions about food and lifestyle. That complexity has the effect of holding both fields back from proving or predicting things that non-experts mistakenly view as our jobs, while also not being able to reject bad ideas to a point where five or ten academics around the world can’t continue to believe it.
This is not some postmodern rejection of absolute truth–there are plenty of ideas in my field I think are flat wrong, and I’m sure there are diets that do more harm than good. The difficulty is that we often can’t reject wrong ideas so irrefutably that their most hardcore supporters are forced to cut and run.
Lizards Eating Chips
The other possibility when we see so many seemingly contradictory ideas is that they don’t contradict each other at all. Perhaps diets have proliferated so dramatically because structure and overall awareness about what you eat are more important than whether you eat meat or vegetables.
Suppose you had exactly two food items in your kitchen: potato chips and broccoli. Standing in the kitchen, the chips are often too tempting even if you know it’s against your better judgement. The broccoli, at least if it were me, would be far easier to resist.
Now picture yourself at the supermarket. My guess is the broccoli-to-chips ratio you buy there is not in line with what happened in the previous paragraph. That’s because we’ve removed instant gratification, which seems to have a hard-wired pull over people (though some more than others), from the equation.
My guess is the conscious decision to go on a diet and follow a few rules is more important, within reason, than the contents of the diet. Even if that bag of potato chips is sitting right there, your brain is forced to count calories or carbs, check what time of day it is, or take a look at the ingredients. What you’ve done in this moment is to take the decision to eat from your visceral, I-want-it-right-now lizard brain, and planted it more firmly in the realm of cognition where self-control is easier to come by.
Psychologist and Nobel laureate Daniel Kahneman provides a convenient framework in which to think about how the brain handles these two different decisions: “‘System 1’ is fast, instinctive and emotional; ‘System 2’ is slower, more deliberative, and more logical.” Ironically, much of the field of behavior economics that grew out of Kahneman’s work with Amos Tversky has disappointed, primarily because it didn’t follow this core insight, instead opting to tweak math problems representing peoples’ decisions one at a time for various psychological biases.
A couple of weeks ago my boss suggested I try intermittent fasting, where you do all your eating in a 6-8 hour window every day. Since I hadn’t asked for dieting tips, I decided it was a sign I’d been too lax for too long with my eating habits. And lo and behold, two weeks later I’m starting to see results–there’s science behind it for sure but even during that window of time I have to think more about what I eat, planning so that I’ll be good to go for the next mini-fast.
Some people eat a perfectly healthy diet without help–typically those people also choose to spend their free time lifting weights or doing yoga with baby goats on their back. I suspect the reason why so many contradictory diets succeed is people like me. With a few rules the decision of what and when to eat moves to the slower, smarter part of our brains.