Shennan, SJ

English

UCL Discovery

Cambridge Archaeological Journalhttp://journals.cambridge.org/CAJAdditional services for Cambridge Archaeological Journal:Email alerts: Click hereSubscriptions: Click hereCommercial reprints: Click hereTerms of use : Click hereStructure and Agency in Complex Adaptive Systems A review of Dynamics in Human and Primate Societies: Agent­based Modeling of Social and Spatial Processes, edited by Timothy A. Kohler & George J. Gummerman, 2000. (Santa Fe Institute Studies in the Sciences of Complexity.) Oxford & New York (NY): Oxford University Press; ISBN 0­19­513168­1 paperback, £28.99 & US$40; ISBN 0­19­513167­3 hardback, £46.99 & US$65, 412 pp., ills.Stephen J. ShennanCambridge Archaeological Journal / Volume 11 / Issue 02 / October 2001, pp 274 ­ 276DOI: 10.1017/S0959774301250161, Published online: 28 November 2001Link to this article: http://journals.cambridge.org/abstract_S0959774301250161How to cite this article:Stephen J. Shennan (2001). Cambridge Archaeological Journal,11, pp 274­276 doi:10.1017/S0959774301250161Request Permissions : Click hereDownloaded from http://journals.cambridge.org/CAJ, IP address: 144.82.107.38 on 22 Aug 2012274Review ArticlesStructure and Agency in ComplexAdaptive SystemsDynamics in Human and Primate Societies:Agent-based Modeling of Social and Spatial Processes,edited by Timothy A. Kohler & George J.Gummerman, 2000. (Santa Fe Institute Studies inthe Sciences of Complexity.) Oxford & New York(NY): Oxford University Press; ISBN 0-19-513168-1paperback, £28.99 & US$40; ISBN 0-19-513167-3hardback, £46.99 & US$65, 412 pp., ills.Stephen J. ShennanArchaeologists, anthropologists and their social phi-losopher predecessors have long been interested inthe processes by which such structures as states andcivilizations emerge from human action. The frame-works within which they have pursued that interesthave varied over time. One of the earlier ones, andcertainly one of the most influential, was the Marxistview of history as class struggle, involving the emer-gence  of  contradictions  between  the  forces  andrelations of production. More recently, the neo-evolu-tionary framework which has been such a dominantinfluence in Anglo-American archaeology over thelast 40 years has emphasized the growth of complex-ity and social hierarchies as a process of group adap-tation. Those groups which developed hierarchicalsocial mechanisms for overcoming subsistence in-stability, or for competing better with rival groups,would be more successful. The adoption of innova-tions such as agriculture, which made it possible forsocieties to harness more energy from their environ-ment, provided the basis for supporting increasinglycomplex social structures.In the 1970s there were attempts to make explo-rations of these adaptive processes, and in particularthe process of morphogenesis, more rigorous and sys-tematic by building computer simulations in whichvariables were defined and their relations with othervariables specified by systems of equations. By andlarge, these attempts were unsuccessful. If structurewas going to emerge, it had to be programmed intothe relations between the variables in the first place.The agent-based approach to the modelling ofsocial processes, of which this book is an excellentexample, attempts to overcome the shortcomings ofthese  earlier  frameworks.  Consequences  of  postu-lated processes are rigorously followed-through bymeans of computer simulation. Structure is not builtin  to  the  model  from  the  beginning  but  emergesfrom the interaction between ‘agents’ through a proc-ess of self-organization. ‘Agents’, as Kohler (p. 2)describes in his introduction, are ‘processes, how-ever simple, that collect information about their en-vironment, make decisions about actions based onthat information, and act’. The outcome of such localactions, based on local knowledge and decision-mak-ing criteria, can often be large-scale patterns unin-tended  by  the  individual  agents.  A  famous  earlyexample showed that individual preferences for spa-tial neighbours of the same type as oneself rapidlyled to the emergence of segregated neighbourhoods.This insight has profound consequences. Thehuman tendency to anthropomorphize the world tendsto lead to the view that complex patterns must be theresult of complex intentions and mental operations aswell as global knowledge. In fact, nothing could befurther from the truth, as te Boekhorst and Hemelrijkillustrate  in  their  description  of  an  example  fromsituated robotics. Their robots are small vehicles thatmove randomly distributed cubes into a central heapand line up the rest against the wall. Far from requir-ing complex object recognition software and capacitiesfor co-ordinating movement and action to achieve this,they need only minimal sensors and movement con-trol. The pattern emerges from simple local responsesto colliding with the cubes and with other robots.Indeed, this is one of the major themes of thebook:  what  are  the  minimal  requirements  for  theemergence of structure from action? Are they purelymechanical, so that structure is simply a function ofthe degree of interconnection between different ele-ments? Is it necessary to assume evolutionary dy-namics of selection and adaptation? What is the roleof conscious intentions and knowledge? These areissues of great significance for the understanding ofboth human history and biological evolution.275Review ArticlesIn addition to their robotics example, te Boek-horst and Hemelrijk cite a simulation of dominanceinteractions  between  individuals  in  which  the  ef-fects of winning and losing are self-reinforcing. Thestudy showed that patterns of cooperation could arisewithout any memory mechanism in the individuals,simply as a result of the fact that fleeing from theattack range of one opponent leads an individualinto  the  range  of  another.  The  observed  series  ofimmediate  reciprocal  actions  of  support  corre-sponded to that of the famous Tit-for-Tat strategy inthe Prisoner’s Dilemma game but without any of theassumptions about the costs and benefits of coopera-tion versus defection that Prisoner’s Dilemma involves.In  a  similar  vein,  Pepper  and  Smuts  use  anagent-based model to examine the evolution of co-operation in a simple ecological context. They showthat patchiness of food distribution can itself createsufficient population structure to generate between-group selection, leading to the spread of group-ben-eficial traits, without any need for the operation ofkin-selection. One of the traits they modelled wasfeeding restraint. In freely-mixing populations theindividual paying the cost of feeding restraint wasonly rarely among those reaping the benefits, in con-trast to the situation when resource patches wereisolated  and  small.  Too  much  isolation,  however,was also problematical. Continuing cooperation de-pended on groups being able to export their increasedproductivity to other patches, otherwise between-group selection did not occur. When within-groupselection is the only relevant process then coopera-tors will die out.Skyrms’  study  of  the  evolution  of  signallingsystems and inference is also a generic one, demon-strating that evolutionary dynamics provides an ac-count of the spontaneous emergence of signallingsystems which does not require pre-existing com-mon knowledge or agreement. Where the individu-als concerned have a common interest, almost anysort of adaptive dynamics leads to successful coordi-nation of a signalling system, because such systemsare powerful attractors in the dynamics. Which sys-tem emerges, however, depends on the vagaries ofthe initial stages of the evolutionary process (Skyrmsp. 84). The evolution of a correct rule of inference inthe context of a signalling system ‘depends on therepeated occurrence of situations where there is apositive payoff for acting on the right conclusion’, inthe interest of both senders and receivers of signals(p. 87). Skyrms’ example is the correct inference of thetype of predator currently presenting a threat, fromalarm calls which are differentiated by predator.The remaining studies in the volume are spe-cific rather than generic. They use multi-agent mod-elling to explore particular situations, based on largequantities of specific information. Lake describes aproject in which multi-agent simulation is linked toGIS  to  explore  the  processes  involved  in  makingforaging decisions. The agents can learn from theirown experience and from others and make decisionsin the light of their knowledge and goals. In this casethe  GIS  link  enables  them  to  have  spatially-refer-enced knowledge in the form of their own cognitivemaps. The system is used to explore the likelihoodthat foraging for hazelnuts was a major determinantof Mesolithic land-use patterns on the Scottish is-land of Islay, and Lake concludes that it was not. Ashe explains, however, the major significance of hisproject  is  that  he  has  created  a  powerful  genericmodelling tool which can be used relatively straight-forwardly by others to model the activities of socialagents in a landscape.The papers by Kohler and colleagues and Deanand colleagues model prehistoric settlement changein southwestern Colorado and northeast Arizona re-spectively, using multi-agent techniques, and go onto compare the results of their models with the set-tlement  histories  actually  observed.  In  both  casesthis is a very illuminating process. Kohler et al. con-clude that towards the end of their period, either theimportance of dry-farming was decreasing, or farm-ers were now settling in locations which were ineffi-cient with regard to access to their fields, perhapsbecause social considerations placed a new impor-tance on community members living in face-to-facecircumstances. Their model does not reproduce thepopulation growth that actually occurred towardsthe end of the period they studied, probably becausetheir agents could not intensify, whereas in fact in-tensification seems to have been exactly what hap-pened,  through  the  increasing  use  of  water  andsediment management techniques. The Long HouseValley, whose settlement history was modelled byDean and colleagues, was abandoned after AD 1300,a process generally seen as the result of drought. Thesimulation  results,  however,  show  that  the  valleyenvironment after 1300 could have supported a re-duced population if people had disaggregated intosmaller communities and dispersed into favourablehabitats. It appears that the environmental factorsonly partially account for the abandonment of thearea. This contrast between the real world and thesimulated one is all the more striking in the light ofthe remarkable success of the simulation in model-ling many other aspects of the valley history, and276Review Articlesadds force to Dean et al.’s claim that agent-basedmodels can be seen as laboratories for testing com-peting explanations.Reynolds’ paper is also a settlement study, ex-ploring  the  role  of  conflict  in  chiefdom  and  stateformation in the Oaxaca valley by means of decisiontrees. These are used to express changes in the fac-tors predicting raiding and warfare targets over time.Lansing explores patterns of cooperation amongBalinese rice farmers and the mechanisms by whichsynchronized planting patterns emerge over largeareas in response to pest and water availability prob-lems. He shows that there is pairwise synchroniza-tion of patterns between upstream and downstreamfarmers but that individuals also imitate those neigh-bours who obtain the best results, a process whichleads  to  uniformity  in  planting  times,  with  highyields, and low variance in yields from one farmer tothe next; in other words, a highly satisfactory state ofaffairs produced by traditional methods, and far bet-ter than the results produced by centralized devel-opment policies.In a very different sort of way, Lehner’s longand detailed paper on ancient Egypt as a complexadaptive system is equally illuminating, although itdoes not involve any modelling at all. His main con-cern is to show that it makes no sense to see central-ised control as the key feature of archaic complexsocieties such as Egypt, not least because the state wasvery limited in its capacity to intervene. The complexadaptive  systems  perspective  directs  ustowards a bottom-up view of Egyptian society, lookingat the connections between people and households. Itis here that complexity lies, for example in the inequal-ity among people in various superordinate and sub-ordinate relationships at different scales, both withinand between households: hence the view of Egypt inthe paper’s title as ‘the fractal house of Pharaoh’.Finally,  Small’s  study  uses  an  agent-basedmodel to explore a classic anthropological issue, theimpact  of  marriage  rules  on  the  degree  of  socialstratification in Polynesia. Where rules prescribe mar-rying non-relations, patterns of stratification are ex-tremely  unstable.  For  stratification  to  emerge,marriage rules restricting kin have to be replaced byrules permitting endogamy; without this, the trajec-tories of chiefly lines through time converge on oneanother. Cross-cousin marriage produces a more sta-ble  system  where  chiefly  lines  tend  to  keep  theirhigh position, but the most stable and rigid system isachieved by permitting brother-sister marriage, suchas occurred in Hawai’i.In his introduction Kohler is very careful not toover-sell the virtues of multi-agent modelling, verywisely in the light of the history of panaceas whichhave come and gone in archaeology since the 1960s.Nevertheless, the papers in this book make a strongcase for the productiveness of agent-based model-ling approaches within a broad complex adaptivesystems framework. While predictions are alwaysdangerous, I would venture to suggest that thesemethods will be one of the main means by which thehierarchically-focused  social  evolutionary  ap-proaches of the last 40 years in archaeology are fi-nally reformulated and superseded.Stephen J. ShennanUniversity College LondonInstitute of Archaeology31–34 Gordon SquareLondonWC1H OPYEmail: s.shennan@ucl.ac.uk

Structure and Agency in Complex Adaptive Systems A review of

Dynamics in Human and Primate Societies: Agentbased

Modeling of

Social and Spatial Processes, edited by Timothy A. Kohler & George J.

Gummerman, 2000. (Santa Fe Institute Studies in the Sciences of

Complexity.) Oxford & New York (NY): Oxford University Press

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