Warfare and the Evolution of Social Complexity: A Multilevel-Selection Approach

Abstract

Multilevel selection is a powerful theoretical framework for understanding how complex hierarchical systems evolve by iteratively adding control levels. Here I apply this framework to a major transition in human social evolution, from small-scale egalitarian groups to large-scale hierarchical societies such as states and empires. A major mathematical result in multilevel selection, the Price equation, specifies the conditions concerning the structure of cultural variation and selective pressures that promote evolution of larger-scale societies. Specifically, large states should arise in regions where culturally very different people are in contact, and where interpolity competition -warfare -is particularly intense. For the period of human history from the Axial Age to the Age of Discovery (c.500 BCE-1500 CE), conditions particularly favorable for the rise of large empires obtained on steppe frontiers, contact regions between nomadic pastoralists and settled agriculturalists. An empirical investigation of warfare lethality, focusing on the fates of populations of conquered cities, indicates that genocide was an order of magnitude more frequent in steppefrontier wars than in wars between culturally similar groups. An overall empirical test of the theory's predictions shows that over ninety percent of largest historical empires arose in world regions classified as steppe frontiers. Introduction: the Puzzle of UltrasocialityWhen World War I broke out in August 1914 patriotic crowds in Vienna, Berlin, and London demonstrated in support of their governments' decision to enter the war. More remarkably, young men volunteered in large numbers for military service. In the United Kingdom 300,000 men enlisted in August alone, and more than 2.5 million throughout the war (Ferguson 1999:198). The British government did not need to institute the draft until 1916. Huge numbers of these men were killed (close to a million in Britain alone); others were physically maimed (1.6 million in Britain) or psychologically scarred for life (Urlanis 1971). Such willingness to sacrifice life and limb for the sake of a nation-an imaginary construct with uncertain boundaries encompassing millions of people, most of whom could never even hope to meet each other-presents a huge problem to the standard evolutionary theory. It is a central part of the "puzzle of ultrasociality"-the ability of humans to form cooperating societies consisting of huge numbers of genetically unrelated individuals (Campbell 1983, Richerson andBoyd 1998).It may seem strange to equate cooperation with warfare, but war encompasses both morally repugnant atrocities and morally uplifting stories of selfless heroism. A fruitful conceptual framework for the study of this human activity, which involves both coercion and cooperation, is offered by the theory of multilevel selection (Sober and

Selection and adaptation in human migration

“…The more obvious examples are large-scale military campaigns with high individual costs. 110 More subtle and variable are the decisions made at the household or kin-group level that reduces risk and increases available capital. Select kin may migrate to gain and remit resources.…”

“…More generally, specific institutions of knowledge production, labor management, and other group features may lead to greater representation through expansion or migration 107 . Systems of expansion may feature norms that mediate resource demands such as the acceptance of credit, 108 agricultural demands, 109 and warfare 110 . Frontier culture is a well‐illustrated concept aimed at capturing the selective process by which individuals adapt to propel themselves beyond a society's well‐worn pathways 111 .…”

“…107 Systems of expansion may feature norms that mediate resource demands such as the acceptance of credit, 108 agricultural demands, 109 and warfare. 110 Frontier culture is a well-illustrated concept aimed at capturing the selective process by which individuals adapt to propel themselves beyond a society's well-worn pathways. 111 The primary observation is that at the margins of society people house greater individualistic attitudes than in the interior.…”

Integrating evolutionary game theory and cross-cultural psychology to understand cultural dynamics.

“…Finally, EGT models can implement group competition not only directly through war or competitive games, but also indirectly through a multilevel selection mechanism (Turchin, 2010). A key tenet of multilevel selection is that selection happens at the individual and the group (or other) levels simultaneously.…”

Evolution of family systems and resultant socio-economic structures

“…The multi-level selection was originally introduced to explain the evolution of cooperative behaviour among eusocial insects by examining the conflict between the fitness of an individual and that of a group 20,21 . This framework is generally applied to the evolution of hierarchical systems and is used to discuss the public good or social structure in biological and social evolution [22][23][24][25][26][27] . We have previously applied the framework to construct a mathematical model for the evolution of kinship structures in clan societies 22 .…”