Contagious: Why Things Catch On

Behavioral residue refers to the physical evidence or traces that remain after someone has performed an action or supported a cause. These lasting markers can take many forms, from reusable shopping bags to wristbands to digital footprints on social media. The concept emphasizes how physical artifacts can continue to spread awareness and influence others even after the initial action is complete. For example, the LiveStrong yellow wristbands served as persistent reminders of cancer support, visible to others long after purchase.

Diminishing sensitivity refers to the psychological principle that the same change in value has a smaller impact as it moves further from a reference point. In the context of marketing and consumer behavior, this principle explains why people respond differently to identical price reductions depending on the original price of an item. For example, saving $10 on a $25 item feels more significant than saving $10 on a $500 item, even though the absolute amount saved is identical. This concept helps explain why consumers might drive across town to save a small amount on an inexpensive item but not make the same effort for identical savings on a costly purchase.

Exclusivity refers to the practice of limiting access to a product, service, or experience to people who meet specific criteria. In Contagious, Berger explains that exclusivity creates value by making things accessible only to certain individuals, whether through membership requirements, special knowledge, or specific connections. The concept differs from scarcity in that it focuses on who can access something rather than how much of it exists. Berger demonstrates this principle through examples such as Rue La La’s invitation-only shopping platform and Please Don’t Tell’s hidden entrance, showing how restricted access can enhance the desirability and encourage word-of-mouth marketing.

Game mechanics are the elements and systems that make activities engaging and motivating by leveraging people’s natural competitiveness and desire for achievement. In Contagious, Berger describes how companies use features like points, levels, and status rankings to encourage participation and sharing. These mechanics tap into people’s desire to excel and outperform others, as demonstrated by airline frequent flyer programs and social media achievement badges. The concept extends beyond traditional games to influence behavior in various contexts, from customer loyalty programs to professional achievements.

Inner remarkability describes the qualities that make a product, service, or idea worthy of discussion and sharing, regardless of its surface-level appeal. Berger explains that inner remarkability can be discovered or created in any offering by identifying or developing surprising, interesting, or notable aspects that people will want to talk about. This concept challenges the notion that only inherently exciting products can generate buzz. Berger illustrates this principle through examples such as Snapple’s surprising facts under bottle caps and Blendtec’s creative product demonstrations, showing how seemingly mundane items can become conversation starters.

Physiological arousal refers to the state of being activated or energized, characterized by increased heart rate, heightened nervous system activity, and general activation of the body’s fight-or-flight response. In Contagious, Berger identifies physiological arousal as a crucial factor in determining whether content goes viral; he explains that high-arousal emotions like excitement, anger, or awe are more likely to prompt sharing than low-arousal states like contentment or sadness. This bodily state creates an action tendency that makes people more likely to share content, forward emails, or post on social media. Berger’s research demonstrates that the level of physiological arousal, rather than whether an emotion is positive or negative, better predicts what content will become viral.

Psychological cover refers to the way stories enable people to share information about products or services without appearing to advertise. This concept explains how narratives create a natural context for discussing commercial products or ideas, allowing individuals to transmit marketing messages while primarily appearing to share an entertaining or interesting story. The term emphasizes how stories provide social permission to discuss brands or products in a way that direct advertising cannot achieve.

Public visibility describes the degree to which others can observe a product, behavior, or idea in everyday life. This concept explains why certain products or behaviors spread more rapidly than others based on how easily they can be seen and noticed by potential adopters. Public visibility affects adoption rates differently across product categories; for instance, clothing choices spread more easily than toothpaste preferences because they are more readily observable to others. The concept encompasses both natural visibility (such as cars being visible on roads) and strategically designed visibility (such as Apple’s decision to orient laptop logos for observers rather than users).

The Rule of 100 is a marketing principle that determines whether discounts should be presented as percentages or absolute dollar amounts based on an item’s original price. For products priced under $100, percentage discounts appear more attractive to consumers than equivalent dollar amounts. For products priced over $100, absolute dollar amounts seem more substantial than equivalent percentage discounts. This principle helps marketers optimize how they present savings to potential customers—for example, advertising a $40 shirt as “25 percent off” rather than “$10 off,” while promoting a $400 television as “$100 off” rather than “25 percent off.”

Scarcity describes the limited availability of a product or service due to factors such as high demand, limited production, or time restrictions. In Contagious, Berger explains how scarcity increases perceived value and encourages sharing by making things seem more desirable. The concept operates through various mechanisms, such as limited-time offers, small production runs, or artificial constraints on availability.

Social currency represents the social value people gain from sharing information or experiences with others. Berger describes it as a metaphorical form of currency that individuals spend to create positive impressions and enhance their status among peers. The concept encompasses the way people use knowledge, experiences, and insider information as tools for social positioning, similar to how they might use material goods to demonstrate status. Social currency motivates sharing behavior because people gain social benefits from being seen as knowledgeable, cool, or in-the-know. This fundamental concept underlies many viral marketing successes, as people naturally share things that make them look good to others.

A social epidemic occurs when products, ideas, or behaviors spread through a population in a pattern similar to viral transmission. These phenomena begin with a small group of individuals or organizations and diffuse through communities via person-to-person transmission. Social epidemics can manifest in various forms, from the adoption of fashion trends and dietary practices to the spread of social movements and technological innovations. Unlike biological epidemics, social epidemics typically involve shorter transmission chains and rely more heavily on conscious human choice in their spread patterns.

Social proof describes the psychological phenomenon in which people look to others’ actions to determine appropriate behavior in ambiguous situations. This principle explains why individuals often make choices based on what they observe others doing, from selecting restaurants to choosing career paths. Social proof can operate both positively and negatively; while it can help beneficial behaviors spread, it can also perpetuate harmful behaviors when people incorrectly assume that observed actions reflect others’ true preferences. The principle becomes particularly powerful when combined with public visibility, as people can only imitate what they can observe.

STEPPS is Berger’s framework for understanding and creating contagious content. It represents six key principles that drive social transmission: Social Currency (people share things that make them look good), Triggers (environmental reminders that keep ideas top of mind), Emotion (content that evokes strong feelings), Public (visible behaviors that can be easily imitated), Practical Value (useful information that benefits others), and Stories (narrative vehicles that carry messages and ideas). This systematic approach provides a practical methodology for analyzing why certain things catch on and designing content that people are more likely to share. Each principle represents a distinct psychological or social factor that contributes to the likelihood of information spreading through social networks.

A trigger is any stimulus in the environment that prompts people to think about a related concept, product, or idea. Triggers act as reminders that bring products or ideas to the top of mind, increasing the likelihood that people will talk about them or share them with others. For example, seeing peanut butter might trigger thoughts of jelly, or hearing a particular song might trigger memories of a specific time or place. Effective triggers are frequent enough to regularly remind people about the product or idea, but not so common that they lose their impact through overexposure.

A Trojan Horse is a marketing narrative that carries embedded commercial messages within an engaging story that people want to share. The term draws from the ancient Greek tale to metaphorically describe how effective marketing stories conceal their promotional intent within entertaining or engaging content. In the context of modern marketing, a Trojan Horse narrative must integrate its marketing message so fundamentally that people cannot tell the story without also conveying information about the product or brand.