Name: How Scientists Build Consensus: Crash Course Scientific Thinking #5
Uploaded: 2026-03-03T22:31:26.000Z
Duration: 10 min 59 s
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We've known for nearly 100 years that vitamin C prevents scurvy.

So why can't I seem to get a straight, once and for all answer on whether chocolate lowers my risk of cancer?

Or what about coffee or soy or even kale?

Maybe you've asked one of them before yourself.

Why does science seem to be so sure about some things, but not others?

Hi, I'm Hank Green and this is Crash course Scientific Thinking.

Today we are going to talk about scientific consensus.

The point when an idea with a strong base of evidence gains broad acceptance by scientific experts.

Consensus is way more than an opinion or a vibe, and

also a very different thing from just letting your Gen Z colleague rename the group chat Meatball Zone because it wasn't worth the fight, which is a real thing that happened.

Scientific consensus happens only when a claim has been run through the gauntlet of skepticism.

Scientists have conducted many, many studies.

They have sifted through the evidence, argued with each other, tested an idea again and again with different techniques and approaches, scrutinized studies that others have done, and

reached a point where the evidence backs one explanation as far more likely than any other.

Consensus happens only after an idea has been poked and prepared, prodded and tested from many angles.

Building a consensus takes a lot of time and effort.

But of course, I've never seen an atom before.

Even the highest resolution picture any human has ever taken of them just kind of looks like blurry, out of focus blobs.

But scientists agree that atoms exist and make up everything because they've worked for centuries collecting evidence and performing experiments to form a consensus around those tiny little building blocks.

To learn more about this, let's get some sage advice.

What a fine day to learn about science, Hank.

So the scientific process for building consensus is kind of like putting together a jigsaw puzzle, but

In the case of atomic theory, it started thousands of years ago as this idea that everything in the universe was made of tiny unseeable particles, what the ancient Greeks called atoms.

At first it was more of a philosophical thought experiment before it was science.

But then this guy, John Dalton, started thinking about it more scientifically.

Dalton, who was a chemist in the early 1800s, started to apply that old philosophical idea to his research into Chemical reactions.

See, the existence of atoms helped explain certain patterns he was seeing in his work.

He put the first puzzle pieces on the board.

He said, here's a thing that could explain what matter is made up of.

It had explanatory power, the ability to explain a phenomenon through evidence.

And that's one of the most important jobs of a scientific theory.

But it was still just a few pieces of the puzzle.

He wasn't anywhere near seeing the whole picture or having scientific consensus.

Flash forward to the end of the 19th century, and

New experiments provided evidence for what scientists had long expected.

There's something even tinier inside of atoms.

That theory gained more and more support until we had more puzzle pieces and an even clearer picture of what an atom looks like.

And not long after that, the nucleus was discovered.

But there were still big questions waiting to be answered, things that we did not understand well enough to agree on exactly.

So weird, and we can't see, say with absolute certainty where they'll be at any given moment.

So in 1926, we started visualizing electrons as clouds of probability.

Scientists agreed, based on all the available evidence, that the best we can say isn't where an electron is, but

Being more precise about what we don't know actually makes the picture clearer.

And naming areas of certainty and uncertainty is what allowed scientists to come to a strong consensus around atomic theory.

So that's how science agrees about stuff.

But why are there things science can't agree on, at least not fully?

Like, how can there be consensus about stuff as tiny and practically invisible as atoms, but

It can feel like there's a lot of mixed messages around food science, but

there's actually quite a bit about basic nutrition that we do have consensus on.

We know that vitamin C prevents scurvy, like I mentioned before, but

also that vitamin D prevents rickets, which we know that fiber is good for us, and

too much saturated fat, salt and sugar can be harmful to our health.

But there can be areas of uncertainty within a broader scientific consensus.

And that's actually where most modern day science happens, in the nooks and crannies of what we don't know within the broader context of what we do know.

Sometimes this means we're seeing details we didn't see before because we didn't have all the pieces.

Other times it means that our idea of what the picture looks like has changed because we've got enough pieces to understand it more clearly.

Important to remember that debate may still happen about smaller details without disproving the main conclusion.

That scientific debate is part of the process.

It's the same with the science of nutrition.

There are some areas that are tricky to bring into perfect focus because they're tricky to measure.

Like say that I want to understand once and for all whether eating chocolate raises or lowers my risk of getting cancer.

it would take a lot of large, high quality studies and someone who cared enough to actually pay for them.

And if the effect wasn't very large, it would be hard to spot it.

Plus, it would still be a challenge to get strong evidence either way, because there's so much we can't control for when studying people and food and health.

Like, you can't really ethically design an experiment where a group of people are on a very specific diet for a very long time and also think about all the different types of chocolate out there, not to mention all the other foods that people eat that could affect the results of a single study.

On top of it all, things like exercise, genetics, and

the environment around you all impact your health too.

Even comparing apples to apples isn't easy.

In other words, it's hard to gather enough evidence to push a consensus around chocolate's health benefits toward an agreement.

So even though the consensus around basic nutrition hasn't changed much for decades, there's still a lot we don't know about the interaction between an individual's health and the food they eat.

And that brings me to something I think can feel frustrating.

Scientists always hold open the possibility that with new evidence, an explanation could shift or become more refined, just like how our model of the atom has.

It would take overwhelming evidence to shift a scientific consensus, which is why consensus is so important.

It's a way of saying, okay, scientists can agree that there will always be a degree of uncertainty in any theory, but

with that caveat, they can agree on some things that are very, very likely to be excellent at explaining the world.

So where do we as consumers of science media usually encounters scientific consensus?