The spread of inequality

Abstract

The causes of socioeconomic inequality have been debated since the time of Plato. Many reasons for the development of stratification have been proposed, from the need for hierarchical control over large-scale irrigation systems to the accumulation of small differences in wealth over time via inheritance processes. However, none of these explains how unequal societies came to completely displace egalitarian cultural norms over time. Our study models demographic consequences associated with the unequal distribution of resources in stratified societies. Agent-based simulation results show that in constant environments, unequal access to resources can be demographically destabilizing, resulting in the outward migration and spread of such societies even when population size is relatively small. In variable environments, stratified societies spread more and are also better able to survive resource shortages by sequestering mortality in the lower classes. The predictions of our simulation are provided modest support by a range of existing empirical studies. In short, the fact that stratified societies today vastly outnumber egalitarian societies may not be due to the transformation of egalitarian norms and structures, but may instead reflect the more rapid migration of stratified societies and consequent conquest or displacement of egalitarian societies over time.

Figure 1. Typical egalitarian and stratified populations in constant environment with baseline parameter values.

Egalitarian populations (A) are able to stabilize, although no logistic or density-dependent growth function was used, while stratified populations (B, C, D) cannot stabilize because upper classes continue to thrive even as resources are being depleted.

Figure 2. Egalitarian and stratified populations in constant and variable environments, with and without storage of surplus.

Black bars show results without storage and white bars with storage. Egalitarians have larger, more stable populations in constant environments (A, C), while storage causes them to overshoot carrying capacity by a greater extent and go extinct (E). In variable environments, egalitarian populations are unstable and go extinct (B, D, F). Stratified groups have smaller, less stable populations in constant environments (A, C), and are more likely to go extinct (E). In variable environments, stratified populations are protected against extinctions because mortality is sequestered in the lower classes; storage of surplus gives further protection (B, D, F).

Figure 3. Representative migration competition outcomes in constant and variable environments.

Solid lines show number of sites held by stratified societies, and dotted lines show number of sites held by egalitarian societies. The two plots on the left (A, C) show rate of occupying sites during the “frontier phase” when uninhabited sites are still available, while the two plots on the right (B, D) show what happens after all sites are occupied (“carrying capacity phase”), and thus expansion can only take place when a site opens up because another population goes extinct. These plots show results using the population decline trigger optimal values (loss of a threshold fraction of population), but results for the other two migration triggers (resource deprivation threshold for individuals, and resource depletion threshold for sites) are similar. Stratified societies always migrate outward more frequently, and thus take over quickly (A, C). Over the long term, in comparison with egalitarian societies they experience a higher rate of group extinctions in constant environments (B), and a lower rate in variable environments (D).