7 BILLION PEOPLE WHOSE BIGGEST CHALLENGE IS OBESITY:

For tens of thousands of years, egalitarian hunter-gatherer societies were widespread. And as a large body of anthropological research shows, long before we organised ourselves into hierarchies of wealth, social status and power, these groups rigorously enforced norms that prevented any individual or group from acquiring more status, authority or resources than others.*

Decision-making was decentralised and leadership ad hoc; there weren't any chiefs. There were sporadic hot-blooded fights between individuals, of course, but there was no organised conflict between groups. Nor were there strong notions of private property and therefore any need for territorial defence. These social norms affected gender roles as well; women were important producers and relatively empowered, and marriages were typically monogamous.

Keeping the playing field level was a matter of survival. These small-scale, nomadic foraging groups didn't stock up much surplus food, and given the high-risk nature of hunting - the fact that on any given day or week you may come back empty-handed - sharing and cooperation were required to ensure everyone got enough to eat. Anyone who made a bid for higher status or attempted to take more than their share would be ridiculed or ostracised for their audacity. Suppressing our primate ancestors' dominance hierarchies by enforcing these egalitarian norms was a central adaptation of human evolution, argues social anthropologist Christopher Boehm. It enhanced cooperation and lowered risk as small, isolated bands of humans spread into new habitats and regions across the world, and was likely crucial to our survival and success.

How, then, did we arrive in the age of institutionalised inequality?  [...]

One line of reasoning suggests that self-aggrandising individuals who lived in lands of plenty ascended the social ranks by exploiting their surplus - first through feasts or gift-giving, and later by outright dominance. At the group level, argue anthropologists Peter Richerson and Robert Boyd, improved coordination and division of labour allowed more complex societies to outcompete the simpler, more equal societies. From a mechanistic perspective, others argued that once inequality took hold - as when uneven resource-distribution benefited one family more than others - it simply became ever more entrenched. The advent of agriculture and trade resulted in private property, inheritance, and larger trade networks, which perpetuated and compounded economic advantages.

It is not hard to imagine how stratification could arise, or that self-aggrandisers would succeed from time to time. But none of these theories quite explain how those aiming to dominate would have overcome egalitarian norms of nearby communities, or why the earliest hierarchical societies would stop enforcing these norms in the first place. Many theories about the spread of stratified society begin with the idea that inequality is somehow a beneficial cultural trait that imparts efficiencies, motivates innovation and increases the likelihood of survival. But what if the opposite were true?

In a demographic simulation that Omkar Deshpande, Marcus Feldman and I conducted at Stanford University, California, we found that, rather than imparting advantages to the group, unequal access to resources is inherently destabilising and greatly raises the chance of group extinction in stable environments. This was true whether we modelled inequality as a multi-tiered class society, or as what economists call a Pareto wealth distribution (see "Inequality: The physics of our finances") - in which, as with the 1 per cent, the rich get the lion's share.

Counterintuitively, the fact that inequality was so destabilising caused these societies to spread by creating an incentive to migrate in search of further resources. The rules in our simulation did not allow for migration to already-occupied locations, but it was clear that this would have happened in the real world, leading to conquests of the more stable egalitarian societies - exactly what we see as we look back in history.

In other words, inequality did not spread from group to group because it is an inherently better system for survival, but because it creates demographic instability, which drives migration and conflict and leads to the cultural - or physical - extinction of egalitarian societies.

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