So I used to have this shirt.

I know at least one of you was watching back in 2013.

[MAN HOOTS] Hey, Kyle.

Now, for the rest of you, you're probably wondering the same thing as me.

What is up with the hair?

What this shirt means is that the microbes in and on our bodies outnumber our own cells.

The most common figure is by 10 to 1, except that's not true.

It's a scientific urban legend.

Yet this continues to be shared and recited as fact.

I'm guilty of it too.

My old video is called You're Mainly Microbe.

And it is literally centered around this erroneous factoid.

It turns out that urban legends like this are surprisingly common, even in science.

And how they begin and why they persist can teach us a lot about how science works and when it doesn't.

[MUSIC PLAYING] At some point, the 10 to 1 bacterial to human cell ratio became common knowledge.

Common knowledge is information that the average educated person in some group-- the general public, scientists, whoever-- accepts as reliable without having to look it up, like how we all know that water freezes at 0 degrees Celsius.

We all know that, right?

Now, somewhere along the line, people stop asking where this common knowledge came from.

There are countless facts in science that have become common knowledge.

If research papers cited an original source for every single fact they presented, it would be an absolute mess.

Say you wrote a paper about synthesizing some new chemical.

Do you have to cite a paper that proves chemicals are arrangements of different atoms?

OK, then do you have to cite something that proves atoms exist, maybe Einstein's 1905 paper on Brownian motion?

Or do you have to go back to John Dalton in the early 1800s?

You can see that things get pretty ridiculous pretty fast.

But sometimes things that aren't true become common knowledge.

Or they're corrected later, but the new information fails to replace the old idea.

Here's an example.

I wouldn't be surprised if at some point in your life, you probably heard that spinach was a particularly excellent source of iron.

I certainly remember being taught that.

I can't even remember where.

[POPEYE THEME PLAYING] You can probably guess where I'm going with this.

It's not true.

In 1981, a biologist named Terry Hamblin studied historical science papers and realized that the iron content in spinach was misreported thanks to a misplaced decimal point way back in the early 1900s, except he didn't cite a source for the misplaced decimal point story either.

And it turns out that that's a myth too.

It turns out the earliest old-school measures of iron and spinach were way too high and wrong, but because of contamination, not a misplaced decimal point.

It's science.

Details matter.

Spinach actually does contain large amounts of iron, as much as red meat, in some cases.

But it also contains compounds that make the iron it does have harder for us to absorb.

So it's not an exceptionally great source of iron.

Incidentally, it turns out Popeye creator EC Segar chose spinach as the sailor man's food of choice for its high vitamin A content, not because of iron.

It's another case where the correction never seems to spread as wide as the lie.

And it's a good reminder that a good story is not necessarily a true story.

And I'm willing to bet at some point in your life, you've taken vitamin C to help cure or prevent a cold.

Yeah, that's not true either.

That myth traces to legendary scientist Linus Pauling.

In 1966, Pauling was convinced by a random dude named Irwin Stone that taking large doses of vitamin C would help him live longer.

And Pauling started taking doses equivalent to hundreds of glasses of orange juice every day and wrote books and articles claiming that the colds he had suffered from his whole life no longer occurred.

Even though Linus Pauling won two solo Nobel prizes in his life, dozens of studies have since proven he was wrong about vitamin C. It doesn't significantly affect colds.

And the only disease it definitively prevents is scurvy.

Yet somehow the cold myth still continues today.

Or maybe you've heard that you lose most body heat through your head.

Well, that urban legend goes back to one military study in the 1950s where people were left out in the cold with no hats on.

You're going to lose most your body heat through your head if that's all that's exposed.

Today scientists know the amount of body heat you lose depends on the total surface area exposed.

But parents everywhere are still making sure you don't leave home without a hat.

You also don't need to drink eight glasses of water a day.

That urban legend probably goes back to one set of dietary recommendations for water intake from 1945, except many people who cited that number ignored the part where it said most people get the majority of water they need from food.

It's important to stay hydrated.

But eight glasses?

I mean, what size of glasses, even?

And one of the most famous is that sugar causes hyperactivity in children.

This one seems totally logical.

But more than a dozen randomized controlled trials have failed to detect different behavior between kids given large doses of sugar and kids who weren't.

That's right.

The cake is actually a lie.

It turns out when parents even think their children have been given a drink containing sugar, even if it's actually sugar-free, they tend to think their kids are being hyperactive.

This particular urban legend traces its origin back to a California allergy doctor, Benjamin Feingold, in 1973, who, with little to no evidence, recommended removing artificial colors and flavors from the diets of hyperactive children.

And I guess people were like, why not sugar too?

I mean, kids are just kids.

And they're going to go nuts sometimes.

Let's go back to that 10-to-1 mainly microbe cell number from the beginning.

In 2010, a couple of researchers went on a deep dive to find the original source.

And the paper cited most often was this one from 1977.

It states the human body contains 100 trillion microbial cells and 10 trillion of its own cells, 10 to 1.

Scroll down to reference number 70, and we find the source of the 100 trillion microbial cell number is this paper by Thomas Lucky, which, when we read the paper, turns out was just a back-of-the-envelope estimate and wasn't based on any actual experiments.

This has nothing to do with the rest of the video.

But I just have to mention Dr. Lucky was literally an honorary samurai, which is awesome.

And going back to the original 1977 paper, the human cell number comes from reference number 27, a textbook by biologist Theodosius Dobzhansky.

I dug through the internet to find a copy of it.

And right there in chapter 1, with absolutely zero supporting evidence, is the claim that a human body contains 10 trillion cells.

And there you have it-- a back-of-the-envelope estimate combined with a totally unsupported approximation to create the very wrong and very widely shared fact that human cells are outnumbered by microbes 10 to 1.

Right about now you're probably wondering what the real numbers are.

First, the original estimate for microbes living inside us was calculated using the volume of the entire intestine.

But the vast majority of your body's microbes live in your colon, which is only a portion of that volume.

And yes, that is where your poop is made.

Using a more accurate volume of the average colon and the number of bacteria that we typically find per volume of poop, in 2016 researchers calculated that your inner microbial population is-- drum roll, please-- 39 trillion, not 100 trillion.

And as for the number of cells in the human body, this is a seemingly simple question that you might assume we biologists have known for a long time.

But the truth is, until very recently, no one really knew.

Over the past couple centuries, estimates have ranged from 5 billion to more than a quadrillion cells in our body.

What makes it so difficult is that our cells vary hugely in size and how tightly packed they are.

So the only way to get a good count is to estimate each organ individually.

And that's what a group of researchers did in 2013.

Based on actual evidence, their new number is 37.2 trillion cells in the average human body.

That makes the ratio of microbe to you more like 1 to 1, pretty much even stevens.

Amazingly, although most of your mass comes from muscle and bone cells, by sheer number, red blood cells make up more than 80% of the cells in your body.

But remember how I said almost all of your inner microbes live in your colon?

Well, you lose almost a third of them every time you have a bowel movement.

So every time you poop, the ratio swings in your favor at least for a few hours until they get their numbers back up.

That doesn't make quite as catchy of a shirt, though.

Things we consider common knowledge can be based on bad information.

And despite the amazing power of science to correct its own mistakes and uncover better and better knowledge over time, that good knowledge doesn't always spread out and replace the bad knowledge.

So how do these scientific urban legends continue to persist?

More scientific journals exist today than ever before.

And we're doing more science today than ever before.

Most of that science is peer reviewed.

But peer reviewed doesn't always mean something is true.

If one false citation makes it into the system, it can set up a domino effect as other people cite that very bad fact instead of verifying the original.

The solution?

Well, for you out there in the general public at least, wherever you can even, if you think something is common knowledge, try to learn where it came from.

You might be surprised by what you find.

But that is easier said than done because most published science today isn't freely available, at least not legally.

Most scientific research today sits behind paywalls.

So even if you wanted to check a source, you couldn't.

Then what about this?

Now, it is easy to dump on Wikipedia.

Anyone can edit it.

And they have an entire page title, "Wikipedia is not a reliable source."

Whoa.

That's a paradox.

Wait, why don't I have a Wikipedia page?

Come on, Kyle.

But Wikipedia represents a collection of our common knowledge.

It's the most widely read and widely accessible information source on Earth.

And at least one study has shown that Wikipedia pages are more likely to cite scientific sources that are freely available.

This isn't an ad for Wikipedia.

It just seems like if you want to get good science out to the broadest audience, making it freely available is not a bad place start.

The point to me, at least, is pretty clear.

If you want common knowledge to be true, you have to let true knowledge be common.

Every one of us carries quite a few pieces of incorrect knowledge in our heads.

And that is nothing to feel bad about.

What matters is being comfortable enough with the idea of not knowing everything that you're able to replace bad knowledge when you find better knowledge.

Stay curious.