Political institutions evolve only incrementally

Image copyright Nature 2010
I really enjoyed this interesting and creative article in Nature this week:

Rise and fall of political complexity in island South-East Asia and the Pacific

Thomas E. Currie, Simon J. Greenhill, Russell D. Gray, Toshikazu Hasegawa & Ruth Mac

Abstract: There is disagreement about whether human political evolution has proceeded through a sequence of incremental increases in complexity, or whether larger, non-sequential increases have occurred. The extent to which societies have decreased in complexity is also unclear. These debates have continued largely in the absence of rigorous, quantitative tests. We evaluated six competing models of political evolution in Austronesian-speaking societies using phylogenetic methods. Here we show that in the best-fitting model political complexity rises and falls in a sequence of small steps. This is closely followed by another model in which increases are sequential but decreases can be either sequential or in bigger drops. The results indicate that large, non-sequential jumps in political complexity have not occurred during the evolutionary history of these societies. This suggests that, despite the numerous contingent pathways of human history, there are regularities in cultural evolution that can be detected using computational phylogenetic methods.

Probably the most fun aspect of the article is that they take some clever statistical techniques to evaluate a complex archeological/anthropological data set that is never analyzed with quantitative methods. Their main result is that it seems very unlikely that political institutions develop complexity with leaps and bounds. Rather, institutions appear to evolve very slowly with only incremental changes in complexity.  However, they find that the reverse is not true: sometimes it looks as though complex political institutions may collapse rapidly to much simpler institutions.

They basically use maximum-likelihood techniques to estimate the probability that political systems make specific transitions in a simple markov-chain model of political institutions.  Because the probability of certain transitions between nodes in the chain seem very unlikely, they are able to rule out certain models of political development (leaps and bounds) that otherwise seemed plausible.  Very interesting.

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