Growing Arti�cial Societies� Social Science from the

Bottom Up� By Joshua M� Epstein and Robert Axtell�Complex Adaptive Systems Series� Washington� D�C�Brookings Institution� Cambridge and London� MITPress� ����� Pp� xv� �� ����� cloth �ISBN ����� ����� ���� paper �ISBN ����� � ���� � ��� CD�ROM �ISBN ����� ������

In the past� tractability issues have generally forcedeconomists to focus their attention on specialized aspectsof economic behavior� without detailed consideration ofpsychological� cultural� demographic� and migratory pro�cesses� The recent development of agent�based compu�tational frameworks� such as the Sugarscape model de�veloped by Joshua Epstein and Robert Axtell at theBrookings Institution� raises new questions concerningthe appropriate bounds of economic analysis� for theseframeworks permit the study of social activity from amuch more inclusive perspective� Indeed� such frame�works may at last provide a common paradigm for socialscience as a whole�

Using Sugarscape animations as illustration� this fas�cinating monograph persuasively makes the case thatan inclusive agent�based computational approach to thestudy of dynamic social systems can be both productiveand fun� The monograph consists of six chapters� withtechnical details largely relegated to three appendices�

Chapter I provides general motivation for the authors�agent�based computational methodology� Noting thatthis methodology has a long lineage� including the semi�nal work of Thomas Schelling� the authors argue that therecent advent of more powerful computational tools has�nally made large�scale agent�based modelling practical�

In particular� object�oriented programming languagessuch as Java� C��� and Object Pascal �the languageused for Sugarscape� permit the construction of arti��cial societies consisting of heterogeneous agents situatedin external environments� Agents and environmental fea�tures �e�g�� resource sites� are instantiated as objects withinternal attributes �states and behavioral rules�� �Non�genetic� attributes may adapt in response to agent�agentand agent�environment interactions� and �genetic� at�tributes may be passed in modi�ed form to descendants�Such an arti�cial society can grow itself over time� withlarge�scale social structures and group behavior emerg�ing from the bottom up� i�e�� from local agent�agent andagent�environment interactions� There is no need to im�pose �ctitious top�down coordinating mechanisms suchas representative agents� market clearing� and environ�mental stationarity�

Sugarscape is introduced in Chapter II in simple form�Agents inhabit a two�dimensional grid of sugar�bearingsites� where sugar is a renewable resource which agentsmust constantly collect and consume in order to survive�Each agent has a �xed� randomly�determined genetic en�dowment consisting of a sugar metabolism �sugar burned

per time step� and a vision level that helps it search forsugar� Any sugar collected by an agent in excess of itsmetabolic needs is added to its sugar holdings �wealth��An agent dies any time its wealth drops to zero or below�Experiments focusing on wealth distribution include anadditional feature� agents die in �nite time even if theymaintain positive wealth levels� and any agent that diesis replaced by another randomly generated agent� De�spite the simplicity of this setting� the migration patternsand distributions of wealth and genetic endowments thatarise and persist are seen to be related in interesting waysto the �xed structural characteristics of the arti�cial so�ciety� namely� sugar capacity and renewability rates ateach site� and local agent search rules�In Chapter III the authors introduce sexual repro�

duction �inheritence by children of parental genetic en�dowments�� cultural transmission �modi�cation of non�genetic attributes via agent�agent interactions�� andcombat �wealth plunder�� The authors are then able togrow crude caricatures of early social history character�ized by the formation of combative spatially segregated�tribes� �cultural groupings��In Chapter IV the authors introduce a second resource�

�spice�� Agents have randomly determined metabolicneeds for spice and sugar� and so have an incentive toengage in sugar�spice trades� Only bilateral Pareto im�proving trades between neighboring pairs of agents arepermitted� at prices determined locally by a bargainingrule� When trade is the only behavioral mode� pricestend ultimately to bunch around an �equilibrium� pricelevel even though potential gains from trade are neverfully extracted� When additional behavioral modes areintroduced� however� such as sexual reproduction andcultural transmission� the markets that emerge tend todisplay persistently higher price volatility with randomdrift� Various other related topics� e�g�� credit arrange�ments and the emergence of �nancial hierarchies� are alsoexplored in this thought�provoking chapter�The construction of a uni�ed immunology and epi�

demiology model is the concern of Chapter V� The au�thors examine the consequences of allowing intra�agentand inter�agent spread of infectious diseases to interactwith social processes such as trade and migration� Theconcluding Chapter VI reviews previous �ndings and dis�cusses future possible research directions�In summary� the authors provide preliminary but in�

triguing evidence that computational frameworks such asSugarscape can provide laboratories for exploring whichmicromechanisms su�ce to generate important socialphenomena� As they note� it is not the generation per sethat is surprising� but rather that simple local rules oftensu�ce for this generation� The acid test of explanationmay thus one day come to be �can you grow it��

LEIGH TESFATSION

Iowa State University