Significance Inequality is one of the main drivers of social tension. We show striking similarities between patterns of inequality between species abundances in nature and wealth in society. We demonstrate that in the absence of equalizing forces, such large inequality will arise from chance alone. While natural enemies have an equalizing effect in nature, inequality in societies can be suppressed by wealth-equalizing institutions. However, over the past millennium, such institutions have been weakened during periods of societal upscaling. Our analysis suggests that due to the very same mathematical principle that rules natural communities (indeed, a “law of nature”) extreme wealth inequality is inevitable in a globalizing world unless effective wealth-equalizing institutions are installed on a global scale.

Abstract Most societies are economically dominated by a small elite, and similarly, natural communities are typically dominated by a small fraction of the species. Here we reveal a strong similarity between patterns of inequality in nature and society, hinting at fundamental unifying mechanisms. We show that chance alone will drive 1% or less of the community to dominate 50% of all resources in situations where gains and losses are multiplicative, as in returns on assets or growth rates of populations. Key mechanisms that counteract such hyperdominance include natural enemies in nature and wealth-equalizing institutions in society. However, historical research of European developments over the past millennium suggests that such institutions become ineffective in times of societal upscaling. A corollary is that in a globalizing world, wealth will inevitably be appropriated by a very small fraction of the population unless effective wealth-equalizing institutions emerge at the global level.

Several societies have seen as little as 1% of their population own approximately 50% of the total wealth. This was the case in many Western countries around 1900, including Britain, France, and Sweden, and some claim that at present, roughly 1% of the population owns 50% of total wealth at the global level (1, 2). Similarly, in natural communities, a small fraction of the total species often makes up most of the biomass; for instance, a recent study of the Amazon rainforest revealed that roughly 1% of the tree species account for 50% of the total stored carbon (3). Although the correspondence between the dominance in society and this famously diverse ecosystem may be a coincidence, it raises the questions of whether there might be generic intrinsic tendencies to such inequality, and what could be the unifying mechanisms behind it.

We first turn to the question of the extent to which patterns in nature and society are actually similar. The natural communities that we analyze range from mushrooms, trees, intestinal bacteria, and algae to flies, rodents, and fish (SI Appendix, section 1). Our societal data consist of estimates for different countries (1, 4⇓⇓–7) (SI Appendix, section 1). We focus on wealth and not income distribution, which is much less unequal and—perhaps surprisingly—poorly correlated with wealth inequalities across countries (SI Appendix, section 2). While income concerns a flow, wealth concerns a stock, just as biomass in species.

As a first illustration of the similarities of patterns in nature and society, consider the wealth distribution of the world’s richest individuals compared with the abundance distribution of the Amazon’s most common trees (Fig. 1 A and B). The patterns are almost indistinguishable from one another. For a more systematic comparison, we also analyzed the Gini indices of a wide range of natural communities and societies (Fig. 1 C and D). The Gini index is an indicator of inequality that ranges from 0 for entirely equal distributions to 1 for the most unequal situation. It is a more integrative indicator of inequality than the fraction that represents 50%, but the two are closely related in practice (SI Appendix, section 3). Surprisingly, Gini indices for our natural communities are quite similar to the Gini indices for wealth distributions of 181 countries (data sources listed in SI Appendix, section 1).

Fig. 1. Inequality in society (Left) and nature (Right). The Upper panels illustrate the similarity between the wealth distribution of the world’s 1,800 billionaires (A) (8) and the abundance distribution among the most common trees in the Amazon forest (B) (3). The Lower panels illustrate inequality in nature and society more systematically, comparing the Gini index of wealth in countries (C) and the Gini index of abundance in a large set of natural communities (D). A complete list of data sources is provided in SI Appendix.

In societies, inequality is also found for other units besides the wealth of single actors or households. For instance, power law-like distributions characterized by high inequality are found in statistics on city sizes, number of copies sold of bestseller books, number of adherents of religious bodies, and number of links to web sites (9). In addition, firm size typically varies widely, with a few companies dominating the market (10, 11). At first glance, firm size may seem comparable on an abstract level to the wealth of households. Indeed, firms may grow and shrink depending on vagaries of markets and other factors. However, there are also important differences. For instance, firms are relatively ephemeral entities that are linked through a global web of shareholders (12) and may be fused or split depending on shareholders’ decisions and antitrust legislation. In this paper, we limit our discussion to the wealth of households for our comparison of nature and society.

The patterns that we describe (Fig. 1) raise the question of whether the similarities between nature and society are a coincidence or might hint at universal underlying processes. Viewed in detail, the complex interplay of mechanisms that govern wealth distribution in society is obviously very different from the processes regulating the abundance of species in nature. However, as we argue, on an abstract level, there are in fact comparable generic processes at play (Fig. 2).

Fig. 2. Four unifying mechanisms that shape inequality and their specific drivers in nature (solid lines) and society (text boxes with dashed borders).

Long-Run Instability of Equalizing Mechanisms Although the four forces that we have highlighted (Fig. 2) may shape much of the observed patterns of inequality, determining their relative importance is not easy. Occasionally, however, perturbations of the balance provide valuable clues. In nature, the importance of repression of dominance (Fig. 2, III) is vividly illustrated by the occasional spectacular population explosion of newly invading species, explained by the release from the natural enemies they left behind: the so-called “parasites lost” phenomenon (32). The balance is typically restored over the subsequent decades as natural enemies catch up with the newcomers. In societies, control of wealth inequality is also notoriously unstable over time. The drop and rebound in inequality over the last century has received much attention (2, 6, 29), but a careful analysis of historical sources reveals several waves of rising and falling inequality in history (4, 7, 33). Some of those cycles look surprisingly regular (33), suggesting that they might be governed by universal basic forces. Indeed, inequality and conflict are common elements across historical analyses, even though precise mechanisms of their interaction differ among cases (7, 31, 33). It is becoming increasingly clear that institutions can play a dominant and long-lasting role in shaping societal prosperity and inequality (34). Indeed, on closer look, several historical cycles of inequality may be explained, at least in part, by the emergence of equalizing institutions followed by periods during which various mechanisms undermined the effectiveness of these institutions (4). An often-overlooked mechanism that may undermine the power of wealth-equalizing institutions is societal upscaling. Focusing on Western Europe, we can see how in the Middle Ages, and especially in the 12th to 14th centuries, local communities reduced inequality by limiting opportunities for transacting and accumulating land and capital, and developing mechanisms of redistribution, through guild or community systems, operating at the local level, where most of the exchange and allocation of land and capital took place (4). However, these town and village communities saw their institutional frameworks eroded by the growth of international trade, migration, and interregional labor and capital markets, as well as by the process of state formation with the rise of more centralized bureaucracies in the (early) modern period, triggering a long episode of rising inequality (5, 7, 35). In the late 19th century and early 20th century, institutions aimed at effectively constraining wealth accumulation were developed at the level of the nation state, with the emergence of tax-funded welfare states. Perhaps the most conspicuous of these institutions is the introduction of the inheritance tax, which limits wealth transfer to the next generation (2). Over the past decades, however, globalization has given way to a more unconstrained use and accumulation of wealth (29). The financial playing field for the wealthiest is now global, and mobility of wealth has greatly increased, providing immunity to national taxation and other institutional obstacles to wealth accumulation.

Prospects Our analysis suggests that even if all actors are equivalent, in the absence of counteracting forces, there is an intrinsic tendency for significant inequality to arise from multiplicative chance effects. Although the surprising similarity between inequality of species abundances and wealth may have the same roots on an abstract level, this does not imply that wealth inequality is “natural.” Indeed, in nature, the amount of resources held by individuals (e.g., territory size) is typically quite equal within a species. While wealth inequality may have emerged as far back as the Neolithic era (31, 36), the relative amount of wealth appropriated by the richest has increased as societies have scaled up. One explanation for this effect is scale itself. Put simply, one can accumulate less wealth in a village than across the globe. However, as we have argued, another explanation is that installing effective institutions to dampen inequality becomes more challenging as scale increases. Excessive concentration of wealth is widely thought to hamper economic growth, concentrate power in the hands of a small elite, and increase the chance of social unrest and political instability (1, 2, 4, 37⇓–39). This raises questions about the prospects for current societies. Phases of upscaling of governance successfully curbed unconstrained growth of inequality first in the communities of late medieval Europe and later in the nation states of the 20th century, but in both cases, this was a lengthy and painful process. Whether scaling up of effective governance can now be done at the global level and, if so, what this new form of governance might look like, remains unclear.

Acknowledgments We thank Diego G. F. Pujoni, Gerben Straatsma, and Willem M. de Vos for assistance with our ecological data search and insightful discussions on inequality in nature, and Jan Luiten van Zanden, Michalis Moatsos, and Wiemer Salverda for discussions on inequality in society. This project was supported by the European Research Council Fund (339647, to B.v.B.). The work of M.S. is supported by a Spinoza Award from the Dutch National Science Foundation.

Footnotes Author contributions: M.S. designed research; B.v.B., I.A.v.d.L., and E.H.v.N. performed research; I.A.v.d.L. and E.H.v.N. analyzed data; and M.S. and B.v.B. wrote the paper.

Conflict of interest statement: Simon A. Levin coauthored a review article published in Critical Care Medicine in 2016 with M.S., I.A.v.d.L., and E.H.v.N.

This article is a PNAS Direct Submission.

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