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The Ostracism Model.
This ABM model [12] replicates and further explores an
analytical model [13]. Both ostracism models investigating the role of norms and
social disapproval by norm followers for cooperation amongst a group of harvesters
that share a common resource. If the group of norm followers that harvest the re-
source sustainably is large enough (i.e., social capital is high), they engage in ostracis-
ing over-harvesting norm violators. The system develops through agents imitating
best performing strategies. When the initial number of norm followers and hence the
social capital of the community is low, norm violators prevail and the resource is
over-harvested. If the number of norm followers is high and defection of norm viola-
tors is not too large, a community of norm followers evolves. If the initial number of
norm followers is high but the defection of the norm violators is large, coexistence
emerges, where a small group of norm violators share the resource with a large group
of norm followers. The ecological and social drivers of system dynamics balance out,
e.g., the gain defectors get from higher resource levels due to high levels of coopera-
tion is balanced with the social disapproval they experience through the community of
norm followers.
The main concepts of the model are shown in Table 1. On the macro level, the
emergent pattern (target) is the level of cooperation, e.g., the proportion of norm fol-
lowers that harvest sustainably. In addition, the proportion of cooperators and defec-
tors, the ostracism costs for defection and resource volume affect the agents in their
aggregated form. On the micro level the agents are characterised by their behavioural
options, to defect or to cooperate. They choose the behaviour that performs best, i.e.,
has the highest utility
4
.
4
The environment (the meetingList) defines with whom the
Table 1.
An overview of the main concepts of the (abstract) cooperation model we depart from
filtered by main ABM dimensions
Macro Level
Emergent pattern
Level of cooperation
Aggregated
variables
Proportion of Cooperators, Defectors [%]
Ostracism costs
Resource volume
Behaviours options: {Cooperate, Defect}
Decision-making: behaviour = imitate if other
has a better strategy
Micro level
Agent
Environment
(topology)
MeetingList [ random ]
Physical environment: Receive utility()
Social environment: Compare utility()
Interaction
4
In the model there is a distinction between payoff and utility. Payoff represents the 'crop
output', whereas utility is the final gain (in money) that an agent can receive. Ostracism af-
fects the step from payoff to utility, which can be illustrated by an agent being blocked access
to the market.
