Contagious: Why Things Catch On

Why do some products, ideas, and behaviors become popular while most fail? Wharton marketing professor Jonah Berger argues that virality is made, not born, and that the secret lies in understanding the psychology of why people talk and share. Drawing on years of rigorous research analyzing thousands of New York Times articles, baby names, car purchases, and word-of-mouth campaigns, Berger distills six principles—Social Currency, Triggers, Emotion, Public, Practical Value, and Stories (STEPPS)—that make content contagious. With memorable examples from a hundred-dollar cheesesteak to blending iPhones to viral Google ads, Contagious gives marketers, entrepreneurs, nonprofits, and anyone with a message a practical, science-based toolkit for engineering word of mouth and making ideas spread.

What it argues

Contagious: Why Things Catch On

Key ideas it contributes

• Social Currency — The degree to which talking about a product or idea makes a person look good, smart, cool, or in-the-know to others, achieved through inner remarkability, game mechanics, and feeling like an insider.
• Triggers — Stimuli in the environment that remind people of a related product or idea, keeping it top of mind so it is more likely to be talked about; depends on the frequency and strength of the cue and its link to the desired behavior.
• Emotion — The extent to which a product, idea, or message evokes feelings in people, particularly high-arousal emotions like awe, excitement, amusement, anger, or anxiety, as opposed to low-arousal emotions like sadness or contentment.
• Physiological Arousal — A state of bodily activation and readiness for action characterized by increased heart rate and blood pressure, which mediates the relationship between emotion (or physical activity) and the drive to share information with others.
• Public Observability — The degree to which a product, behavior, or idea can be seen by others when used or adopted, enabling social proof and imitation; encompasses self-advertising design and behavioral residue that persists after use.
• Practical Value — The usefulness of information for helping others save time, save money, improve health, or have better experiences, made more shareable through remarkable deals and packaged expertise.
• Stories — Narratives that carry information, lessons, or brand messages embedded within them, acting as Trojan horses that people pass along while transmitting the embedded content under the guise of idle chatter.
• Valuable Virality — The degree to which the brand or desired message is integral to the narrative such that people cannot tell the story without mentioning it, ensuring that sharing benefits the sponsoring product or organization rather than just spreading the content.
• Social Transmission (Word of Mouth) — The act of people talking about, sharing, forwarding, or imitating a product, idea, or behavior with others through word of mouth and social influence, both online and offline.
• Popularity / Catching On — The outcome in which a product, idea, or behavior diffuses through a population, becoming widely adopted, talked about, purchased, or imitated, constituting a social epidemic.