imagine the movement of agents in the environment of the model some-
what similarly to the pieces of a board-game moving in a pre-defined,
structured space. However, unlike the pieces of the game that are moved
by the players, the agents in a computer model follow simple (or, given
recent gains in computational power, increasingly complex) rules of
behavior. Computer games (in which many characters might follow behav-
ioral rules rather than the instructions of players) provide another analogy
to agent-based models, except that graphics and appearance are not rele-
vant factors in agent-based models. An agent-based model can be pro-
grammed without any graphical interface, agents being represented by
abstract programming objects. However, some programming languages,
such as the popular NetLogo environment used in this chapter, do provide
graphical representation by default—which often proves itself a useful
feature for both educational and research purposes.
Both Mitchel Resnick’s bookTurtles, termites, and traffic jams(1994) and
Uri Wilensky and William Rand’s Introduction to Agent-based modeling
(2015) provide accessible, hands-on introductions to the NetLogo environ-
ment, including many sample projects.^3 The agents in NetLogo are represent-
ed by so-calledturtlesthat can move around in themodel world, found in the
Interface tab of the programming environment, as instructed by the program
code entered into the Code tab. The turtles are moving onpatches, which can
be thought of asfloor-tiles on the surface of the model world. Patches cannot
move but they can store information and turtles can interact with patches.
The position of both turtles and patchesis given by horizontal x and vertical
y coordinates (as in a Descartes system ofcoordinates). Theinterface tab can
be also used to select and adjust parameters, as well as display values and
diagrams that are updated as the model runs, as instructed by the code. Most
importantly, in most models, the interface will include two standard buttons:
thesetupbutton and thegobutton that activate the setup and go procedures
of the code, respectively. The setup procedure will set the parameters of the
world, populate it with turtles, and set other initial parameter values. The go
procedure will instruct turtles to perform calculations, make decisions, and
move within the modeling world. The go button can be also set to run the go
procedure repeatedly.
The Interface of the NetLogo model“Mission”can be seen in Figure 9.1.^4 At
the top of the interface, there is a slider called“speed,”which serves to adjust
(^3) One possible drawback of using Resnick’s very didactic but older book is that it is based on
an early version of NetLogo and the code samples need adjustment to work in recent editions of
the software.
(^4) The model has been adapted to function in NetLogo Web. The programfile is available as
supplementary material to this book at the url http://www.religionandcognition.com/netlogo/.
The model can be run in an internet browser without installing NetLogo on the computer.
192 Cognitive Science and the New Testament