American dieters have a bad case of one-thing-itis.

Every diet that has gotten traction in my lifetime, an unconscionably long time, has boiled weight loss down to one thing. Sure, the thing changes, but the oneness doesn’t. You know the litany. Fat, carbs, sugar, meat, gluten.

There are two problems with one-thing-itis. One’s obvious, and the other, not so much.

First, the obvious one: It’s not an effective weight-loss strategy. Decades of research shows failure after failure. People lose weight in the first six months, or maybe the first year or two, and then they stop. And then they regain.

The proof of the failure of these diets is all around us, in obesity rates that just won’t quit. If any one thing actually was the answer for everyone who’s trying to be thinner, we would have reversed the trend already. (There are, of course, health implications that aren’t weight-related, a column for another day.)

Yet the media — particularly social media — is filled with people shouting from the rooftops that the diet that worked for them is the One True Diet, the answer to our prayers. Which brings me to the less obvious problem with one-thing-ness, which I will let Dr. Seuss explain.

Remember the Sneetches?

“Now, the Star-Belly Sneetches

Had bellies with stars.

The Plain-Belly Sneetches

Had none upon thars.”

The starred Sneetches decide they’re the better Sneetches and want nothing to do with the Plain-Bellies. Enter entrepreneur Sylvester McMonkey McBean, with a machine to put stars on bellies. The Plain-Bellies pay to get stars upon thars, to the horror of the Star-Bellies, who then pay McBean to get their stars taken off. And so on.

If you’re on Twitter, perhaps you’ve noticed people who identify themselves by their diet. Sometimes, it’s right there in their profile. Sometimes, it’s even in their name — a little C (for carnivore) or V (for vegan) in a circle.

Sound familiar?

I am not unsympathetic to this. I have lost weight, and kept it off, and it’s a very compelling experience. The tendency to believe that the thing that worked for you will work for others is strong, and I understand wanting to shout it from the rooftops. I’ve done it!

But making your diet part of your identity has unfortunate repercussions: It’s bad for discourse, and it’s bad for you. Unfortunately, we’re hard-wired to do it.

Katherine Kinzler is a University of Chicago psychology professor and the author of “How You Say It: Why You Talk the Way You Do — And What It Says About You.” She studies why Sneetches care about the stars on the bellies — er, I mean in-group and out-group thinking — by studying children and infants.

“Human infants have to learn about what to eat,” she told me, “and a lot of what we learn is through social modeling.” If each of us had to learn what was safe by trial and error, we would be in big trouble. “You’re not trying all the foods, you’re mimicking other people.”

Which raises the question: Which other people?

“From early in life, kids are thinking about the social nature of food,” Kinzler told me. Given the choice, she said, babies prefer a food they’ve seen a native speaker eat over a food eaten by a nonnative speaker.

We associate food with “us” and “them” from a very early age. From there, it’s just a hop, skip and jump to putting little letters after your name on Twitter.

And it’s downhill from there. As Yale psychology associate professor Yarrow Dunham wrote to me in an email, “groups become more acrimonious when they are defined in opposition to one another,” a framing “that often leads in-group favoritism to also blossom into out-group derogation.”

Spend some time on diet-related social media and you’ll see what he means. It’s very dispiriting when people call each other vile names because they disagree about meat’s impact on LDL cholesterol. Or some other one thing.

Of course, the fight isn’t just about the one thing. It’s about us. And also them. Because that’s how humans divide people up. “Once you have a category and imbue that category with meaning,” Kinzler explained, you end up with a kind of categorization mission creep. “It’s not just that people are making different choices. The underlying essence is different, and that’s what produces the surface-level differences.” This tendency, she said, takes root “in a way that you can’t observe, but feels deep and real.”

So vegans aren’t just people who don’t eat animal products. They’re the kind of [fill-in-the-blank] person who makes that choice. And that makes them so much easier to insult!

We all read “Lord of the Flies” in junior high school, so we know all about our tendency to divide ourselves into essentially arbitrary groups and then build a hill so we can die on it. Social media just makes it easier.

That’s not the worst of it. Sure, elevating diet to be part of your identity is bad for discourse. But the reason it’s bad for discourse is that it’s bad for people engaging in the discourse; once you define yourself by your diet, it gets much harder to see evidence clearly.

We’re seeing that dynamic play out in our response to covid-19. You wouldn’t think that your political affiliation would affect your ideas about virus transmission, but sure enough, a recent poll found that 32 percent of people with Republican stars on their bellies, but only 3 percent of those with Democratic stars, believe masks don’t help limit the spread of the novel coronavirus.

Once a particular belief gets associated with something you consider essential about yourself — your values, your affiliations, your identity — confirmation bias kicks into high gear, and your chance of figuring out if your one thing is wrong plummets.

Diet preferences aren’t quite as arbitrary as Sneetch stars, of course, They’re also personal. It doesn’t matter if a diet doesn’t outperform others in trials if it works for you. And if you’ve found one that works, I’m delighted for you! Weight loss is really hard, and congratulations on your success. I get that you want to spread the word, and I’m not asking you not to.

Here’s what I’m asking, not for the first time: civility. The public conversation is so much better without name-calling and sneering and smug superiority. We might have a fighting chance, together, of figuring out what’s actually true, if we’re good to one another. But the biggest reason I try to be civil in public — and if you look, I guarantee you will find instances where I’ve failed — is that it leaves the door open for me to change my mind. Meanness digs you in.

The Sneetches learned the lesson the hard way. They paid Sylvester McMonkey McBean to take stars off and then put them back on until the money ran out. But, by then, they had lost track of who was who, and they all just went back to being Sneetches. Broke Sneetches, but still. And they started being nice to one another.

Next month: the green eggs and ham diet.

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