The Great Divide is a series about inequality.

Another dubious first for America: We now employ as many private security guards as high school teachers — over one million of them, or nearly double their number in 1980.

And that’s just a small fraction of what we call “guard labor.” In addition to private security guards, that means police officers, members of the armed forces, prison and court officials, civilian employees of the military, and those producing weapons: a total of 5.2 million workers in 2011. That is a far larger number than we have of teachers at all levels.

What is happening in America today is both unprecedented in our history, and virtually unique among Western democratic nations. The share of our labor force devoted to guard labor has risen fivefold since 1890 — a year when, in case you were wondering, the homicide rate was much higher than today.

Is this the curse of affluence? Or of ethnic diversity? We don’t think so. The guard-labor share of employment in the United States is four times what it is in Sweden, where living standards rival America’s. And Britain, with its diverse population, uses substantially less guard labor than the United States.

In America, growing inequality has been accompanied by a boom in gated communities and armies of doormen controlling access to upscale apartment buildings. We did not count the doormen, or those producing the gates, locks and security equipment. One could quibble about the numbers; we have elsewhere adopted a broader definition, including prisoners, work supervisors with disciplinary functions, and others.

But however one totes up guard labor in the United States, there is a lot of it, and it seems to go along with economic inequality. States with high levels of income inequality — New York and Louisiana — employ twice as many security workers (as a fraction of their labor force) as less unequal states like Idaho and New Hampshire.

When we look across advanced industrialized countries, we see the same pattern: the more inequality, the more guard labor. As the graph shows, the United States leads in both.

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Note that, in 1979 (shown by the pink dot), the United States was less unequal and employed less guard labor. In the graph, inequality in income takes account of payment of taxes and receipt of government transfers such as Social Security. (We measure inequality by the Gini index, a measure that varies from 0 for complete equality — that is, if all families have the same income — to a value of 1 if a single person has all of the income.) The data shown are the most recent for all nations on which comparable measures of inequality and guard labor are available.

For the same countries, guard labor is also more common where those starting out in life face a sharply tilted playing field, such as America, Britain and Italy. These are countries in which the income of a father is a good predictor of the income of his adult son. The countries with the least guard labor are those in which there is greater equality of economic opportunity by this measure: These are Denmark and Sweden, countries in which knowing the father’s income does not enable a very accurate guess of the son’s income when he grows up.

Nobody has a good explanation of why the United States is a standout when it comes to guard labor. Some of the differences in the guard-labor fraction across nations arise because, in many countries, the job of getting people to play by the rules is not left up to enforcement specialists. Anyone who has tried jaywalking in Germany will know what we mean: It’s not the police who are on your case, but your fellow pedestrians. In the United States, when the neighbor’s boisterous party is disturbing sleep, it’s often the police who will get the irate call, not the neighbor. Some of the increase in guard labor over the past century in America may reflect a shift from the informal enforcement of social norms to their enforcement by specialists in uniform.

Does the graph show that inequality causes a country to devote more of its labor force to guard labor? It is hard to be sure. It could be that people with a strong commitment to economic justice are, for some unknown reason, also more law-abiding, explaining the difference between Denmark and the United States. But the correlation evident in the graph could be evidence that economic disparities push nations to devote more of their productive capacity to guarding people and property. Fear and distrust of one’s neighbors and fellow citizens fuel the demand for guard labor. Economic disparities can contribute to both. Among the countries shown, a common measure of distrust of strangers is strongly correlated with both the guard-labor fraction and inequality.

Social spending, also, is strongly and inversely correlated with guard labor across the nations shown in the graph. There is a simple economic lesson here: A nation whose policies result in substantial inequalities may end up spending more on guns and getting less butter as a result.

Nobody doubts that the work of guard labor is essential. One of us, Samuel Bowles, knows this firsthand: His son-in-law is a corrections officer whose work is skilled, demanding and necessary. Every society divides its labor between those who produce things and those who guard the store. But how much guard labor is too much?

“You have money spent on guarding stuff rather than making stuff,” said Michael Hood, an economist at Barclays Capital. “There’s a large population standing around in blue blazers rather than engaged in more productive activities.” He was talking about Latin America, but could have been describing things in the United States.

“It is lamentable to think,” wrote the philosopher John Stuart Mill, in 1848, “how a great proportion of all efforts and talents in the world are employed in merely neutralizing one another.” He went on to conclude, “It is the proper end of government to reduce this wretched waste to the smallest possible amount, by taking such measures as shall cause the energies now spent by mankind in injuring one another, or in protecting themselves from injury, to be turned to the legitimate employment of the human faculties.”

This venerable call to beat swords into plowshares resonates still in America and beyond. Addressing unjust inequality would help make this possible.

Sources: Further information on data and methods is here.

Samuel Bowles, a research professor of behavioral sciences at the Santa Fe Institute, is the author of “The New Economics of Inequality and Redistribution.” Arjun Jayadev, an associate professor of economics at the University of Massachusetts, Boston, also teaches at Azim Premji University, Bangalore, and works at the Institute for New Economic Thinking, New York.