A Survey Of Agent-Based Models

Federico Bianchi together with Flaminio Squazzoni accept published a real useful survey of the evolution together with uses of agent-ba...

Federico Bianchi together with Flaminio Squazzoni accept published a real useful survey of the evolution together with uses of agent-based models inwards the social sciences over the past times twenty-five years inwards WIREs Comput Stat 2015 (link). The article is a real useful reference together with give-and-take for anyone interested inwards the applicability of ABM inside sociology.

Here is their full general Definition of an ABM:

Agent-based models (ABMs) are figurer simulations of social interaction betwixt heterogeneous agents (e.g., individuals, firms, or states), embedded inwards social structures (e.g., social networks, spatial neighborhoods, or institutional scaffolds). These are built to discovery together with analyze the emergence of aggregate outcomes. By manipulating behavioral or interaction model parameters, whether guided past times empirical show or theory, micro-generative mechanisms tin hold out explored that tin delineate of piece of work concern human relationship for macro-scale organization behavior, that is, an existing fourth dimension serial of aggregate information or certainly stylized facts. (284)

This Definition highlights several of import features of the ABM approach:

• unlike traditional rational selection theory together with microeconomics, it considers heterogeneous agents
• it explicitly attempts to stand upward for concrete particulars of the social environs inside which agents act
• it is a micro to macro strategy, deriving macro outcomes from micro activities
• it permits a substantial marker of "experimentation" inwards the shape of modification of base of operations assumptions
• it is possible to supply empirical show to validate or invalidate the ABM simulation of a given aggregate outcome 

Bianchi together with Squazzoni banking concern complaint that the primary areas of application of agent-based models inwards social-science inquiry include a relatively express attain of topics. The start of these topics included uncoordinated cooperation, reciprocity, together with altruism. Robert Axelrod's piece of work on repeated prisoners' dilemmas represents a key instance of modeling efforts inwards this surface area (link).

H5N1 peculiar shape of altruism is punishment: imposition of a toll on non-cooperators past times other actors. Without penalization the exploitation strategy to a greater extent than oftentimes than non extinguishes the cooperation strategy inwards a attain of situations. H5N1 "reciprocator" is an musician who is opened upward to cooperation exactly who punishes previous non-cooperators on the adjacent interaction. Bianchi together with Squazzoni spend fourth dimension describing an ABM developed by Bowles together with Gintis (link) to evaluate the 3 strategies of Selfish, Reciprocator, together with Cooperator, together with a derived Shirking charge per unit of measurement inwards a hypothetical together with heterogeneous population of hunter-gatherers. Here is Bowles together with Gintis' hypothesis:

The hypothesis nosotros explore is that cooperation is maintained because many humans accept a predisposition to punish those who violate group-beneficial norms, fifty-fifty when this reduces their fitness relative to other grouping members. Compelling show for the beingness together with importance of such altruistic penalization comes from controlled laboratory experiments, especially the written report of world goods, mutual puddle resource, ultimatum, together with other games.

And hither is their key finding, according to Bianchi together with Squazzoni:

They flora that the robustness of cooperation depended on the coexistence of these behaviors at a grouping storey together with that strong reciprocators were functional inwards keeping the storey of cheating nether command inwards each grouping (see the shirking charge per unit of measurement every bit a mensurate of resources lost past times the grouping due to cheating inwards Figure 1). This was due to the fact that the higher the issue of cooperators inwards a grouping without reciprocators, the higher the direct chances that the grouping disbanded due to high payoffs for shirking. (288)

Here is the graph of the incidence of the 3 strategies over the start 3000 periods of the simulation published inwards the Bowles together with Gintis article:

 

This graph represents the relative frequency of the 3 types of hunter-gatherers inwards the population, along alongside a calculated shirking charge per unit of measurement for each period. The Selfish profile remains the most frequent (between 40% together with 50%, exactly Reciprocators together with Cooperators hand relatively stable levels of frequency every bit good (between 30% together with 40%, together with betwixt 20% together with 30%). As Bowles together with Gintis argue, it is the robust presence of Reciprocators that keeps the Selfish grouping inwards check; the willingness of Reciprocators to punish Selfish actors keeps the latter grouping from ascent to total domination.

In this simulation the frequencies of Selfish together with Shirking laid about high (>85%) together with rapidly turn down to a relatively stable rate. After G iterations the 3 strategies achieve relatively stable frequencies, alongside Selfish at almost 38%, Reciprocator at 37%, Cooperator at 25%, together with a shirking charge per unit of measurement at almost 11%.

It is tempting to read the written report every bit representing a population that reaches a crude oil equilibrium. However, it is possible that the appearance of equilibrium conveyed past times the graph higher upward is deceptive. Other areas of complex phenomena heighten the possibility that this is non a longterm equilibrium, exactly rather that around time to come combination of percentages of the 3 strategies may ready off a chaotic redistribution of success rates. This is the key characteristic of a chaotic system: modest fluctuations inwards parameters tin Pb to major deviations inwards outcomes.

Also interesting in Bianchi together with Squazzoni's review is their handling of efforts to utilisation ABMs to model the diffusion of cultural together with normative attitudes (293ff.). Attitudes are treated every bit local "contagion" factors, together with the finish of the simulation is to model how different adjacencies influence the blueprint of spread of the cultural features.

Agents interacted alongside neighbors alongside a probability theme on the issue of identical cultural features they shared. H5N1 machinery of interpersonal influence was added to align 1 randomly selected dissimilar cultural characteristic of an agent to that of the partner, later interaction. (294ff.)

Social network characteristics accept been incorporated into ABMs inwards this area.

Bianchi together with Squazzoni besides consider ABMs inwards the subject areas of collective behaviour together with social inequality. They depict a issue of useful conclusions almost the potential role that ABMs tin play inwards sociology, including especially the importance of considering heterogeneous agents:

At a noun level, these examples present that exploring the fundamental heterogeneity of private behaviour is of paramount importance to sympathize the emergence of social patterns. Cross-fertilization betwixt experimental together with computational inquiry is a useful process. It shows us that past times conflating the concept of rationality alongside that of self-interest, every bit inwards measure game theory together with economics, nosotros cannot delineate of piece of work concern human relationship for the subtle social nuances that characterize private behaviour inwards social contexts. (298)

And they believe -- possibly unexpectedly -- that the sense of edifice ABMs inwards a attain of sociological contexts underlines the importance of institutions, norms, together with social context:

Moreover, these ABM studies tin assistance us to sympathize the importance of social contexts fifty-fifty when looking at private behaviour inwards a to a greater extent than micro-oriented perspective. The role of social influence together with the fact that nosotros are embedded inwards complex social networks accept implications for the type of information nosotros access together with the types of behaviour nosotros are exposed to. (301)

This is a useful contribution for sociologists, every bit a foundation for a 3rd choice betwixt statistical studies of sociological phenomena together with high-level deductive theories of those phenomena. ABMs accept the potential of allowing us to derive large social patterns from good chosen together with empirically validated behavioral assumptions almost actors.

I mentioned the mutual finding inwards complexity studies that fifty-fifty fairly elementary systems possess the capacity for abrupt instability. Here is a simulation of a three-body gravitational organization which illustrates periods of relative stability together with and thus precipitous destabilization.

Three-body occupation 3D from Daniel Piker on Vimeo.

ABMs permit us to model populations of interactive adaptive agents, together with oftentimes the simulation produces of import together with representative patterns at the aggregate level. Here is an interesting predator-prey simulation on YouTube using an ABM approach by SSmithy87:

The writer makes a key hollo for at 2:15: the blueprint of variation of predator together with prey presented inwards the simulation is a well-known characteristic of predator-prey populations. (Red is predator together with blueish is prey.)

But the equations representing this human relationship were non built into the model; instead, this characteristic blueprint is generated past times the model based on the elementary behavioral assumptions made almost prey together with predators. This is a bright demonstration of the novelty together with importance of ABM simulations.