V. Ahl & T.F.H. Allen: Hierarchy Theory (1996)
A Systems Library, Vol. 8
Has it ever occured to you that we are surrounded by biological and social systems that exhibit a very particular architecture? They are hierarchical, i.e. composed of one or more levels of interrelated subsystems. Hierarchy theory explains why that is and how it helps to better understand complexity. This book by Valerie Ahl and Timothy Allen is a classic in this niche of the systems literature.
Sometimes one wonders why certain, ostensibly very simple phenomena take so long to be recognised and formalised. Take the mechanism of circular causation, or feedback loops. Humanity has been confronted with these phenomena for ages. Engineers and craftsmen from all eras have worked with these mechanisms, without however understanding their wider import. It was only at the end of World War II that Norbert Wiener codified it as a feedback phenomenon and thereby established the new discipline of Cybernetics.
Hierarchy Theory is the result of a similar, belated discovery. It was Herbert Simon who pointed out, in the 1960s, that many complex living and non-living systems featured a hierarchical structure. By ‘hierarchic’ he meant “a system that is composed of interrelated subsystems, each of the latter being in turn hierarchic in structure until we reach some lowest level of elementary subsystem.” For instance, an animal includes several organs; the organs include lots of tissues; the tissues include zillions of cells, etc. Simon wondered how the prevalence of these hierarchic structures could be explained. It turned out that systems that included loosely coupled subsystems (or ‘intermediate stable forms’) could evolve faster in the process of natural selection. In other words: if we would start with a population of systems of comparable complexity, some of them hierarchical and others not, but all being subjected to similar frequencies of mutation, the hierarchical systems would increase their fitness through evolutionary processes much faster than the remaining systems, and would soon come to dominate the entire population. Simon’s famous parable on the two watchmakers made this clear in a very intuitive fashion.
This insight put in place the foundation for Hierarchy Theory. Also this book by Ahl and Allen takes its cue from Simon’s point. As an aside: the ideas expounded in it are basically Allen’s, a botanist and ecologist by training. Valerie Ahl, a clinical psychologist, presumably lent her husband a more fluid pen to produce a relatively short and more accessible book than Allen’s original 1982 tome (co-authored with T. Starr) on the same subject.
The contribution of this book is twofold. It explains how relationships between sub-entities (or sublevels) in hierarchical systems can be understood and it does so from an emphatically non-realist, constructivist perspective.
It is the interaction between these two elements that makes for an arduous read (don’t be fooled by the book’s numerous, sometimes droll illustrations).
Mainly, levels in hierarchies can be ordered based on scalar characteristics — in space and time — of the entities included in them. This leads to higher levels operating more slowly and at a lower frequency than lower levels; and higher levels also exhibiting a constraining influence on lower levels. This applies to systems of all sorts. In an ecological system layers can be identified that show different spatio-temporal dynamics, ranging from slow biome-scale phenomena to high-frequency changes at the level of a patch. The US Federal Supreme Court is a hierarchically superior, stable institution by virtue of the vast professional experience invested in the nine justices, their life tenure, the relative independence from political pressures, and the judicial restraint they are expected to observe. It has been designed to have a dampening, constraining influence on institutions at lower levels.
Now, the point forcefully made by Allen is that the choices that determine these scalar characteristics are observer-dependent. The temporal and spatial limits of our observations are, consciously or unconsciously, defined by us as a result of the kinds of questions that we dream up as scientists. “Observing is an active process. To make an observation one must search but cannot begin without an idea of what it is one might find. (…) As soon as the question is formulated, certain constraints immediately follow. The question directs and structures the observation process by posing hypothetical entities and their roles.”
As a result, Hierarchy Theory in Allen’s conception presents itself as an epistemological (meta-)theory that emphatically brings in the role of the observer in any formal study of complex systems. We don’t have access to the real world. We are confronted with a material world that is blind, purposeless, active, ever-changing. The best we can do is to be very explicit and transparent about the vantage point that we construct for ourselves to give us a handle on this complexity.
But why on earth would we want to make our lives so difficult? Wouldn’t it be easier to simply assume that what we see is really what is out there? According to Allen the problem is that we will run into paradoxes (or, as others would say, into ‘errors of logical typing’), and because we don’t understand where these inconsistencies come from, we will find ways to ‘save the appearances’ rather than to improve our predictive theories. Biologists are particularly skillful in developing new vocabulary in response to observed mismatches between what they a priori define and what they see. For instance, when large organisms meet large numbers of very small organisms the concept of ‘parasitism’ is invoked to define away the inconsistency with a conventional hierarchy that puts organism above population. Hence, “biology has rich typology but a poor record of prediction in new situations because it is slow to identify observed empirical entities and associated levels of observation.”
So, complexity does not exist independently of an observer’s questions. Instead, complexity is the product of asking questions in a certain way. In this book, Allen offers a conceptual framework, not to ask better questions, but to help us articulate the observational implications of a given question and to separate out the contributions of observer and observed in the generation of data.
Now, where does the lead us? While I am ready to acknowledge the cogency of these ideas, in this book they don’t really gel into a practicable approach. And I’m not sure Allen is interested in that. In a videotaped interview he gave a rather cavalier response to the question how hierarchy theory was used in policy development: “I am just a theorist who dumps ideas on my peers who then need to find out how they can put them into practice.” Frankly, it is a missed opportunity.
A second point of criticism is the structure of the book’s argument. The main thrust is clear: the first three chapters explain Allen’s epistomological stance, the next four introduce a basic framework to think about hierarchies (revolving around core concepts such as levels, constraints, nestedness, filters and surfaces), and the final chapter explores the the dynamics of hierarchical systems. But on that journey I got confused at numerous junctures by plethoric jargon, lots of conditional statements, frequent iterations on familiar points and by scattered and superficially treated examples. In my humble opinion, stricter editorial oversight might have produced a better book.
Finally, and this is a more general point with a bearing on the stature and relevance of hierarchy theory, I wonder whether it is appropriate to narrow the perspective on complex systems to a hierarchical template, to the exclusion of other and equally potent architectural principles. I am thinking more particularly on the notion of networks (or meshworks). It is not unreasonable to hypothesise that combining (vertical) hierarchies and (horizontal) networks would provide us with much more leverage in explaining the dynamic behaviour of complex systems. (Manuel DeLanda did something of the sort in his materialist version of the last Thousand Years of Nonlinear History) Freeing itself of this observational blind spot would perhaps unholster Hierarchy Theory from the theoretical niche in which it has been confined up to the present day.
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