Contagious by Jonah Berger

The Big Idea: word of mouth relies on six principles: social currency, triggers, emotion, public, practical values, and stories.

  • Word of mouth is the primary factor behind 20-50 percent of all purchasing decisions.
  • Word of mouth is more effective than advertising because it is more persuasive and more targeted.
  • As widespread as social media is, only 7% of word of mouth happens online.
  • Malcolm Gladwell argues that social epidemics are driven mavens/connectors, but science says that word of mouth happens because shareable messages exhibit certain traits that make them contagious.


Principle 1: Social Currency

  • Knowing about cool things (like a blender that can tear through an iPhone) makes people seem sharp and in the know.
  • Leverage game mechanics to give people ways to achieve and provide visible symbols of status that they can show to others.
  • Examples mentioned: Will It Blend, Snapple Caps, The Blair Witch Project, Foursquare, Rue La La, Disney Vault, McRib, Please Don’t Tell bar.

Principle 2: Triggers

  • Prompt people to think about related things.
  • Design products and ideas that are frequently triggered by the environment and create new triggers by linking our products and ideas to prevalent cues in that environment.
  • Examples mentioned: Cheerios, BzzAgent, Mars Bar, voting in schools, Kit-Kat and coffee, Budweiser’s Wazzup, hot dogs and July 4th, Weekends are made for Michelob, Friday song.

Principle 3: Emotion

  • Craft messages and ideas that make people feel something.
  • Emotional things often get shared.
  • Examples mentioned: The Mysterious Cough, Susan Boyle, Google’s “Parisian Love”.

Principle 4: Public

  • Make products and ideas more public.
  • Design products and initiatives that advertise themselves.
  • Examples: Macbook’s upside down logo, rejected kidney donations, No Shave November, Sent from my iPhone, Livestrong, I Voted stickers, Tiffany and Victoria’s Secret shopping bags,

Principle 5: Practical Value

  • People like to help one another.
  • Highlight the incredible value of what we offer — monetarily and otherwise.
  • Increase value using principles of behavioral economics: smart pricing, limited availability, Rule of 100.
  • Examples mentioned: Ken Craig’s Clean Ears Everytime, Vanguard Funds

Principle 6: Stories

  • People don’t just share information, they tell stories.
  • Make your message so integral to the narrative.
  • Examples mentioned: Trojan Horse, The Three Little Pigs, Jared Fogle and Subway, Barclay Prime’s $100 cheesesteak, Egyptian dairy company Panda, Vietnamese nail salons.


  1. Social Currency: Does talking about your product or idea make people look good? Can you find the inner remarkability? Leverage game mechanics? Make people feel like insiders?
  2. Triggers: Consider the context. What cues make people think about your product or idea? How can you grow the habitat and make it come to mind more often?
  3. Emotion: Focus on feelings. Does talking about your product or idea generate emotion? How can you kindle the fire?
  4. 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?
  5. 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 will want to disseminate?
  6. 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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