ABM Applications to the Social Sciences

1
ABM Applications to the Social Sciences

• Lars-Erik Cederman
• Department of Government, Harvard
• EITM Workshop, July 18, 2002

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Outline Applications

• Three types of agent-based models
• Example Barabasis Preferential Attachment Model
• Applications to political science
• Example GeoSim and the democratic peace
• Validation

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Three types of emergent effects
Emergent behavioral patterns
D
C
actor
actor
actor
Emergent boundaries and networks
actor
actor
Emergent cultural configurations
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Barabasis Preferential Attachment Model

• A. L. Barabasi et al. 1999. Mean-field theory
  for scale-free random networks. Physica A 272
  173-187.

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Applications to political science

• Cooperation theory Axelrod etc.
• Voting and party politics Kollman, Miller,
  Page 1992
• Ethnic conflict Bhavnani Backer 2000 Epstein
  et al 2001 Lustick 2000 Cederman 2001
• Geopolitical models Bremer Mihalka 1977
  Cusack and Stoll 1990 Cederman 1997

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Modeling the democratic peace with agent-based
modeling

• Assume the democratic peace hypothesis to hold at
  the micro-level
• How can the democratic peace spread to the entire
  state system?
• Reference Modeling the Democratic Peace as a
  Kantian Selection Process Journal of Conflict
  Resolution (August 2001).

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Outline

• 1. Modeling geopolitics
• 2. Adding tags
• 3. Adding alliances
• 4. Adding collective security
• 5. Replications
• 6. Conclusions

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Modeling geopolitics GeoSim

• Hobbesian geopolitical environment
• Cederman 1997 Emergent Actorsgt RePast
• 15 x 15 grid
• local combat and conquest
• two types of actors
• non-democratic states power-seekers
• democratic states conditional cooperators

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A dynamic network on a grid
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Tagged decision rule for democratic state i

• for all external fronts j do
• if i or j fought or j attacked an ally of i
  then
• attack j else cooperate with j Grim
  Trigger
• if there is no action on any front then
• randomly select a non-democratic neighbor state
  j
• with probability p(i,j) factoring in alliances
  do
• launch unprovoked attack against j

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Threshold functions
Probability
Decision to attack p(i,j)
Combat victory
Force ratio
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Structural change conquest

• Conquest follows victorious battles
• Each attacker randomly selects a battle path
  consisting of an attacking province and a target
• The outcome depends on the targets nature
• if it is an atom, the whole target is absorbed
• if it is a capital, the target state collapses
• if it is a province, the target is absorbed

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Guaranteeing territorial contiguity
Conquest... resulting in... partial state
collapse
"near abroad" cut off from capital
Target Province
Agent Province
j
i
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Geopolitical sample run
Time 0
Time 1000
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Sample run with tags
Time 0
Time 1000
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Sample run with alliances
Time 12
Time 1000
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Sample run with collective security
Time 76
Time 1000
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Share of democracies at t1000
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Conclusions from DP-Model

• It is indeed possible to grow the democratic
  peace in a Hobbesian world
• All three Kantian mechanisms contribute to the
  democratic peace
• Spatial context crucial for cooperation
• But tagging does not always suffice
• Counter-intuitive finding democracy may
  undermine itself

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Four types of validation
Object of validation End point
Process
Mode of validation Qualitative Distribu- tiona
l
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The limits of ABM?

• ad hoc assumptions
• failure to yield predictions
• fragility of results
• lack of cumulation

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General readings on agent-based modeling

• Axelrod, Robert. 1997. The Complexity of
  Cooperation Agent-Based Models of Competition
  and Collaboration. Princeton Princeton
  University Press.
• Casti, John L. 1997. Would-Be Worlds How
  Simulation Is Changing the Frontiers of Science.
  New York Wiley.
• Cederman, Lars-Erik. 1997. Emergent Actors in
  World Politics How States and Nations Develop
  and Dissolve. Princeton Princeton University
  Press.
• Epstein, Joshua M. and Robert Axtell. 1996.
  Growing Artificial Societies Social Science From
  the Bottom Up. Cambridge, Mass. MIT Press.
• Holland, John H. 1995. Hidden Order How
  Adaptation Builds Complexity. Reading, Mass.
  Addison-Wesley.
• Special issue on Computational Modeling, The
  Political Methodologist, Fall 2001.
• See also web pages http//www.courses.fas.harvard.
  edu/gov2015 and http//www.courses.fas.harvard.ed
  u/gov2016