Chapter 1In the Thick of It

Dyads agglomerate to form

large interconnected webs

The simplest network is a dyad or pair

Node = a person

Line = a relationship between two people

“embedded”: the degree to which a person is connected within a network

more embedded = central

less embedded = periphery

Linear

Mutual ties (flow in both directions)

The Bucket Brigade

Photo from FEMA

Information is directional (inbound and outbound ties)

The Telephone Tree

Creates cascade effect

Reduces number of steps

Squad is more intra-connected

than inter-connected with other squads

The Military SquadTightly interconnected groups

Two-way tie between all members of squad

TermsA Social Network: consists of all the connections and ties within a group or collection of groups

A group is a collection of individuals defined by a common attribute (it need not contain information about connections)

TermsThe Shape of a network = its “structure” or “topology”

Terms• Contagion: what flows across ties (germs, money, violence, fashions, organs, happiness, obesity, etc.)

• Connection: who is connected to whom

(ties to family, friends, co-workers, etc.)

• Homophily: the tendency to associate with people who resemble ourselves

(“love of being alike”)

We shape our network1. We determine structure of our network:

how many people we are connected toThe average American has 4 close social contacts known as their “core discussion network”The same number of people can be arranged and connected in different ways, and have different topologies

We shape our network2. We influence the density of

interconnections between friends and family

we introduce friends from separate groups to one another

We shape our network3. We control how “central” we are within the

social network

Are you the life of the party? Or the wall flower?

TransitivityTransitivity: a relationship is transitive when all those involved know each other (form a triangle)

Those with high transitivity (e.g. person A) are deeply embedded within a single group

Those with low transitivity (e.g., person B) act as a bridge between different groups, connected with people who do not know one another

InfluenceContagion: what flows across ties (germs, money, violence, fashions, organs, happiness, obesity)

Dyadic Spread: the tendency of effects to spread from one person to their direct social ties

Hyperdyadic spread: the tendency of effects to spread from person to person to person (outside a person’s direct social ties)

What Are You Looking At?

Stanley Milgram’s sidewalk experiment exploring the importance of reinforcement from multiple people

• Planted actors or “stimulus crowds” of different size to stare up at a window across the street• Passersby are filmed to record reactions--stop and stare/glance up• Crowd of 1: 4% of pedestrians stopped• Crowd of 15: 40% stopped, 86% glanced up• Crowd of 5 induced almost the same effect as 15+!

Passersby were influenced in deciding to copy a behavior by the size of the crowd exhibiting it Photo by David Sim

Six Degrees of Separation

Photo by Dan Coulter

• A citizen in Nebraska was instructed to mail a letter to someone who they thought would know a particular business man in Boston

•The goal was to measure the number of steps for the letter to reach the business man

• On average, required six steps

• Replicated on global scale with same results!

It’s a small world after all

Stanley Milgram experiment showing that people are all connected by an average of six steps

Three Degrees of Influence

The influence of actions ripples through networks 3 degrees(to and from your friends’ friends’ friends)

You are affected by people you don’t even know!

Influence dissipates after 3 degrees because:

-Intrinsic decay: corruption of information (like the game telephone)

-Network Instability: social ties become unstable at 4+ degrees of separation

-Evolutionary Purpose: we evolved in small groups where everyone was connected by 3 degrees or less

Photo by Taro Taylor

Three Degrees of Influence

If we are connected to everyone by 6 degrees and influence those up to 3 degrees, then we can reach halfway to the whole world!

Photo by Wonderworks, Flikr

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