Chapter 1
In the Thick of It
The simplest network is a dyad or pair
Dyads agglomerate to form
large interconnected webs
Line = a relationship between two people
more embedded = central
less embedded = periphery
Node = a person
“embedded”: the degree to which a person is connected within a network
The Bucket Brigade
Linear
Mutual ties
Photo from FEMA
(flow in both directions)
The Telephone Tree
Information is directional (inbound and outbound ties)
Reduces number of steps
Creates cascade effect
The Military Squad
Tightly interconnected groups
Two-way tie between all members of squad
Squad is more intra-connected
than inter-connected with other squads
Terms
A 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)
Terms
The 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 network
1. We determine structure of our network:
how many people we are connected to
The 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 network
2. We influence the density of
interconnections between friends and family
we introduce friends from separate groups to one another
We shape our network
3. We control how “central” we are within the
social network
Are you the life of the party?
Or the wall flower?
Transitivity
Transitivity: 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
Influence
Contagion:
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
Stanley Milgram experiment showing that people are all
connected by an average of six steps
• 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!
Photo by Dan Coulter
It’s a small world after all
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!
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Photo by Wonderworks, Flikr