Agent Based Models - Social Legislation - Lecture Slides, Slides of Introduction to Sociology

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Agent Based Models

in Social Science

The Big Picture: Collective Action

• Cooperation

• Alternative Models of Participation

• Social Networks

Alternative Models of Political Participation

• Computational Models of Adaptive Voters and

Legislators

• Parties, Mandates, and Voters: How Elections Shape the Future 2007
• Policy-Motivated Parties in Dynamic Political Competition JTP 2007
• Habitual Voting and Behavioral Turnout JOP 2006
• A Tournament of Party Decision Rules
• Empirical Models of Legislator Behavior
• Dynamic Responsiveness in the U.S. Senate AJPS 2005
• Elections and Markets: The Effect of Partisan Orientation, Policy Risk, and Mandates on the Economy JOP 2006
• Parties and Agenda-Setting in the Senate, 1973-

Alternative Models of Political

Participation

• Experiments
  • Altruism and Turnout JOP 2006
  • Patience as a Political Virtue: Delayed Gratification and Turnout Political Behavior 2006
  • Beyond the Self: Social Identity, Altruism, and Political Participation JOP 2007
  • Social Preferences and Political Participation
  • When It's Not All About Me: Altruism, Participation, and Political Context
  • Partisans and Punishment in Public Goods Games
• Genetics
  • The Genetic Basis of Political Participation
  • Southern California Twin Register at the University of Southern California: II Twin Research and Human Genetics 2006

Political Social Networks

• Court Precedents

• The Authority of Supreme Court Precedent Social Networks, forthcoming
• Network Analysis and the Law: Measuring the Legal Importance of Supreme Court Precedents Political Analysis , forthcoming

Other Social Networks

• Political Science PhDs

• Social Networks in Political Science: Hiring and Placement of PhDs, 1960- 2002 PS 2007

• Academic Citations

• Does Self Citation Pay? Scientometrics 2007

• Health Study Participants

• The Spread of Obesity in a Large Social Network Over 32 Years New England Journal of Medicine 2007
• Friends and Participation
• Genetic Basis of Social Networks

What is an Agent Based Model?

• “Boids” are simulations of bird flocking behavior

(Reynolds 1987)

• Three rules of individual behavior

• Separation
  • avoid crowding other birds
• Alignment
  • point towards the average heading of other birds
• Cohesion
  • move toward the center of the flock

• Result is a very realistic portrayal of group motion in

flocks of birds, schools of fish, etc.

What is an Agent Based Model?

• Comparison with formal models

– Same mathematical abstraction of a given

problem,

– but uses simulation rather than mathematics to

“solve” model and derive comparative statics

• Comparison with statistical models

– Same attempt to analyze data,

– but uses simulation data rather than real data

Disadvantages of Agent Based Modeling

• Models too simple
  • Could be solved in closed-form (Axelrod 1984)
  • Closed-form solution always preferable
• Models too complicated
  • Not possible to assess causality (Cederman 1997)
  • What use is an existence proof?
• Coding mistakes
  • Many more lines of code than lines in typical formal proof
• Data analysis
  • What part of the parameter space to search?

My Approach to Agent Based

Modeling

• Write down model

• Solve as much as possible in closed-form

• Justify simulation with mathematical description of

the complexity problem

• Use real world to “tune” model

• Make predictions

• Check predictions against reality

• Do comparative statics near real world parameters to

assess causality

Tournament ABM test-bed

• We advertised a computer simulation tournament

with a $1000 prize for the action selection rule

winning most votes, in competition with all other

submitted rules over the very long run.

• Tournament test-bed (in R) adapted from Laver

( APSR 2005)

• The four rules investigated by Laver were declared pre-entered

but ineligible to win: Sticker, Aggregator, Hunter and Predator

• Submitted rules constrained to use only published

information about party positions and support levels

during each past period and knowledge of own

supporters’ mean/median location

Departures from Laver (2005)

• Distinction between inter-election (19/20) and

election (1/20) periods

• Forced births (1/election) at random locations , as

opposed to endogenous births at fertile locations, à la

Laver and Schilperoord

• De facto survival threshold (<10%, 2 consecutive

elections)

• Rule designers’ knowledge of pre-entered rules
• Diverse and indeterminate rule set to be competed

against

Tournament algorithm portfolio

• Center-seeking rules: use the vote-weighted centroid or median
  • Previous work suggests these are unlikely to succeed, a problem exacerbated in a rule set with other species of the same rule
• Tweaks of pre-entered rules : eg with “stay-alive” or “secret handshake”

mechanisms (see below)

• Sticker is the baseline “static” rule for any dynamic rule to beat
• Hunter was the previously most successful pre-entered rule
• “ Parasites ” (move near successful agent): have a complex effect
• Split successful “host” payoff so unlikely to win – especially in competition with other species of parasite
• But do systematically punish successful rules
• No submitted rule had any defense against parasites
• No submitted parasite anticipated other species of parasite

Tournament algorithm portfolio

• Satisficing (stay-alive) rules : stay above the survival

threshold rather than maximize short-term support

• Substantively plausible but raise an important issue about agent time preference – which only becomes evident in a dynamic setting
• “Secret handshake” rules : agent signals its presence to

other agents using the same rule (e.g. using a very

distinctive step size), who recognize it and avoid attacking it

• Substantively implausible (?) but, given 29 rules and random rule selection, there was smallish a priori probability that an agent would be in competition with another using the same rule
• Inter-electoral explorers : use the 19 inter-election periods

to search (costlessly) for a good location on election day

• Substantively plausible but raise an important issue about relative costs of inter-electoral moves