How Scientists Build Consensus: Crash Course Scientific Thinking #5

0:00 We've known for nearly a hundred years that vitamin C prevents scurvy.

0:03 So why can't I seem to get a straight once

0:05 and for all answer on whether chocolate lowers my risk of cancer?

0:09 Or what about coffee or soy or even kale?

0:14 These are fair questions.

0:15 Maybe you've asked one of them before yourself.

0:17 Why does science seem to be so sure about some things but not others?

0:22 Hi, I'm Hank Green and this is Crash Course Scientific Thinking.

0:26 I don't care if it's good for me.

0:31 [music] Today we are going to talk about scientific consensus.

0:36 The point when an idea with a strong

0:38 base of evidence gains broad acceptance by scientific experts.

0:42 It's how science moves forward.

0:45 Consensus is way more than an opinion or a vibe

0:49 and also a very different thing from just

0:51 letting your Gen Z colleague rename the group

0:53 chat Meatball Zone because it wasn't worth the fight,

0:56 which is a real thing that happened to me.

0:58 Scientific consensus happens only when a claim

1:00 has been run through the gauntlet of skepticism.

1:04 Scientists [music] have conducted many, many studies.

1:06 They have sifted through the evidence, argued with each other,

1:09 tested an idea again and again with different techniques and approaches,

1:13 scrutinized studies that others have done and reached a point where

1:17 the evidence backs one explanation as far more likely than any other.

1:21 Consensus happens only after an idea has been

1:24 poked and prodded and tested from many angles.

1:28 And getting there isn't easy.

1:30 Building a consensus takes a lot of time and effort.

1:33 Think about atoms.

1:34 Not those atoms, these atoms.

1:36 But of course, I've never seen an atom before.

1:39 Even the highest resolution picture any human has ever taken

1:42 of them just kind of looks like blurry out of focus blobs.

1:45 But scientists agree that atoms exist and make

1:48 up everything because they've worked for centuries

1:51 collecting evidence and performing experiments to form

1:54 a consensus around those tiny little building blocks.

1:57 To learn more about this, let's get some sage advice.

2:01 [music]

2:08 What a fine day to learn about science and puzzles.

2:10 Ooh, puzzles.

2:11 I sense an analogy coming, Sage.

2:14 So the scientific process for building consensus is kind of like putting

2:18 together a jigsaw puzzle but without seeing the picture on the box.

2:21 In the case of atomic theory,

2:22 it started thousands of years ago as this idea that everything in the universe

2:26 was made of tiny unseeable particles with the ancient Greeks called atoms.

2:31 Clever name.

2:32 I like it.

2:33 Now at first it was more

2:34 of a philosophical thought experiment before it was science.

2:37 But then this guy John Dalton started thinking about it more scientifically.

2:42 Exactly.

2:43 Dalton, who was a chemist in the early 1800s,

2:45 started to apply that old philosophical

2:47 idea to his research into chemical reactions.

2:50 See, the existence of atoms helped explain

2:52 certain patterns he was seeing in his work.

2:54 He put the first puzzle pieces on the board.

2:56 He said, "Here's a thing that could explain what matter is made up of.

3:01 It had explanatory power, the ability to explain a phenomenon through evidence,

3:05 and that's one of the most important jobs of a scientific theory."

3:08 But it was still just a few pieces of the puzzle.

3:10 He wasn't anywhere near seeing the whole picture or having scientific consensus.

3:15 Flash forward to the end of the 19th century,

3:17 and new experiments provided evidence for what scientists had long expected.

3:21 There's something even tinier [music] inside

3:24 of atoms that's negatively charged, electrons.

3:27 That theory gained more and more support until we had more puzzle

3:31 pieces and an even clearer picture of what an atom looks like.

3:35 And not long after that, the nucleus was discovered.

3:38 But there were still big questions [music] waiting to be answered.

3:40 Things that we did not understand well enough to agree on.

3:44 Exactly.

3:44 Like electrons are weird.

3:46 So weird.

3:47 And we can't say with absolute certainty where they'll be at any given moment.

3:51 So in 1926, we started visualizing electrons as clouds of probability.

3:57 Scientists agreed based on all the available evidence

4:00 that the best we can say isn't where an electron is,

4:03 but where it's most likely to be.

4:06 Being more precise about what we don't know actually makes the picture clearer.

4:10 And naming areas of certainty and uncertainty is what allowed

4:14 scientists to come to a strong consensus around atomic theory.

4:17 And that's today's sage advice.

4:21 So that's how science agrees about stuff.

4:23 But why are there things science can't agree on?

4:26 At least not fully.

4:27 Like how can there be consensus about stuff as tiny and practically

4:29 invisible as atoms but not about the food I put in my body?

4:34 It can feel like there's a lot of mixed messages around food science,

4:37 but there's actually quite a bit about

4:39 basic nutrition that we do have consensus on.

4:42 We know that vitamin C prevents scurvy,

4:44 like I mentioned before, but also that vitamin D prevents ricketetts.

4:47 We know that fiber is good for us, and too much saturated fat,

4:51 salt, and sugar can be harmful to our health.

4:53 But there can be areas of uncertainty within a broader scientific consensus.

4:57 And that's actually where most modern-day science happens,

5:00 in the nooks and crannies of what we don't

5:03 know within the broader context of what we do know.

5:06 Sometimes this means we're seeing details we didn't

5:09 see before because we didn't have all the pieces.

5:12 Other times it means that our idea of what the picture looks

5:15 like has changed because we've got enough pieces to understand it more clearly.

5:19 It's important to remember that debate may still

5:22 happen about smaller details without disproving the main conclusion.

5:26 that scientific debate is part of the process.

5:28 It's how scientists refine a consensus.

5:31 It's the same with the science of nutrition.

5:33 There are some areas that are tricky to bring

5:36 into perfect focus because they're tricky to measure.

5:39 Like, say that I want to understand once and for all

5:41 whether eating chocolate raises or lowers my risk of getting cancer.

5:46 It wouldn't be impossible, but it would take a lot of large,

5:49 highquality studies and someone who cared enough to actually pay for them.

5:53 And if the effect wasn't very large, it would be hard to spot it.

5:57 Plus, it would still be a challenge to get strong evidence either way because

6:01 there's so much we can't control for when studying people and food and health.

6:04 Like, you can't really ethically design an experiment where a group

6:07 of people are on a very specific diet for a very long time.

6:11 And also, think about all the different types of chocolate out there.

6:14 Not to mention all the other foods that people

6:16 eat that could affect the results of a single study.

6:19 On top of it all, things like exercise, genetics,

6:21 and the environment around you all impact your health, too.

6:24 Forget comparing apples to oranges.

6:26 Even comparing apples to apples isn't easy.

6:29 In other words, it's hard to gather enough evidence

6:33 to push a consensus around chocolate's health benefits toward an agreement.

6:38 So, even though the consensus around

6:39 basic nutrition hasn't changed much for decades,

6:42 there's still a lot we don't know about the interaction

6:45 between an individual's health and the food they eat.

6:48 And that brings me to something I think can feel frustrating,

6:51 but in science is very important.

6:54 Scientists always hold open the possibility that with new evidence,

6:58 an explanation could shift or become more refined,

7:01 just like how our model of the atom has.

7:04 But it wouldn't happen overnight.

7:06 It would take overwhelming evidence to shift a scientific consensus.

7:11 Which is why consensus is so important.

7:13 It's a way of saying okay scientists can agree that there

7:16 will always be a degree of uncertainty in any theory

7:19 but with that caveat they can agree on some things

7:22 that are very very likely to be excellent at explaining the world.

7:26 So where do we as consumers

7:28 of science media [snorts] usually encounter scientific consensus?

7:34 It's often in statements like 99% of scientists agree on something.

7:39 Like 99% of scientists agree that climate change is caused by human activities.

7:44 Let's unpack what that means.

7:45 First off, there's no real threshold for consensus.

7:48 Even though we often say things like 99% of scientists,

7:52 all that really tells us is that we have a strong consensus.

7:56 But you could have a strong consensus with 95% or 83%.

8:00 Overwhelming consensus of any degree means that a claim

8:03 has cleared a very, very high bar.

8:06 It's like we've got 3,000 pieces of this puzzle and 2,996

8:10 are spelling out climate change is real and we are causing it.

8:14 It would not just be weird,

8:16 but wildly unlikely to uncover evidence that overturns this fact.

8:20 It would be as strange as learning that atoms aren't real.

8:23 Now, does this mean that we know

8:24 everything there is to know about climate change?

8:27 No.

8:27 There's still plenty of stuff for [music] scientists to debate and discover,

8:31 but they're working on the unsettled

8:33 details within the broader scientific consensus.

8:36 Scientists aren't debating if climate change

8:38 will bring serious warming to the planet.

8:40 They're debating things like the importance of pee bogs for carbon

8:44 storage or how fast methane [music] breaks down in the atmosphere.

8:48 Science can tell us a lot about

8:49 the world from atoms to nutrition to climate change,

8:52 but it can't tell us everything.

8:54 That's just not how science works.

8:56 Science is the process of becoming less wrong over time,

9:00 of always being open to new evidence,

9:02 but also recognizing when an idea is very good

9:05 at explaining all of the weird phenomena of our world,

9:09 has stood up to challenges from every angle,

9:11 and is extremely unlikely to be wrong.

9:14 Consensus is all the big stuff that scientists agree on.

9:18 They understand atoms enough to know what their properties are.

9:20 They understand nutrition enough to offer overarching health guidelines.

9:24 and they understand climate change enough to know that we are causing it.

9:28 But they still don't have a complete model of the atom.

9:32 They still can't say in every case what chocolate will do to your body.

9:36 And there's more to study about how humans are affecting the climate.

9:39 At the end of the day, scientific consensus is what emerges when

9:43 an idea has consistently explained the evidence,

9:46 predicted outcomes, and stood up to challenge.

9:49 It's the big stuff.

9:51 The things that we keep seeing confirmed again and again.

9:54 We know so much more about the world than we did even a hundred years ago.

9:59 Our knowledge continues to build and grow.

10:02 And those puzzles are getting more complete

10:04 [music] and more detailed all the time.

10:06 But science never stops pushing at those edges of what we

10:10 don't [music] know and asking questions about the stuff we do.

10:15 In our next episode, we're going to take a deeper look at the ways

10:18 you can fact check the world around you.

10:20 I'll see you then.

10:21 This episode of Crash Course Scientific Thinking

10:23 was produced in partnership with HHMI Bio Interactive,

10:26 bringing real science stories to thousands

10:28 of high school and undergrad life sciences classrooms.

10:31 If you are a teacher, visit their website for resources that explore

10:35 the topics we discussed in the video today.

10:37 Thanks for watching this episode of Crash Course Scientific Thinking,

10:39 which was filmed in Missoula,

10:40 Montana, and was made with the help of all these nice people.

10:43 [music] If you want to help keep Crash Course free for everyone forever,

10:47 you can check out our Patreon.

10:48 There's a link in the description.