Contagious: Why Things Catch On by Jonah Berger

Reading Time: 9 minutes


This book is perfect for anyone who wants to learn more about how to spread ideas, increase brand awareness, or grow their customer base. In it, Wharton marketing professor Jonah Berger explains what makes certain ideas, products, and stories popular. He explains 6 mechanisms that contribute to virality: social currency, triggers, emotion, practical value, public, and stories.

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Key Takeaways

While quality, price, and advertising contribute to products and ideas being successful, word of mouth drives 20 to 50 percent of all purchasing decisions. Word of mouth is more effective than traditional advertising for two reasons:

  1. It’s more persuasive. Word of mouth recommendations are more objective and candid, so we are more likely to trust and listen to them.
  2. It’s more targeted. We share information and stories with people who will find our information relevant and interesting.

Contagious explains what makes products, ideas, stories, and news likely to spread from person to person via word of mouth and social influence. There are 6 principles:

Principle Why it Works
Social Currency How does it make people look to talk about a product or idea? Most people would rather look smart than dumb, rich than poor, and cool than geeky.
Triggers How do we remind people to talk about our products and ideas? Triggers are stimuli that prompt people to think about related things.
Emotion When we care, we share. So how can we craft messages and ideas that make people feel something?
Public Can people see when others are using our product or engaging in our desired behavior? Making things more observable makes them easier to imitate, which makes them more likely to become popular.
Practical Value How can we craft content that seems useful? People like to help others, so if we can show them how our products or ideas will save time, improve health, or save money, they’ll spread the word.
Stories What broader narrative can we wrap our idea in? People don’t just share information, they tell stories.

Principle I: Social Currency

“We share things that make us look good”

Social currency helps people look good to others. Most people would rather look clever rather than dumb, hip rather than dull, and cool rather than geeky. There are three ways to generate social currency:

1. Find inner remarkability

Remarkable things are unusual, extraordinary, or worthy of notice or attention. Something can be remarkable because it is novel, surprising, extreme, or just plain interesting. 

You can make your product remarkable by breaking a pattern that people have come to expect. For example, it’s remarkable that the Blendtec blender can destroy an iPhone, and that a ball of glass will bounce higher than a ball of rubber. It’s also remarkable that a low-cost airline like Jetblue provides first-class amenities to all passengers.

2. Leverage game mechanics

Game mechanics include rules and feedback loops that make things fun. 

Building game mechanisms into your product (e.g., Airline frequent flier programs) lead people to engage in certain behaviors (e.g., flying one airline even when it’s not convenient) and to talk about their achievements (e.g., achieving Diamond status with an airline). Along the way, they spread the word about brands.

3. Make people feel like insiders

People enjoy feeling like insiders. There two mechanisms that make people feel like insiders:

  • Scarcity: how much of something is offered. Scarce things are less available, due to high demand, limited production, or restrictions on a time/place to buy. For example, Please Don’t Tell (NYC Speakeasy), only has 45 seats available.
  • Exclusivity: the availability of something based on particular criteria. For example, for an invite-only website, you need to know an existing site member to get access.

By making people feel like insiders, scarcity and exclusivity boost word of mouth.

Caveat for driving referrals

In some cases, people who refer others to your product or service for free will stop referring people once you start paying them. This happens because you have taken away the person’s intrinsic motivation to share something helpful with a friend with a financial incentive that may feel “dirty.”

How to evaluate social currency

  • Does talking about your product or idea make people look good?
  • Can you find the inner remarkability?
  • Can you leverage game mechanics?
  • Can you make people feel like insiders?

Principle 2: Triggers

“Top of mind, tip of tongue”

How do we remind people to talk about our products and ideas? Triggers stimulate people to think and talk about your product, idea, or service.

When triggering people to think about your product (and to think about it at the right time), they will be more likely to engage with and share your product.

When it comes to triggers, there are two important factors:

  1. Frequency: how often the trigger happens. For example, if your product is associated with coffee, people think about and consume coffee daily, so it’s a frequent trigger. Hot chocolate, on the other hand, is a seasonal drink, so an association with hot chocolate will have a lower trigger frequency.
  2. Strength of link: A more unusual link will be stronger than a common link. The color red, for example, is associated with many things (Valentine’s Day, berries, Coca-Cola, roses, blood, etc.), whereas something like peanut butter has fewer, but stronger links (e.g. Jelly).

Trigger Examples

  • Mars Candy Company: In 1997, a NASA mission to Mars gained traction in the news, and sales of Mars candy bars spiked. Despite the candy company having nothing to do with the NASA mission, the frequent news cycle triggered the idea of the candy in people’s minds.
  • Kit Kat: The chocolate company increased sales by linking eating Kit Kats with coffee breaks at work. A huge number of people drink coffee and take breaks at work, so Kit Kat became triggered in people’s mind while they went about and did their daily activities. 

Triggering people at the right time is important. For example, if you trigger someone to brush their teeth while they’re driving a car, then it’s not very effective. If you do it while they’re in the bathroom, it’s more likely they’ll engage in the action.

Note: Not only do products with triggers get more immediate word of mouth, but they also get more word of mouth on an ongoing basis.

How to evaluate triggers

  • Consider the context.
  • What cues make people think about your product or idea?
  • How can you grow the habit and make it come to mind more often?

Principle 3: Emotion

“When we care, we share”

Emotions compel us to action. They make us laugh, shout, share, talk, and buy. So how can we craft messages and ideas that make people feel something?

Both positive and negative emotions can incite action. What drives action is the level of arousal that we feel with a particular emotion. High-arousal emotions (positive and negative) drive us to action, and low-arousal emotions (positive and negative) keep us in a state of inaction.

High-arousal emotions: Drive us to action.

Emotion Type Example
Awe  + After a stunning, four-day hike in the Andes Mountains, you rave to friends and family about how they need to go.
Excitement + You tell your friends about a great ski or surf day.
Amusement (Humor) + You share a funny Instagram meme with friends.
Anger You share a news article that covers a policy decision that makes you angry.
Anxiety You check your work 5x before a big client presentation.

Low-arousal emotions: Keep us from acting.

Emotion Type Example
Contentment + When you are satisfied with a good meal, a long hot shower, or a relaxing massage, you don’t feel like doing anything. You relax.
Sadness After not getting the offer for your dream job, you want to sit on the couch and do nothing.

More arousal = more sharing.

Note: Getting your message in front of people when they are physiologically aroused (e.g., when getting off a treadmill or after turbulence on a plane) may increase sharing.

How to evaluate emotion

  • Focus on feelings.
  • Does talking about your product or idea generate emotion?
  • How can you kindle the fire?

Principle 4: Public

“Built to show, built to grow”

Can people see when others are using our product or engaging in our desired behavior? Making things more observable makes them easier to imitate, which makes them more likely to become popular.

The Power of Social Proof

When people are free to do what they want, they often imitate others. They look to other people to determine what is good or right about a product or situation. That’s why we assume that the longer the line at a restaurant, the better the food.

The Two Components of Social Proof

  1. Self-advertising: Making a product, idea, or behavior that advertises itself shows people that other people are using the product or engaging in the behavior. For example, Apple includes a large and visible logo on its computers so that people can easily identify others using Apple computers in cafes, libraries, and other places.
  2. Behavioral residue: Physical traces that actions or behaviors leave in their wake. Behavioral residue shows us what behaviors others engage in. For example, people wearing an “I voted” sticker after voting shows others that they have voted.

Many brands have successfully leveraged social proof.

  • Movember Foundation: The organization got people to care about an important, but under-discussed topic (men’s health) by starting a trend where people grow mustaches in the month of November to support men’s health. The people with mustaches become walking billboards for the cause.
  • Livestrong bracelet: The yellow color brought attention to cancer. The band represented the race leader’s jersey in the Tour de France, but it is also gender-neutral and not a popular color, so it made it easy for both genders to wear the band and for people to notice it since yellow sticks out.
  • Lululemon: The company makes its shopping bags high-quality bags that feel bad to throw away, so people use them to carry groceries, to travel, or to do other errands. The brand gets social proof as people bring Lululemon bags throughout their daily lives.

How to evaluate public

  • Does your product or idea advertise itself?
  • Can people see when others are using it?
  • If not, how can you make the private public?
  • Can you create behavioral residue that sticks around even after people use it?

Principle 5: Practical Value

“News you can use”

How can we craft content that seems useful? People like to help others, so if we can show them how our products or ideas will save time, improve health, or save money, they’ll spread the word.

Practical value is about the person receiving information. If you can help someone save time and money, make them seem better to others, or help them have a more enjoyable time, you’re adding practical value.

Reference Points and Value

People use reference points, rather than the absolute value of a product, to assess the value of something. For example, imagine a gallon of milk cost $1.50 when you were growing up. If the gallon of milk now costs $3.50, how you feel about that price depends on your reference point. If your reference point is the $1,50 you paid as a kid, you might feel that it’s a bad deal.

Reference points explain why Amazon includes the original price of a book next to the discounted price that a customer can buy the book for on Amazon. The original price becomes the reference point, and it influences how a customer feels about a purchasing decision.

Diminishing Sensitivity and Discounts

The same change has a smaller impact the farther it is from the reference point. If you get a $20 product for $10, you will be much more satisfied with the deal than if you get a $1,000 product for $990. While you save $10 in both cases, your reference point is different. In the first case, the final price of $10 seems much further away from the original $20 price. In the second case, $1,000 and $990 seem roughly the same.

The principle of diminishing sensitivity informs how brands should frame discounts. When discussing a discount, a brand can frame the discount in terms of a percentage or in absolute terms. For a $50 product, you can offer a 20% discount or $10 off. The dollar amount is the same, but how the discount is framed determines how the customer feels about the value.

The rule of 100

If a product costs less than $100, price reductions seem more significant when framed in percentage terms. For products that cost more than $100, framing price reductions in dollar terms make it seem like a better deal.

How to evaluate practical value

  • Does talking about your product or idea help people help others?
  • How can you highlight incredible value, packaging your knowledge and expertise into useful information others want to disseminate?

Principle 6: Stories 

“Information travels under the guise of idle chatter”

People don’t just share information, they tell stories. Stories help us transmit information to others. They engage our emotions, motivating us to feel and take action. 

As an information vessel, stories have the capacity to change our attitudes, beliefs, and behaviors in ways that no other medium can.

To spread an idea, you need to wrap the idea into a broader narrative.

Customer Service

If you receive exceptional customer service from a company that helps you get what you want, you might tell a friend about that service. In helping you solve your problem, the company has created a story for you to tell others that will spread the word about their brand.

Dove Commercial

Dove, the personal care brand, created a video (“Evolution”) that showed how much makeup, photoshopping, and work went into making people look beautiful for advertisements. They encouraged viewers to feel comfortable in their own skin. 

In doing so, Dove tapped into an important topic for many people: unrealistic beauty norms. Whereas many people may not have been comfortable discussing beauty norms openly, the video and story around it gave people an avenue to discuss an emotional and important topic.

Weaving your brand into the story

If you want to get people talking, you can hire someone to do a crazy stunt. If it’s crazy enough, people will talk about it. But if your brand is not an integral part of the story, people won’t talk about your brand. 

With stories, people often miss superfluous details. So to get people sharing a story that also spreads the word of your brand, your brand needs to be an integral part of the story.

Panda, a cheese brand, did this well when they created a series of commercials where a man in a Panda suit disrupted everyday situations (grocery shopping, a kid’s birthday party, a hospital lunch, etc.). They had the tagline “Never say no to Panda.”

These stories were entertaining and worth sharing, but they were effective because the brand was an integral part of the story. If it was a man in a gorilla suit, it may have been a funny commercial to talk about, but it wouldn’t have helped spread the word about Panda cheese.

How to evaluate stories

  • What is your Trojan Horse?
  • Is your product or idea embedded in a broader narrative that people want to share?
  • Is the story not only viral, but also valuable?

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