Emojis of laughter, hearts, and thumbs-up.

The Snowball Effect

Why do some companies become household names, while others flame out? How do certain memes go viral? And why do some social movements take off and spread, while others fizzle? Today on the show, we talk with sociologist Damon Centola about social contagion, and how it can be harnessed to build a better world.

Additional Resources


Change: How to Make Big Things Happen, by Damon Centola.

Claudette Colvin: Twice Toward Justice, by Phillip Hoose. 

Research Studies:

Beaman, Lori, et al. “Can Network Theory-based Targeting Increase Technology Adoption?” 2021, conditionally accepted, American Economic Review

Centola, Damon, et al. “Experimental evidence for tipping points in social convention.” Science. 2018 Jun 8;360(6393):1116-1119. 

Centola, Damon. 2013. “Social Media and the Science of Health Behavior.” Circulation. (127) 2135-2144. 

Granovetter, Mark S. “The Strength of Weak Ties.” American Journal of Sociology, vol. 78, no. 6, 1973, pp. 1360–1380. SNAP: Stanford.

Grab Bag:

Friday, by Rebecca Black, YouTube, 2011.

Bill Gates doing the ice bucket challenge, YouTube, 2014.

The transcript below may be for an earlier version of this episode. Our transcripts are provided by various partners and may contain errors or deviate slightly from the audio.

Shankar Vedantam: This is Hidden Brain, I'm Shankar Vedantam. Back in 2011, a 13 year old girl posted a music video on YouTube. It sat there for about a month without getting much notice. But, then, something totally unexpected happened.

Rebecca Black (singing): It's Friday, Friday, gotta get down on Friday!

Shankar Vedantam: The girl's name was Rebecca Black and the song she was singing in the video was called Friday. More and more people started watching it, leaving comments and sharing it. The song went viral. Friday became the most watched YouTube video of 2011 with 167 million views. Plenty of people watched the video to make fun of it, perhaps not knowing or caring that Rebecca Black was only 13-years-old. But a decade later, she's gotten the last laugh with a new remix of the song. That version, it's also gone viral, with millions of views in the first few days after its release. This week on Hidden Brain, what makes certain songs, memes, and even social movements go viral? We look at why they spread and how social contagion can be harnessed to build a better world.

Shankar Vedantam: How is it some social movements take off and spread, while others find themselves stuck? Why do some companies soar and become household names, while others flame out? Can you predict success and failure, or is it all a matter of luck? These are questions that have long fascinated Damon Centola. He's a sociologist at the University of Pennsylvania, and the author of the book, Change. Damon Centola, welcome to Hidden Brain.

Damon Centola: Thank you for having me.

Shankar Vedantam: I want to start with one model of how something can spread very, very rapidly. In late 2019, Damon, a few people got infected with the COVID-19 virus in Wuhan, the Chinese city. Paint a picture for me of how a virus can go from infecting a small number of people in one Chinese city to a global pandemic.

Damon Centola: Yeah. The way that viruses typically spread is through social contact, so that means that a few people in Wuhan will infect their friends and neighbors, the people they interact with directly. But it also means that if any one of those people gets onto an airplane and travels, say, to Tel Aviv or to Berlin, then that infected person can infect the people around them on the airplane just by mere proximity. And then, obviously, those people, when they get off the plane, will infect their friends and neighbors and so forth. And if any of those people get on an airplane, then, of course, this spreads to a new part of the world. And so when we think about how the spreading process grows and how it spreads from one town to the globe, we really sort of can imagine it as a series of firework explosions, where one infected community basically infects everyone that they come in contact with and then all of those people become sort of the center of their own fireworks explosions, and what you get is this cascading series of fireworks that, very quickly, through rapid communication and transportation networks, spread around the world.

Shankar Vedantam: So one of your central contentions is that the way people are connected to each other, their social networks, influence how things spread or how they fail to spread, but let's just drill down a second deeper. Let's look at the social networks that allow a virus to spread exponentially. Let's say I'm at an airport, you're at an airport, we're sitting next to each other at a gate waiting for our flights. We don't know each other, we don't talk to one another, but this chance meeting allows a virus to leap between us. So for this network, transmitting the virus requires very brief contact between us, almost like a glancing connection, right?

Damon Centola: Yeah, that's exactly right. And in many ways, we think about other kinds of contagions that way too, when we think about the spread of information. If you're sitting next to me at an airport and you tell me about a great new NPR show or a great new app that I can download, then I may do that and I can tell someone else that once I get home, and that's exactly how diseases spread as well. And so we can think about this epidemiological model, the spread of diseases, as a metaphor or as an underlying guide for thinking about all kinds of spreading processes and how just simple contact with a stranger can transmit really anything around the world.

Shankar Vedantam: So it's very appealing to think that most things spread like viruses. So you have the example of the COVID-19 virus or a song like Friday and it's easy to think that maybe everything spreads this way. Can you talk for a moment about how this fairly simple model of how things spread, how that model, in some ways, has, if you will, gone viral?

Damon Centola: So the notion of influencers, which are highly connected people that we're all familiar with on social media sites, like Instagram and Facebook and Twitter, and we tend to think of these influencers as the key to spreading any kind of innovation, idea or social change process. And that comes from viral thinking because if one person who interacts with a lot of other people gets COVID-19, then that person is going to be a super-spreader for the virus. So this idea that a highly connected person can be very effective for spreading something to lots of other people, and therefore, kind of set off an epidemic all by themselves, is an intuition that sits underneath viral marketing campaigns and the whole sort of influencer marketing approach. And so for nearly a century, actually, the assumption has been that everything spreads just like a virus does.

Shankar Vedantam: Yeah. And I feel like you hear this narrative very often in the news. You cite the example of Oprah Winfrey sharing word in April 2009 about a new social media platform called Twitter. Listen to this tape about that moment.

Oprah Winfrey: Hi, it's Friday Live and I'm on Twitter for the first time. Sitting next to me is Evan Williams, the co-founder of Twitter. Can you believe all this tweedlee dee stuff going on? How did it come to life?

Shankar Vedantam: So, Damon, what is the conventional story of how Oprah's first tweet produced exponential growth of the Twitter platform?

Damon Centola: It's exactly what you would imagine, which is that Twitter wasn't growing very fast and then what we see in the timeline after and around Oprah's adoption of Twitter is that Twitter has, by, I think, at the end of that month, 28 million users.

Shankar Vedantam: Mm-hmm (affirmative).

Damon Centola: And so the natural thing to do or the kind of intuitive justification or rationale for what we see happening is to say oh, that company was successful in getting a high profile influencer to endorse their product and as a result, their product was then adopted by a lot of people.

Shankar Vedantam: So I want to spend a moment defining a couple of terms that are going to come up repeatedly in the conversation. Let's return to that moment in the airport. Sociologists call the connection that you and I had at the gate a weak tie. What are weak ties, Damon? And can you give me some other examples of weak ties?

Damon Centola: Sure. I should mention that the term "weak ties" and its complement, "strong tie" come from the sociologist Mark Granovetter, who is personally one of my intellectual heroes. He defined these terms back in 1973. Now the glancing contact that happens in the airport, Mark Granovetter referred to as a weak tie, because if you and I are sitting next to each other in an airport, we're not likely to know each other's friends or family, we're not likely to even ever meet again. But that connection can act as an important, in some senses, vital transmission point for a piece of information from your community to my community. And so what Mark did that was so interesting is noticed that these weak ties that don't play a huge role in how we conceptualize or think about our social networks are largely invisible, but nevertheless, they're playing this important role for the transmission of ideas and information and diseases, of course, from community to community and across nations and ultimately, across the world.

Shankar Vedantam: So Mark Granovetter's influential paper was called "The Strength of Weak Ties" and besides being a very catchy title, it was about this very important idea that weak ties, in some ways, can spur things to travel very quickly and spread very fast. Can you give me some examples of the strength of weak ties in everyday life?

Damon Centola: Yeah. Mark's actually initial example was looking at how people find out about jobs and, of course, when you're looking for a job and you're unemployed, you talk to your friends, you talk to your family, you sort of try to source out what available options there are in what you think of as your network. But, oftentimes, everyone in your community knows about the same openings because everyone in your community has the same friends and they know the same people, and so it's very hard to learn about a new job opening or a new opportunity from the people that you already know. And so what Mark realized was that when people learned about new opportunities, it's often from these glancing contacts, where you're just talking casually in the airport and someone mentions, "Oh, I think that this division at Microsoft is hiring," or "I heard that actually, Amazon's opening up a new branch in this area and that might be interesting to you," that your friends and family might not know about, but that is, of course, incredibly relevant for you doing a job search. And so these weak ties are incredibly effective and you can think of them as efficient for information transmission. As new information comes up, it's very easy for that to spread across these sorts of casual contacts.

Shankar Vedantam: You started to see evidence that this model of how things spread can have serious problems, and I want to point to a couple of moments of insight that you say that you had. You grew up in a Quaker community, where you were part of a lot of marches for social justice, but you noticed that your classmates at school didn't always care about the same issues. How did this speak to the ability or the inability of weak ties to spread ideas?

Damon Centola: Yeah. Well, what I noticed was that across different groups or communities, it wasn't just they didn't care about them, they just didn't even know about them. There wasn't engagement. And if someone were to talk about an issue, like water conservation or fair gender practices in the workplace, at least in my middle school, it wasn't something that people understood. And so you could announce that there's some march or rally or you could talk about the importance of these issues, but it would largely fall on deaf ears because the community of classmates and the way that people talked about topics didn't really engage with or understand those ideas.

Shankar Vedantam: If everything spreads like a virus, how is it some ideas don't leap from group to group? Damon was exposed to ideas about social justice at home, so why didn't they leap through him to his classmates? Why didn't he, if you will, infect them with those concepts? Damon also noticed something else. Over time, some ideas that were popular in one community did spread to others, but the way this happened did not look anything like a virus. Sometimes, it took decades for those ideas to spread. Take, for example, the support for renewable energy or gay rights.

Damon Centola: I noticed that some of those ideas actually had caught on by the time I started college, a lot of people in the mainstream were talking about some of these issues in a regular, everyday way. They were normal topics to think about. One of the most striking examples was the organic foods movement, which was a very marginal sort of fringe idea in the early 1980s that you would go to a grocery store and only buy organic wholegrain foods and had to go to a bulk bin to get it. And a decade later, it was normal for grocery stores to have an aisle that was reserved for what they called health food, then, of course, Whole Foods came along. And so the influence of these peripheral movements on the mainstream was surprisingly effective for some things, but never landed for others, right? Movements like water conservation, which was also a big one among some of the activists that I knew growing up, never really caught on in the mainstream in the US and still haven't. And so the question that was raised for me, which I think is the really important question, is why? Is it just random or is there some underlying sort of scientific explanation for the success of some movements and the failure of others?

Shankar Vedantam: As a young scholar, you started to examine the growth and success of the Civil Rights Movement and you started to see problems with the conventional story of viral spread of how that movement took off. Can you describe that insight to me using the examples of Rosa Parks and a woman named Claudette Colvin?

Damon Centola: Yeah. The Civil Rights Movement became one of the most interesting areas for me to study because of how the story is told, that with the Montgomery bus boycott, Rosa Parks refused to move to the back of the bus and she was arrested and, in many senses, this becomes a kind of galvanizing moment for the Civil Rights Movement. And so the story that's told, which is analogous in many ways to the sort of story of Twitter growing because of its moment on Oprah, the story that's told is one person stands up and then a movement follows and then the whole system of racial segregation collapses. But the truth of it is that lots and lots of women stood up intentionally against racial segregation on public buses. Claudette Colvin is one of them, but there are dozens of others, of women who took a stand against that racial injustice, but they were arrested and nothing much came of it.

Shankar Vedantam: Claudette Colvin was 15 years old when she was arrested and dragged off a bus in handcuffs in Montgomery, Alabama. This happened nine months before Rosa Parks did exactly the same thing.

Claudette Colvin: When the news reporters asked me why didn't I get up, I said, "History had me glued to the seat." I said, "It felt as though Harriet Tubman's hands were pushing me down on one shoulder, and Sojourner Truth's hands were pushing me down on another shoulder."

Shankar Vedantam: When we come back, how social networks explain the difference between the impact of Rosa Parks and the impact of Claudette Colvin.

Shankar Vedantam: This is Hidden Brain, I'm Shankar Vedantam. Damon Centola is a sociologist at the University of Pennsylvania. He studies how change unfolds within communities, within companies, within countries. He argues that the conventional models we have about virality don't describe why one social movement takes off, while a second stalls. Damon, you draw a very important distinction in your book between what you call "simple contagion" and "complex contagion." What do you mean by those terms?

Damon Centola: Well, if we think of social change as a contagion process, some people accept an idea and then the people they come in contact with it accept it and so forth, it's natural to think of that in a viral way, spreading just like a virus would. And that is really the notion of simple contagion, which is just about everyone that an infected or activated or excited person comes in contact with also becomes excited and activated. And so that's how a meme spreads or that's how a contagious news story spreads from person to person. And when we think about simple contagions, what we're really talking about is ideas that are familiar, products that we already really know, things that make sense to us intuitively. And so when we see a new one, it's not a lot of work, emotionally or cognitively or any other way, to just adopt it and pass it on. It just seems normal. Complex contagions, by contrast, are the kinds of social change initiatives or the kinds of ideas or the kinds of products that we resist initially, that aren't familiar, that are weird or a little bit different. Just coming into contact with one person who's advocating for it or who has adopted it isn't enough to convince us that we should do it too. And a great example of this is something like joining a protest march, where you might join a protest march because it's a good idea and you're onboard with the movement. But if you have no idea about the movement, you're not oriented to it all, you don't really care about it, and if there's the possibility of being arrested or the possibility of police retaliation, there's some risk involved with that choice. And so just because someone you know is doing it doesn't mean you're going to be convinced that you should do it too. And this is sort of the big difference between a simple contagion and a complex contagion. For a simple contagion to spread, all it takes is contact with one person. You're just basically finding out about it. You found out about the new meme and you can watch it and spread it. But for a complex contagion, you need to be convinced.

Shankar Vedantam: One of the hallmarks of complex contagions is they require people to accept certain costs. If you want people to change their behavior in some important way, you want them to recycle more or drive more slowly or practice safe sex, these changes can involve personal costs. They're not like sharing a video of someone doing the ice bucket challenge or passing on the Friday song, which has almost zero costs. Why would it be that the social networks that prompt people to share cat videos are different from the social networks that can get someone out on the street to protest racial injustice? To explain, Damon cites an example from 2013, where the Human Rights Campaign launched an initiative on Facebook to build support for same-sex marriage.

Damon Centola: Because the Supreme Court was hearing this case about same-sex marriage, and the initiative was to have people change their profile photo to an equals sign that was pink and red to show support for the movement.

Woman speaking about equal sign profile picture: : And then I started looking through my feed and I just couldn't believe it, it was literally just a red symbol over and over and over again. It was about 24 hours when we knew that we were really onto something, that it had gone viral.

Damon Centola: Now, this initiative spread to almost three million Facebook users in under a week, so that sounds like something that's viral, but some data scientists and physicists looked more closely at the data and what they saw was that the spreading of this social contagion, this sort of change initiative to support same-sex marriage, was a complex contagion, even though it spread really fast. And the signature of this complex contagion was that before people adopted it from one of their neighbors, they waited for more of their neighbors to adopt it. And if you think about your life on Facebook, you're connected to lots of different people. You may see your grandparents, you may be connected to your parents, your friends from college, your friends from high school, people you know from work, people from your neighborhood, and all of those people are sort of evaluating your behavior. So, there's some sort of social risk on adopting something that's kind of controversial and may not be accepted by these people, or you may even run the risk of starting in a flame war or an argument on your social media site. And so what people did is they waited until two, three, four, five of their contacts adopted and then that gave them enough confidence that this new behavior was legitimate and then they would adopt too.

Shankar Vedantam: Mm-hmm (affirmative).

Damon Centola: Now, that idea that you need social reinforcement from several people to convince you to adopt something means that you need networks that create a kind of reinforcing pressure from peers. If we remember, when we think about the spread of the coronavirus, it spread like a fireworks explosion. So one person adopted it and infected lots of people, and then those people infected lots of other people that the first person may not know and so on and so forth. And so a fireworks explosion doesn't have a lot of redundancy in it. It's got a ton of reach and so it can get across the world really fast, but it doesn't have a lot of that kind of social convincement from people adopting and kind of sharing one idea with each other and creating a lot of reinforcement for it.

Shankar Vedantam: You've actually touched on several different ideas here that are really important, that we should actually unpack somewhat slowly, Damon.

Damon Centola: Okay.

Shankar Vedantam: At the core of your book is a very important insight, the central reason people change their behavior in any important way is because they see the people around them changing their behavior. So psychologists sometimes call this social proof. I get on Facebook because other people are on Facebook. I march on the streets because I see other people like me marching on the streets. If you apply that insight to the science of change, the real question to ask might not be how do I make this person change, but how can I show this person that the people around them are changing? Is that, in some ways, the goal of the person who's actually trying to drive this kind of complex contagion?

Damon Centola: Yeah. There are two goals. One is, of course, to have new adopters get excited about the idea, and one way to do that is to have several of their peers and contacts adopt, which creates credibility for the idea and makes it seem more accepted by their friendship group and can also generate emotional excitement about getting onboard. But there's a second part, which is that if the idea is fairly contentious, people who adopt early on, if they see that no-one else has adopted, may wind up giving it up. They may wind up saying, "Well, this is actually not a good thing for me to be a part of because no-one else is supporting it." And so having connections to existing adopters and to people who are showing support for an idea keeps people engaged and it sort of sustains membership in the idea. So if you think about it, it's really more like a fishing net than a fireworks display. And so in a fishing net, you've got lots of triangles connecting these different neighborhoods and people know people and they know their friends of friends and so forth. And that redundancy gives people confidence that the idea's worthwhile or that this new behavior is accepted, and then allows them to coordinate with other new adopters to then convince other people to adopt and so on and so forth.

Shankar Vedantam: Mm. So you ran an interesting experiment that sort of tested the power of redundancy in driving behavior. In some ways, it sort of compared what happens in a network that is in the shape of fireworks, where I'm talking to three people, those three people are talking to three other people and so on, versus a fishing net example, where I'm connected in a network with other people and I'm constantly hearing from the same people in the network over and over again. You ran an experiment trying to drive a health intervention, looking at these two different kinds of social networks. What did you find about the relationship between redundancy and the effectiveness of actually getting people to change their behavior?

Damon Centola: Yeah. Well, what I did is created a fireworks explosion network with about 150 people in it and then a similar network for the same number of people, but everyone was connected into a fishing net. And everyone was given the opportunity to know that they would learn about a new health technology, and then I put the health technology with one person into each network and then wanted to just see which of those contacts would go sign up for that new health technology.

Shankar Vedantam: Which social network would trigger behavioral change, the one organized like a fireworks display, where people are connected to others in an expanding chain, or the fishing net, where people were connected in a closed loop? Damon found that although information spread more slowly in the network organized like a fishing net, more people in that network actually changed their behavior.

Damon Centola: Yeah. I think that although it took a little bit longer to reach around the network in terms of the informational signal, the actual growth of adoption spread much faster and much farther across these fishing net networks than across the firework network. Now, I ran this same experiment six different times and I got the same result, which means you can draw a very powerful kind of conclusion, which is you can make a strong scientific claim that by structuring a communication network among a group of people online, you can change people's likelihood of adopting a new behavior.

Shankar Vedantam: So just to step back and look at the big picture for a second. We started by talking about viruses and how viruses spread, and we showed that viruses spread through this network of weak ties. People know each other slightly, they have these glancing contacts, very quickly, something can go from a city in China to all the way around the planet. And then we looked at this other kind of change, which involves much more difficult behavioral change, whereas people actually have to change their behavior or their attitudes or their norms in some important way. And in order to do this, they often need reinforcement not just from one, but from multiple people who are connected to them more closely, sort of a network of strong ties. And you make the case that for complex contagion, which is actually the kind of change that most of us are actually talking about when we talk about change, you actually need these strong ties rather than the network of weak ties. When you looked at something like the Civil Rights Movement, did you find that people changed their behavior through weak ties or through strong ties?

Damon Centola: Yeah, that was one of the most striking findings about the Civil Rights Movement is that the evidence all works in favor of strong ties. And that was a revelation at the time because the theories had all said that social change and, in particular, the Civil Rights Movement would be explained by weak ties in social networks, it would allow information about activities to spread far and wide. But it turned out that although the information spread far and wide, actual participation didn't. Participation spread through friends of friends who were clustered together, who could provide social reinforcement, who could reduce the risk of participation and ultimately, could generate a strong emotional contagion among new participants and grow a movement from a cluster in the periphery of the network to reach across the nation.

Shankar Vedantam: Now, all of this, in some ways, produces a puzzle, Damon. If people want to see others change before they change, the reasonable question is how does anything ever change, because what gets the ball rolling? So some people might say okay, that's where the celebrity or the influencer comes in, that's where Oprah comes in. If a celebrity comes out in favor of something, others will follow. You call this a myth, in many ways. Why are celebrities not very effective at sparking behavioral change? And they might be effective at sparking awareness, but why are they not very effective at sparking this kind of complex contagion?

Damon Centola: Well, there are two things you need to think about when you're dealing with what we refer to as influencers, and one is that what makes influencers so effective for spreading simple contagions is they've got lots and lots of ties. But when it comes to complex contagions, again, ideas that are unfamiliar or ideas that maybe go against existing norms, the social influencer is highly observed. A lot of people are paying attention to their behavior, which means they're fairly careful about deciding what they're going to do and not do. Someone doesn't become highly connected by ignoring their social ties, they become highly connected by paying a lot of attention to what people expect of them. And this is true in entertainment and it's also true in business. And so what that means is that when people are evaluating which new ideas to take on, which new products to adopt, particularly products that are public, products that people talk about, they have to take into account how everyone around them is going to sort of perceive that decision, which means that when it comes to complex contagions, all of the non-adopters are actually forces against adoption, they're not just neutral.

Shankar Vedantam: This is a crucial idea. Remember how we said the central goal in behavior change is to show people that those around them are changing? Turns out the same thing goes for celebrities and influencers. They wait to see what their fans think. It may be counterintuitive, but if they don't see those people changing, they are hesitant to go first.

Damon Centola: And so this makes highly connected influencers into, in some cases, a kind of a roadblock and you see that most of the successful initiatives for social change, and this includes novel technologies like Twitter and Facebook, but also includes social movements like Black Lives Matter, Arab Spring and the Civil Rights Movement, grew from clusters of people in the network periphery who kind of grew a critical mass, also they had enough support, that they wind up tipping the center of a network and then it spread to everyone. But the origin story is always in the periphery, not in the center.

Shankar Vedantam: You can see the same thing in the rise of former president, Donald Trump. When he first announced his candidacy for president back in 2015, most people in the Republican establishment laughed him off. Some media organizations declined to cover his campaign because they thought it was a joke. But Trump caught on with people in the periphery of the Republican Party and gradually, his growing support from the margins overwhelmed the resistance of the establishment. The change did not start from the center and move out, it started from the outside and moved in. Damon says the exact thing happens outside the political sphere. Take the example of something we talked about earlier, the rise of Twitter.

Damon Centola: Yeah, the origin story of Twitter is quite interesting because it is a story of peripheral growth, but it's also a story, surprisingly, of neighborhood growth, which, when you think about internet technologies and social media, it's almost a truism that they would spread in the ether and not be constrained at all by local neighborhood geography. But, in fact, Twitter grew initially in the San Francisco Bay Area just through sort of friends and neighbors adopting from each other. And so that's sort of the strong tie, complex contagion story, but it gets more interesting because what ultimately happens is that there's an earthquake one day and all of a sudden, it's not a big earthquake for the Bay Area, but, still, it's an earthquake, and Twitter became very, very useful instantly because people could coordinate with one another to figure out where shocks and aftershocks were hitting, what their expectations were for what to do and where family members were and how they were handling it. And so this sort of network really kind of galvanized. And then what happened, that's pretty striking and is most surprising, is that Twitter, once it caught hold in San Francisco, didn't jump to LA or Chicago or New York, it actually started to spread across the countryside of California, it started to spread kind of south to San Jose and sort of north to Marin and to sort of the east to other parts of the Bay Area and surrounding communities. And what you saw was that, as Twitter grew, it wasn't spreading virally through networks online, it was spreading spatially through neighborhoods. And then something really unexpected happened, which is that as it filled out around the Bay Area, then Twitter showed up, again, not in New York or LA, but it showed up in Cambridge, Massachusetts and that's where it sort of next took hold. And the reason is because a lot of people who live in San Francisco and Cambridge went to school together, they went to college, they studied technology and they work in these different sectors, either along Route 28, 128 in Boston, which is a major technology area, or in Silicon Valley. And so these two areas wind up being highly connected, socially, with lots of reinforcing ties across them and essentially form a network that's like a fishing net going from one part of the country to another. And then once it landed in Cambridge, it spread out again, locally, in Boston around the Boston area. And then when it sort of amplified and took off and jumped from Cambridge, Massachusetts to Seattle, which is another high-tech hub where people were connected to other people and so forth and sort of gained traction in more and more cities. Then, ultimately, it reached a tipping point around January 2009 and once it hit that tipping point, it just took off with hyper-exponential growth, month after month. It went from eight million to 20 million to 28 million very, very quickly. And that's the point in which Oprah adopted was during that period of massive growth. And even after that period, it continued to grow, but didn't grow as fast. That was the window for Twitter's major explosion. And so Oprah's sort of at the tail end of that process, not at the beginning.

Shankar Vedantam: And it's almost as if to get the celebrity endorser ... The celebrity endorser is coming onboard not in order to spur the avalanche, but because the avalanche has actually already gathered momentum. In some ways, the celebrity influencer is very often surfing the avalanche that's already formed, not the one who's actually generating it. That's what I'm hearing from you.

Damon Centola: Yeah, and I think that's an important way to think about social change more generally. It makes sense that someone at the center of the network with lots and lots of connections and lots of reputational skin in the game, right, high stakes for the reputation is going to wait until something gains enough traction in the periphery of the network, that there's a substantial critical mass that convinces them that this is actually a real thing that they should invest in as well. And so it makes sense that the costs for someone who's highly connected or higher in some way, so they wait longer. Whereas the people in the periphery are sort of more easily excited and engaged by these sorts of ideas that are contentious or novel or different, and it's one of the reasons why social change almost always originates in the periphery of the network.

Shankar Vedantam: How does this idea that change starts at the periphery apply to the rise of one of the largest social movements in recent years, Black Lives Matter? In 2020, the movement spread rapidly after George Floyd was pinned down and killed by Minneapolis Police.

News Clip: Protests breaking out in Philadelphia, Washington, D.C., Atlanta, and Oakland, California.

News Clip: As outrage spreads over the killing of George Floyd in the US, protesters have taken to the streets around the world.

News Clip: In Brazil, demonstrators have been raising their voices against the killing of black people mainly in poor neighborhoods. Demonstrations are continuing this weekend, with protests held in East Asia and large crowds expected in European cities.

Damon Centola: I think the tragic death of George Floyd is one of the moments that, I think, our generation will never forget. But what's interesting and, I think, what's most important for understanding Black Lives Matter is that people often explain that process by saying well, there was a viral video of a white police officer killing George Floyd and that's what set off this sort of public outrage.

Shankar Vedantam: But Damon reminds us that six years earlier, another video, every bit as disturbing, also made headlines. It showed the death of Eric Garner, a 43 year old black man, at the hands of a white police officer in New York City. When that video became public, thousands of people expressed their outrage on social media.

Damon Centola: And the term Black Lives Matter, that hashtag was used about 600 times, but no major social change and certainly nothing on a global scale emerged. The real change for Black Lives Matter, when it became a movement that galvanized not just the nation, but also the world, was in August 2014. Michael Brown was killed in Ferguson, Missouri. And the important thing to understand about what happened to make Black Lives Matter such a powerful movement is that the networks of people talking about this topic were really quite disconnected prior to Ferguson. There were communities of Black activists, there were communities of white liberals, communities of Black youth, communities of celebrities, communities of international media and white US media, and all of them were talking about these topics, but internally. Basically, a series of different bubbles. And then what wound up happening over the course of the several days or the week that the protests erupted in Ferguson is that the people who really got involved in social media and who really took over the conversation were regular citizens, just college students in the street. Posting notifications about what was happening and the police escalation and other citizens chiming in, talking about ways in which we could interpret the news that was reporting on this. Because at the beginning, the conversation that was happening on social media through news sites, like CNN, referred to the initial protests as a mob that was protesting the alleged shooting of a man. And immediately, just members of the community, regular people started responding to these tweets and engaging directly with mainstream media, saying, "Why mob? Why not citizens or why not community members?" And he had just graduated from high school, he wasn't a man, and the alleged shooting was an actual shooting. And so this conversation had never really taken place before and as media showed up and started engaging online, something really interesting happened in Ferguson, which is that activist groups started interacting directly with mainstream white media. And then black youth, who weren't really engaged in these activist conversations, started becoming engaged as well, along with the regular citizens who were on the street in Ferguson and other citizens from around the country. And so what wound up happening was that these bridges between these different social groups became wider, the relationships became thicker and people started to kind of influence each other. And one of the early victories that didn't get a lot of attention at the time, but it was really important for the growth of Black Lives Matter, was that by the end of that first week, mainstream media was reporting on those protests differently. They were referring to Michael Brown as someone who had just graduated high school, they were talking about it as a shooting and they were talking about it as community members and citizens protesting rather than a mob. And so, all of a sudden, hashtag Black Lives Matter, which had only been used 600 times prior to Ferguson, was used over a million times and it ballooned into something where all these different communities were coordinated on a way of talking and a way of thinking that identified police violence and civil infractions as something that united all these different deaths. All of a sudden, New York City had something specific in common with Ferguson, Missouri. And then in the months after that, others, Sandra Bland and Tamir Rice, those deaths, instead of being isolated incidents, became unified under a core message of the Black Lives Matter Movement and everyone understood it that way. And this really changed the national conversation. All of a sudden, by 2019, the hashtag Black Lives Matter was used 17,000 times a day. It just became this sort of consistent way that people across different groups were talking about it, and that actually set the stage for what happened in 2020.

Shankar Vedantam: What do you think it is that when we hear about these incidents and we look at the Black Lives Matter Movement or we hear about the Rosa Parks story and we see the Civil Rights Movement, what do you think prompts us to want the simple story that basically says Rosa Parks refused to give up her seat in the bus and that triggered the Civil Rights Movement, or George Floyd was killed and that triggered the Black Lives Matter Movement instead of seeing the very complex social networks that had to be created in order, in some ways, for these flashpoints to have the effects that they did?

Damon Centola: The way of thinking that's familiar to us is thinking about spreading as a viral process or as an epidemic process largely because we understand it so well. There's been just a century of science explaining how contagion happens and explaining what we can expect as a fluid spreads around the world, and so we use those as metaphors for interpreting things that we don't understand and I think that's entirely reasonable. One of the exciting things about the science of sociology is that it's really only in the last two decades that we've been able to do experiments or analyze large-scale data in real time. And that's what social media has given us, is a way of sort of observing these processes in real time and to see how they unfold and really show that the same kinds of theories that we can use to develop new models and new policies are the same kinds of theories that really explain changes in the world around us, like Black Lives Matter.

Shankar Vedantam: The idea that a group of people on the periphery can change entire societies is powerful. It's like the image of a rolling snowball that eventually produces an avalanche. So how many people do you initially need to get that snowball rolling? How small can that committed minority be to drive change? The answer will surprise you. That's after the break. You're listening to Hidden Brain. I'm Shankar Vedantam.

Shankar Vedantam: This is Hidden Brain, I'm Shankar Vedantam. It happens all the time. A company, an idea or a social movement comes out of seemingly nowhere and becomes widely popular. Sometimes, even the people with the idea don't fully understand what happened. They've been laboring for decades in obscurity before suddenly striking it big. Sociologist Damon Centola has explored how such revolutions unfold. In his book, Change, he offers a roadmap for policymakers, entrepreneurs and change agents on how to deploy social networks to bring about change. Damon, we've looked at different models of how things spread. At a very simple level, a virus can spread through a network of casual acquaintances, what sociologists might call weak ties. Some ideas spread because a celebrity makes an announcement, but merely getting information to spread far and wide does not always translate to action. If changing behavior is your goal, it often makes more sense to generate change in small groups on the periphery and slowly get the movement to expand from there. You've run experiments where you test whether a committed, small minority can change the behavior of people in a group. Do you find that they can? And how small can this committed minority be to actually have an impact?

Damon Centola: The study we ran examined different sizes of committed minorities or groups who had tried to advance a social change, and we looked at groups from 17 percent through 25 percent up to 30 percent to see what fraction of a population would be required to change everyone else's mind, and what we found was a really clear cut off at 25 percent. Pretty much every group below 25 percent, whether that was 17 percent or 20 percent or 21 percent, had almost no impact on the behavior of the rest of the group. But as soon as the committed minority or the activists reached 25 percent, everyone else in the group changed their behavior to do what the committed minority was doing. And this worked, of course, for 28 percent and for 29 percent and for 30 percent. And so this provides kind of a really clear cut off point when we can say even though a group that was at 22 percent seems hopeless, it seems like they're not making much impact, with just a little bit more support, they can actually have an enormous impact on what everyone else is doing.

Shankar Vedantam: So the fascinating thing is when we think about these tipping points, this might actually explain why it is a movement might be struggling in obscurity for a long time. And then it reaches this tipping point and suddenly, something changes and something that seemed impossible seems like it's inevitable.

Damon Centola: Yeah. It really answers the puzzle, why is social change so abrupt? Because it seems intuitive to think well, something gains traction and it should sort of increase incrementally, so it will get more and more and more popular and then eventually, everyone will be doing it. But we don't really see that with social change. We see, basically, that there's a hidden process that most people are not aware of. There's a fringe movement or there's an activist group that seems not to be having much impact and then almost all of a sudden, everyone sort of takes this idea seriously. In 2014, people were polled about their opinions and they didn't support the protest and they thought that there was really no problem of policing in black communities. And then after George Floyd in 2020, the vast majority of Americans, both Republicans and Democrats, said that they thought that the Black Lives Matter protests were justified and there was a problem with policing in black communities. And that's an enormous change in sort of public opinion and public consciousness in just six years.

Shankar Vedantam: You can see many other examples around the world of the power of social networks. In one study, the researcher Lori Beaman analyzed an intervention in the African country of Malawi. The government wanted to spread use of a new farming technique. The researchers found the most effective way to spread the idea was to concentrate persuasion efforts on a small number of villages and get the idea to take hold. As those areas began to see success, other villages started to copy them and the change spread organically. We could easily capitalize on this insight for all kinds of change. If I'm a city government and I want to get more people to install solar panels on their roofs, I might be tempted to say all right, I have a grant that can give out solar panels to 300 houses. I'm going to spread my money as widely as possible. The better strategy, however, might be to concentrate my panels on houses on a few streets. The goal is to get neighbors to see what's happening and want to get solar panels of their own.

Damon Centola: Yeah. I think that one of the most striking results about solar panel adoption is that it's so incredibly localized. It's really a neighborhood effect. And governments, particularly in Europe, have done an excellent job of incentivizing people to adopt solar panels and incentivizing companies to build solar panels and make them available, but they found that that wasn't sufficient to actually generate a tipping point or a change in the norms. And in some sense, that's the crucial issue with tipping points, everyone's waiting for everyone else to go first. So one thing that the German government did that was so clever is they started targeting neighborhoods and incentivizing homeowners to just install, within certain neighborhoods, solar panels on their homes. And this shifted the social norms within those communities and created kind of a social pressure on the neighborhoods to realize that this was actually normal in their community now, and then they voluntarily went and adopted solar panels. And the most remarkable part about this process is that it spilled over from community to community and neighborhood to neighborhood and wound up becoming the dominant social norm throughout Germany, that everyone was sort of expected to have these solar panels on their homes. And we've seen similar effects in Britain, similar effects in Japan, even in certain neighborhoods in California and Connecticut. And so these sort of neighborhood effects in terms of homeowners' expectations about what they should and shouldn't be doing are very powerful and actually impacted a national scale adoption of something like solar panels.

Shankar Vedantam: Now, even as there are people who don't understand these lessons about what drive change, there are others who seem to understand these lessons very well. Some authoritarian regimes are cleverly using these ideas not to spur change, but to suppress change. Can you tell me about China's 50 Cent Party?

Damon Centola: Yeah. And I think this is one of the more important things about social science. Sociology's now approaching this point where we can understand with enough rigor and insight what governs these kinds of change processes that we also have to worry about what the implications are for governments who might use these ideas, and China has done exactly that. They've done something that is clever, but also destructive, which is when people on social media in China, whenever an uprising takes place or a protest event takes place, you would think that the government would use social media to promote its own sort of fake news or false information about what was actually happening, but instead they devised a different strategy, which was essentially strategic non sequiturs. So they would have agents, who are masquerading as real people, as members of the social media community. And as people started talking about a protest event or about some sort of problem with the government or its policy, these social media agents, which were called the 50 Cent Party because they were paid 50 cents in Chinese jiao, they started talking about a local parade or they would start talking about some principle of communist doctrine. And the idea was if you have enough of these people talking to each other, it basically creates a critical mass, where the other people talking about legitimate protest issues or legitimate grievances feel like their conversation isn't relevant anymore, what's relevant is the sort of celebration of a local parade or a fireworks display. And so it winds up changing the expectations of what we can and can't talk about on social media just by virtue of creating a critical mass of people who are doing something different than other people were doing. And it's been incredibly effective for derailing activists' efforts on Chinese social media.

Shankar Vedantam: It's sort of a remarkable way of actually suppressing dissent without actually burning books or basically, actually censoring people in sort of the ways that totalitarian regimes used to do the middle part of the 20th century.

Damon Centola: Yeah. The way I talk about it in the book is I say they're not so much censoring seditious books as much as flooding the market with the appeal of pulp fiction.

Shankar Vedantam: As I step back and I think about this broad work, Damon, to the extent that non-sociologists think about social networks, I think we tend to think of them as pipes or these are sort of pipes that deliver us information. I get information from this set of people on Twitter, I get information from that set of people on Facebook. You're saying, in many ways, that that's the wrong way to think about the power of social networks. They're not pipes, but something else.

Damon Centola: The pipe metaphor really comes from disease transmission, which is to say if I sit next to you in an airport and you're infected with the coronavirus, now I get the coronavirus, there's nothing sort of deep or interpersonal or meaningful about that process, it's really just a transmission. And when we think about that transmission process and we think about things like highly connected influencers, it's just a person with lots and lots of pipes going out to other parts of society and they can spread whatever content to those people and so on and so forth. But when we look at things like the spread of news, it's not so simple. The networks through which people filter these ideas or new scientific information are actually doing a lot of work to determine how receptive you are to the information, whether you believe it, whether you reject it, or whether you just interpret it very differently than was originally intended. And so this is the way in which social networks act as prisms that refract and shape the beliefs that we hold by virtue of the people that we interact with. And so one of the things that we think about in terms of solving social problems is how we can shape the interactions people have or maybe structure them in ways that are more productive. One thing you can do is to take a centralized echo chamber and make the networks more egalitarian. What we've found is that this is unbelievably effective for changing the actual beliefs of people in a community. In fact, when you take similar groups of Democrats or Republicans who are highly polarized on issues like immigration or climate change or gun control and you let them interact within networks that are more like fishing nets, opinions and ideas from the periphery of those communities can actually gain traction and be reinforced. And what winds up happening is that both communities independently move toward each other and you wind up generating a kind of reconciliation of ideas just by virtue of the fact that good ideas are often in the periphery of the social network, but they wouldn't have been able to catch on or have any influence when they're being blocked by a highly powerful central influencer.

Shankar Vedantam: Damon Centola is the author of Change, a book about social movements and entrepreneurial success. He's a sociologist at the University of Pennsylvania. Damon, thank you for joining me today on Hidden Brain.

Damon Centola: Thank you. It was my pleasure.

Shankar Vedantam: Hidden Brain is produced by Hidden Brain Media. Our production team includes Brigid McCarthy, Laura Kwerel, Kristin Wong, Ryan Katz, Autumn Barnes and Andrew Chadwick. Tara Boyle is our executive producer. I'm Hidden Brain's executive editor. Our unsung hero today is the Library Journal. It's a publication read by librarians around the United States. Some months ago, the Library Journal invited me to their annual conference to talk about my new book, Useful Delusions. One of my fellow panelists was Damon Centola. I so enjoyed hearing about his ideas that I reached out to him shortly afterwards and asked him about coming on Hidden Brain. Librarians have a special place in our heart at Hidden Brain. Books are the lifeblood of our show. I'm grateful to the Library Journal for introducing me to Damon Centola's work and for all the other work the organization does to draw attention to writers and ideas. For more Hidden Brain, sign up for our weekly newsletter, where we explore ideas related to this week's show and share other links. Sign up at news.hiddenbrain.org. If you liked today's show, please consider supporting us. Go to hiddenbrain.org and click on Support. I'm Shankar Vedantam. See you next week.


Subscribe to the Hidden Brain Podcast on your favorite podcast player so you never miss an episode.


Go behind the scenes, see what Shankar is reading and find more useful resources and links.