(This post is part of Politics for Worldbuilders, an occasional series.)
The very earliest groups of people in prehistory, as far as we can surmise, were small bands of nomadic foragers. Such bands have continued to exist down to the present day, though they are becoming increasingly hemmed in by powerful states who prefer people to be stationary, formally employed, and taxable. Still, many of our social intuitions were formed in an ancestral environment of such nomadic bands, so we should discuss them first and foremost.
Assuming that a band does not possess livestock, its members can only own what they can carry. As a result, the social structure is relatively flat; there are no wide class distinctions as are typical in “civilization.” (Civilization does have its advantages, of course, but that is not our present topic.) That is not to say that people are equal; all primates are acutely sensitive to status distinctions, humans included, and any social group will have its pecking order. More successful hunters or warriors will accumulate trophies, jewelry, or marks of prestige, and probably higher-status mates as well. Still, compared to more complex societies, we can still describe such bands as broadly egalitarian.
That doesn’t guarantee that they will stay that way. Commonly, such bands will have a leader or big man (as the anthropologists would call him—and barring magic or some other equalizer of the sexes, he will almost certainly be a man), who has the respect of the others even without having formal authority or privileges. Over time, a canny big man can formalize his position and even pass it on to his sons, becoming a true chief. Initially, the chief or big man would be expected to use his power to redistribute possessions among the band, rather than enriching himself; but with enough political skill, a chief can build a cadre of supporters who will back him as he does in fact become more wealthy (as will they!). Thus does an egalitarian band develop political structures and social classes.
Those bands that remain egalitarian usually manage the feat because of an explicit aversion to hierarchy. To prevent hierarchies from emerging, or to constrain nascent hierarchies as they form, egalitarian bands often discourage inequality with several strategies. The first is an overwhelming social environment of envy. Anyone becoming conspicuous by gaining social power or wealth could expect to be the subject of malicious gossip, petty acts of uncooperation (in James C. Scott’s term, “weapons of the weak”), and later, public disapproval, political opposition, and even magical curses or physical violence. Attempting to dominate an egalitarian band is a risky business.
Second is expecting those with many possessions to be generous with them. This could be through public feasting, or socially required gifts to others, or sacrifices to the gods. (The anthropologist David Graeber has a long and amusing discussion of such mandatory gift-giving.)
This expectation persists even in a hierarchical setting. In most societies, the wealthy and powerful are expected to foster patron-client relationships, in which the powerful patron is served by the weaker clients, and in return the client can expect the patron’s support and protection. You can think of feudalism as a formalized patron-client relationship; the vassals owe taxes and service to their lords, but the lords are expected to defend the rights of the vassals in return. Another example would be large landowners in places like precolonial Southeast Asia; the landowners often took very high percentages of the crop from their sharecropper farmers, but if times were bad, the farmers would expect the landowners to give them food from their storehouses (or risk getting lynched!).
Third, if conditions within a band became intolerable for some of its members, they would simply leave. The band could split, with the dissidents moving somewhere else and leaving any would-be strongman with a vastly diminished pool of manpower. (In the literature, this is called fission.) Obviously, this would be traumatic to the people involved, and would only be a last resort; but the threat of fission does much to keep ambitious leaders in check.
It is no accident that developed states often arose in cramped geographic areas that made it hard to escape, or else at a time when the society was facing outside invasion, which would likewise make it difficult (practically as well as morally) to simply leave. Mobility gives choices; choices constrain political domination. The lack of choice means that band members have little recourse when their chief decides to cement his power. (This concept is applicable even within developed states; the American West played the role of an escape valve for the urban centers of the Northeast, threatening a population drain in response to the more obnoxious schemes of politicians. See James C. Scott for more examples, in Southeast Asia and elsewhere.)
The concepts here offer much to authors. Here are a few thought-provoking questions, in building your setting: What social expectations does your society place on the wealthy? How far are they tolerated, before risking violence from those with less? If the society is egalitarian, how does it stay that way? What role do gossip, threats of violence, or malicious charms and curses play in keeping powerful figures in check? Do political leaders risk driving off their populace if their policies are too harsh, or foolish? Did your protagonists come from somewhere else, and if so, why did they leave? What attitudes or personality traits does that convey, or were taught to them by their experience?