We present a model of the creation of social networks, such as political parties, trade unions, religious coalitions, or political action committees, through discussion and mutual persuasion among their members. The key idea is that people are influenced by those inside their network, but not by those outside. Once created, networks can be rented out' to politicians who seek votes and support for their initiatives and ideas, which may have little to do with network members' core beliefs. In this framework, political competition does not lead to convergence of party platforms to the views of the median voter. Rather, parties separate their messages and try to isolate their members to prevent personal influence from those in the opposition.

Andrei Shleifer

Kevin Murphy

English

Research Papers in Economics

NBER WORKING PAPER SERIES
PERSUASION IN POLITICS
Kevin Murphy
Andrei Shleifer
Working Paper 10248
http://www.nber.org/papers/w10248
NATIONAL BUREAU OF ECONOMIC RESEARCH
1050 Massachusetts Avenue
Cambridge, MA 02138
January 2004
We thank Robert Bates, David Laibson, Jesse Shapiro, Kenneth Shepsle, Daniel Treisman, and Janet Yellen
for helpful comments. The views expressed herein are those of the authors and not necessarily those of the
National Bureau of Economic Research.
©2004 by Kevin Murphy and Andrei Shleifer.  All rights reserved. Short sections of text, not to exceed two
paragraphs, may be quoted without explicit permission provided that full credit, including © notice, is given
to the source.Persuasion in Politics
Kevin Murphy and Andrei Shleifer
NBER Working Paper No. 10248
January 2004
JEL No. D72, D78
ABSTRACT
We present a model of the creation of social networks, such as political parties, trade unions,
religious coalitions, or political action committees, through discussion and mutual persuasion among
their members. The key idea is that people are influenced by those inside their network, but not by
those outside. Once created, networks can be “rented out” to politicians who seek votes and support
for their initiatives and ideas, which may have little to do with network members' core beliefs. In
this framework, political competition does not lead to convergence of party platforms to the views
of the median voter. Rather, parties separate their messages and try to isolate their members to
prevent personal influence from those in the opposition.
Kevin Murphy
Graduate School of Business
University of Chicago
1101 East 58
th Street
Chicago, IL 60637
and NBER
murphy@gsb.uchicago.edu
Andrei Shleifer
Department of Economics
Harvard University
M9 Littauer Center
Cambridge, MA 02138
and NBER
ashleifer@harvard.edu1
Recent research on social psychology and public opinion identifies a number of empirical
regularities on how people form beliefs in the political and social spheres.   First,  beliefs are
flexible and can be relatively easily influenced, particularly in areas where people do not have
significant personal involvement (Doris Graber 1984, John Zaller 1992).  Second, social
influence shapes decisions: people are often persuaded by those they personally interact with
(Mark Grasnovetter 1973, Robert Cialdini 1984).  Such influence from friends, co-workers and
other “discussants” significantly affects the decisions on whether and how to vote (Paul Beck,
Russell Dalton, Steven Greene, and Robert Huckfeldt 2002).  Third, in the political arena,  voter
awareness of specific issues is quite low, and hence susceptibility to persuasion is high (Zaller
1992).
We present a model of the creation of social networks, and of their use by politicians to
obtain support, motivated by these empirical findings.  These networks can be political parties,
trade unions, religious coalitions, political action committees, or even listeners of Rush
Limbaugh’s radio show.  The key idea is that people are influenced by those inside their
network, but not by those outside, because those inside a network talk to and persuade each
other.   Networks are created by entrepreneurs using core issues that are centrally important to
members, such as religious beliefs or union wages, but can then be “rented out” to politicians
who seek votes as well as support for other initiatives and ideas, which might have little to do
with their members’ core beliefs. 
I.  A Simple Model.  
We assume that there is a continuum of agents, uniformly distributed on the interval2
[0,1].  There is only one issue over which agents hold prior beliefs.   The location i of the agent
determines his prior belief   on the issue in question.  Agents interact with those in their xi
0
network, and in the process influence, as well as become influenced by, the views of others.  Let
 denote the equilibrium beliefs of agent i, after he has interacted with those in his network, xi
*
and let  be the average equilibrium belief of those in his network, or the network’s xi
equilibrium message.  Let  be the size of the network that agent i is in, and let   be the Ni () fN i
influence function, defined by:
(1)   () () xxf N x x ii i i i
* =+ −
00
with   () ff f 00 0 0 => < ,' , ' ' .  and 
Such personal influence may consist of exchange of information, but also appeals to friendship,
emotion, or group identity (as in “Democrats don’t let friends vote Republican”).  The more
people are in a network, the stronger is its overall influence on a person’s beliefs, yet the weaker
is the influence per capita.  Moreover, the change in beliefs is proportional to the distance
between the average equilibrium beliefs in a network and the original beliefs of its members. 
Networks pool the beliefs of their members closer together. 
Finally, we assume that two people influence each other if and only if their equilibrium
beliefs are less than d apart, so d is the maximum network size.  That is, for two agents to be in
the same network (i.e., influence each other), their equilibrium beliefs cannot be more than d
apart.  In this  model, all people inside the network influence each other equally.  We only look
at equilibria in which networks do not overlap.  For two agents in different networks not to
influence each other, their equilibrium beliefs must be at least d apart, i.e., the networks must be
at least d apart. 3
We are thus looking for the possible equilibrium structures of networks on the [0,1]
interval, in which two equilibrium conditions hold: a) connectedness – the size of any network
cannot be larger than d, and b) separation – networks must be at least d apart.   We examine the
conditions for different equilibrium configurations of networks.  
Grand Coalition
Can the grand coalition, including the whole population, form in this model?  It is easiest
to form such a coalition with the centrist equilibrium message, so  =  .  Let   be the xi
1
2 xL
equilibrium belief of the agent whose prior is zero, and   be the equilibrium belief of the agent xH
whose prior is 1.  Since we have the grand coalition, separation is not an issue.   The only
question is: can connectedness hold in this coalition?  We can compute that 
(2)  () xf L =.5 1
(3)                                          () xf H =−  15 1 .
The connectedness condition holds that:
(4)  () xx f d HL −= − < 11.
How can this condition be interpreted?  A higher maximum size of the network, d, means either
that people are capable of listening to a broader range of views, or, equivalently, that the range of
views in the society is smaller relative to whom people talk to.  The value   refers to the () f 1
maximum total influence in the grand coalition.  Condition (4) says that if the society is
sufficiently connected that people at the opposite ends of the grand coalition can still influence
each other, then the grand coalition is an equilibrium.  Obviously, the grand coalition is more
likely to form when d is high: severely segmented societies are unlikely to form such networks.4
Two Equal-Sized Networks 
Can two equal sized networks, around   and  , be an equilibrium.  Let   and   
1
4
3
4 xL xH
be the lowest and highest equilibrium beliefs, respectively, of the members of the low (left)
network.  There is an identical in size network around  .  In a symmetric equilibrium, the size
3
4
of each network,  ,  must be smaller than d, and the distance between them,  , xx HL − 12 − xH
must be higher than d.    We can compute that:
(5)  xf f L =+

 

  −

 

  =

 

 

 

  0
1
2
1
4
0
1
4
1
2
,
(6)  xf f H =+

 

  −

 

  =

 

  −

 

 

 

 
1
2
1
2
1
4
1
2
1
2
1
4
1
2
.
The equilibrium conditions then become:
(7)     xx f d HL −= −

 

  <
1
2
1
2
1
2
, or,
(8)  1
1
2
2 −

 

  < fd ;
and 
(9)      12 11
1
2
1
2
−= − +

 

  > xf d H , or,
(10)  fd
1
2
2

 

  > .
When (8) and (10) both hold, two equal-sized networks form an equilibrium.  For a given
influence function f, there may exist a range of parameter values d for which the grand coalition
and two equal sized networks are both equilibria.  These d’s are high enough that the grand
coalition can form, but low enough that it is possible to keep the two networks separated. 
Multiplicity of equilibria is pervasive: we can also have asymmetric equilibria, equilibria with
small fringe networks and large mainline networks, and so on. The higher is d, the more likely
are equilibria with large networks, but for highly polarized societies, with small d’s, we can only
have equilibria with many small networks.  In a dynamic framework, some networks may5
become unstable and disappear, because the channels of influence among their members weaken,
or because they become invaded and influenced by different opinions. 
In summary, the model shows how networks of like-minded individuals can form in
equilibrium.  These networks are not like telephone networks, where communication channels
are established technologically, and then can be filled with any messages.  Rather, here the
issues, and the resulting shared beliefs, bind the network together.  People may start apart from
each other in their views, but through communication they converge closer to each other, and
further away from people with different beliefs.   (Obviously, if   is high enough, there is an f '
equilibrium where everyone converges to identical beliefs.)    Thus unions might bring members
together around beliefs about their shared economic interests, religious coalitions might bring
networks together around shared faith, and political action committees might bring together
devotees of specific causes.  Political parties have some of the same features: Republicans might
keep supporters  together through patriotism, opposition to abortion and support of tax cuts,
while Democrats bind supporters by emphasizing civil liberties (for the elites), the right to
choose, and redistribution.  This model of networks shaped by shared beliefs is closely related to
George Akerlof and Rachel Kranton’s (2000) model of identity and Robert Putnam’s (1993) idea
of social capital of a community.
We described the possibility of multiple stable equilibria, with one, two, or even more
networks forming.  In these equilibria, people in different networks are too far apart in their
beliefs to persuade each other.  Such separation is essential for the leaders of the networks: if a
network comes too close to others, its members might come under foreign influence, and as a
consequence the ideological coherence of the network is endangered.  Organizers of extremist6
networks do not want mainstream opinion to come close to their views: by infecting the beliefs
of their members, such convergence can destroy the network altogether.  Consistent with this
prediction, organizers of extreme movements often insist on total control of their members’
beliefs as well as physical separateness from the rest of the world, to prevent the dilution of
beliefs by the mainstream opinion.
If networks are thought of as political parties, our results suggest both how parties form
and why the Downsian prediction of convergence in their platforms to the median voter’s beliefs
may not be valid.   In our model, if party platforms converge, members of the competing parties
begin to influence each other, and the parties lose control over the beliefs and preferences of
their members.  It might be electorally advantageous to choose more extreme platforms, and to
thus keep the members isolated from the influence of the opposition.  This would be especially
true if one party thinks that its coalition is sufficiently large to win the election: convergence
toward the center then unambiguously reduces the chances of electoral success.  There are strong
incentives in this model to polarize, and to avoid the formation of the grand coalition with a
tossup in the election. 
  
II.  Networks and Political Leverage. 
Networks offer political leverage.  Some of the messages that are broadcast over these
networks corroborate the core beliefs of their members and reinforce network identity.  Through
mutual persuasion, these messages bind members of a network  together.  Suppose now that
there is another, totally orthogonal, issue that the members of the network have no views about,
because of their political apathy or “low awareness.”   The network provides a communication7
structure for persuading its members of a particular position with respect to this new issue.   If
the members’ prior is very diffuse, their posterior belief becomes that of the network as a whole. 
The network is a cheap and efficient mechanism for sending such messages, because it leverages
the persuasive capacity and credibility of its own members. 
This characteristic of networks makes them valuable to politicians, who need only to
persuade the leadership of the network on any of such non-core issues, and get the whole
network to support their positions.  A politician can persuade the leadership by endorsing the
network’s position on its core belief, by offering resources, or simply by persuading its
leadership.   Networks thus become powerful political forces--both through votes and through
other forms of support--even on issues that their members do not care much about. 
Two examples illustrate this principle.  Conservative and religious organizations support
President Bush because he promotes  their beliefs.  But they also support him on the war in Iraq.  
Now, the Republican networks would not necessarily be in favor of that war, since they have
often taken isolationist positions, including most recently on international trade, the Kyoto
agreement, and the U.N. endorsement of the very same war.   Yet once the President has
convinced the leaders of his principal networks to support the war, the persuasion channels
supplied by these networks generated broad support from members who could have equally
enthusiastically taken the isolationist position (and have supported such positions on other
matters).   Likewise, President Clinton managed to get some of the core Democratic networks,
which he supported through  social and other policies, to endorse NAFTA, a policy toward
which their members were at best neutral. 
Our analysis points to a somewhat different view of political competition.  In this view,8
social entrepreneurs such as union or religious leaders are the brokers creating the networks
using issues that bind members together.  They can then “rent out” their networks to candidates
who seek support.  Voting support in an election is itself a good example of this: many voters are
rather indifferent among the candidates, and can be persuaded how to vote.  By receiving
endorsements from network organizers, politicians get access to members who can then persuade
each other to support them.  
Thinking of political competition in terms of networks also points to some characteristics
of a desirable communications strategy for a candidate.  The best positions bind the networks
that support you, and divide those of your opponent.  For example, what would be the issues for 
presidential candidates to promote in the 2004 election?  The binding issues for key Republican
networks are religion, opposition to abortion, and tax cuts.   Among those, the opposition to
abortion is probably least attractive, because the opposing position also unites and binds many
Democrats.  On the other hand, some religious issues, such as the opposition to gay marriage, are
tremendously attractive to the Republicans, because they both unify their core networks and
divide the Democratic ones.  Likewise, tax cuts are attractive as a message because they both
bind the Republican networks, and divide the Democratic ones: many Democrats support the tax
cut to the middle class.    
For the Democratic party, it is harder to find such binding issues that also divide the
opposition.  A key message in the current campaign might be protectionism, of “fair trade,” an
issue that unifies many Democratic networks, but divides at least some of the Republicans.   In a
similar spirit, we would expect the Democrats to run a big “safety net for the middle class”
campaign on matters such as health and education, both because it unites their networks and9
divides those of the opposition.  Opposition to the war in Iraq is more problematic: it is only a
good strategy if the Democrats are united this view, and Republicans are divided.  And the civil
liberties concerns about the Bush administration that bind the elite Democrats are poor campaign
issues because they divide the Democrats’ own networks. 
III.  Conclusion.
This paper has presented a simple model of formation of social networks through which
people persuade and influence each other.  The key feature of the model is that social
entrepreneurs can organize networks around core beliefs that bind members together, and then
rent out these networks to politicians seeking support.  We argued that this framework is useful
for thinking about efficient messages in political campaigns, including the present one. 
More generally, this paper is part of a growing body of research in economics that
assumes that individual beliefs on many issues are flexible, and that as a consequence people are
vulnerable to persuasion and influence.  Hard information is only one tool of such influence,
opinions and stories of trusted authorities, firms, media, and friends are as important.   This
perspective has had some influence on economic research in finance (Harrison Hong, Jeffrey
Kubik, and Jeremy Stein 2004), education (Michael Kremer and Andrei Sarychev 2000), social
beliefs such as hatred (Edward Glaeser 2002), and media (Sendhil Mullainathan and Andrei
Shleifer 2003).  It may very well be broadly useful for thinking about politics.10
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Glaeser, Edward,  2002, “The Political Economy of Hatred,” NBER Working Paper # 9171.
Graber, Doris, Processing the News: How People Tame the Information Tide. New York, NY:
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Grasnovetter, Mark,“The Strength of Weak Ties,” American Journal of Sociology, 1973, 78(6)
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