Social Networks - Lecture 15

Contagion SIR model Cascades Influence Obstacles to diffusion Social influencers

What is contagion in network terms?

The spread of behaviors, ideas, or emotions through social ties.

What does contagion rely on?

Connections between people that allow behaviors or beliefs to spread like a “social infection.”

What are examples of social contagion?

Viral TikTok trends, Protests and social movements, Health behaviors (like quitting smoking)

What does it mean when contagion is “cumulative”?

Adoption increases as more people participate, like peer pressure building over time.

What does the SIR model stand for?

Susceptible, infected, recovered

How is the SIR model used in networks?

To show how people move through stages of adopting or rejecting a behavior or idea, like disease modeling but for social behaviors.

What is R₀ in the SIR model?

The average number of new infections caused by one infected person in a fully susceptible population.

What does a higher R₀ mean?

Faster and wider spread of the idea, behavior, or disease.

What is a cascade in a social network?

A chain reaction where one person’s behavior influences others, causing widespread adoption.

What are some real-world examples of cascades?

A product suddenly going viral, Everyone dropping a class after one person does, A protest growing rapidly from a few early joiners

What factors affect whether a cascade will happen?

The threshold (how many peers must join before someone else does), Position in the network, How many connections are early adopters

What are the risks or flaws of cascades?

can be wrong, can be based on very little information, they can be fragile and collapse quickly, they depend heavily on thresholds

What is influence?

The ability of one person to affect the beliefs or behaviors of another through a connection.

What is the two-step flow model of influence?

1.Info goes first to opinion leaders, 2.Then spreads from leaders to the general public

What can block the spread of ideas or behaviors?

high thresholds, lack of bridging ties, network fragmentation, weak influence of early adopters

What is a social influencer in network terms?

Someone whose actions or opinions have a big impact on others’ behaviors.

How can influencers affect diffusion or contagion?

They can speed up or block the spread of behaviors or ideas, depending on their reach and trust level.