Complex Adaptive Systems andComplexity Theory: Inter-relatedKnowledge Domains byRebecca Dodder and Robert DareESD.83: Research Seminar in Engineering SystemsMassachusetts Institute of TechnologyOctober 31, 2000IntroductionThis paper provides a description of two highly interrelated knowledge domains:Complex Adaptive Systems (CAS) and Complexity Theory. The initial sections providean overview, descriptive characteristics, background and social/institutional outlines forthe Complex Adaptive Systems knowledge domain. The next four sections providedescriptive material on applications of CAS thinking in the disciplines of physics,biology, economics and political science. While CAS has implications for many otherdisciplines, these sections illustrate how CAS thinking has found its way into ongoingtheory development in a representative set of fields. The next sections of this paperdescribe the highly related knowledge domain of Complexity Theory, providing materialon identifying and measuring complexity, and the relationship of complexity toengineering systems. The last section provides some closing thoughts on the outlook forthese two closely related knowledge domains.Overview of Approach to Complex Adaptive SystemsThe rise of “complex adaptive systems” (CAS) as a school of thought took hold in themid-1980’s with the formation of the Santa Fe Institute, a New Mexico think tank formedin part by former members of the nearby Los Alamos National Laboratory. Participantsat the Institute have come from such diverse disciplines as economics, physics, biology,ecology and archaeology. The Institute formed to draw from and further developthinking over the previous twenty years in a variety of disparate fields on the issue ofcomplexity. More information about the formation of the Santa Fe Institute is included inthe background section below.One important emphasis with CAS is on crossing of traditional disciplinary boundaries.CAS provides an alternative to the linear, reductionist thinking that has ruled scientificthought since the time of Newton. The new discipline has been distinguished byextensive use of computer simulation as a research tool.In his book, “Complexity: the Emerging Science at the Edge of Order and Chaos”,author M. Mitchell Waldrop describes the objectives associated with the developmentand use of CAS concepts. Santa Fe members sought to pursue a common theoreticalframework for complexity and a means of understanding the spontaneous, self-organizingdynamics of the world.Examples of CAS are widespread in both the natural and human world. In the naturalworld, brains, immune systems, ecologies, cells, developing embryos, and ant colonies allfall under the category of CAS. In the human world, political parties, scientificcommunities and the economy are examples.CharacteristicsCAS have several common characteristics that recur in a number of natural and humancontexts. The most commonly repeated characteristics noted in the literature are asfollows:• CAS are balanced between order and anarchy, at the edge of chaos. As Waldrop(1992) describes, “…frozen systems can always do better by loosening up a bit, andturbulent systems can always do better by getting themselves a little more organized.So if a system isn’t on the edge of chaos already, you’d expect learning and evolutionto push it in that direction…to make the edge of chaos stable, the natural place forcomplex, adaptive systems to be.”• CAS are composed of a network of many agents gathering information, learning andacting in parallel in an environment produced by the interactions of these agents.• The system co-evolves with its environment.• Order is emergent, instead of pre-determined, always unfolding and always intransition (perpetual novelty).• CAS tend to exist in many levels of organization in the sense that agents at one levelare the building blocks for agents at the next level. An example is cells, which makeup organisms, which in turn make up an ecosystem.• Finally, CAS, by their nature, have a future that is hard to predict.These characteristics are illustrated in the examples of CAS application to differentdisciplines included below.Background of CASThinking about CAS has its roots in many different disciplines. Waldrop (1992)indicates that efforts at the Santa Fe Institute to conceptualize a “common theoreticalframework for complexity” were built upon past work in the fields of neural networks,ecology, economics, artificial intelligence, chaos theory and cybernetics.Long before the Santa Fe Institute got underway, Belgian Nobel laureate Ilya Prigoginewas exploring questions about the sources of order and structure in the world. Waldrop(1992) indicates that Prigogine had been studying self-organizing structures in naturesince the 1960’s. He observed that atoms and molecules are exposed to energy andmaterial flowing in from the outside, partially reversing the decay required by the secondlaw of thermodynamics. As a result, systems are able to spontaneously organizethemselves into a series of complex structures. This work resonated with many of theSanta Fe founders and represented some of the early thinking on self-organization ofsystems.Many of the key figures in CAS have a strong affiliation with the Santa Fe Institute.These include institute founder and first president, George Cowan who had previouslyworked on the Manhattan project and headed research at Los Alamos. Cowan wasdescribed by Waldrop (1992) as “…a fervent and determined man…to Cowan, the SantaFe Institute was a mission…a chance for science as a whole to achieve a kind ofredemption and rebirth.”Murray Gell-Mann, winner of the Nobel Prize in physics for his work on sub-atomicparticles, was a fervent supporter of the formation of the institute, and has retained astrong affiliation over the years. Stuart Kauffman was a leading figure in biology, whowrote about the relevance of adaptation in addition to Darwinian selection in theevolution of species. Major works written about complexity theory by Gell-Mann (“TheQuark and the Jaguar”) and Kauffman (“At Home in the Universe”) are described below.John Holland’s involvement at Santa Fe stems back to early workshops at the institutewhere he shared his thinking on key ideas on adaptation that he had been pursuing inrelative obscurity for a quarter of a century. Holland’s book entitled “Hidden Order” isanother major work