... Distinct as the inequality we face today is, inequality itself is not something new. The concentration of economic and political power was in many ways more extreme in the precapitalist societies of the West. At that time, religion both explained and justified the inequality: those at the top of society were there because of divine right. To question that was to question the social order, or even to question God’s will.

However, for modern economists and political scientists, as also for the ancient Greeks, this inequality was not a matter of a preordained social order. Power—often military power— was at the origin of these inequities. Militarism was about economics: the conquerors had the right to extract as much as they could from the conquered. In antiquity, natural philosophy in general saw no wrong in treating other humans as means for the ends of others. As the ancient Greek historian Thucydides famously said, “right, as the world goes, is only in question between equals in power, while the strong do what they can and the weak suffer what they must.

Those with power used that power to strengthen their economic and political positions, or at the very least to maintain them. They also attempted to shape thinking, to make acceptable differences in income that would otherwise be odious.

As the notion of divine right became rejected in the early nation-states, those with power sought other bases for defending their positions. With the Renaissance and the Enlightenment, which emphasized the dignity of the individual, and with the Industrial Revolution, which led to the emergence of a vast urban underclass, it became imperative to find new justifications for inequality, especially as critics of the system, like Marx, talked about exploitation.

The theory that came to dominate, beginning in the second half of the nineteenth century—and still does—was called “marginal productivity theory”; those with higher productivities earned higher incomes that reflected their greater contribution to society. Competitive markets, working through the laws of supply and demand, determine the value of each individual’s contributions. If someone has a scarce and valuable skill, the market will reward him amply, because of his greater contribution to output. If he has no skills, his income will be low.

Technology and scarcity, working through the ordinary laws of supply and demand, play a role in shaping today’s inequality, but something else is at work, and that something else is government. ...