For most of the history of civilization, the practice of slavery was the rule rather than the exception. It was upheld in the Hebrew and Christian Bibles, and was justified by Plato and Aristotle as a natural institution that was essential to civilized society. So-called democratic Athens in the time of Pericles enslaved 35 percent of its population, as did the Roman Republic. Slaves have always been a major booty in wartime, and stateless people of all races were vulnerable to capture. The word slave comes from Slav, because, as the dictionary informs us, "Slavic peoples were widely captured and enslaved during the Middle Ages." States and armed forces, when they were not used as enslaving devices, were used as enslavement-prevention devices, as we are reminded by the lyric "Rule, Britannia! Britannia rule the waves. Britons never, never, never shall be slaves." Well before Africans were enslaved by Europeans, they were enslaved by other Africans, as well as by Islamic states in North Africa and the Middle East. Some of those states did not abolish legal slavery until recently: Qatar in 1952. Saudi Arabia and Yemen in 1962; Mauritania in 1980.

For captives in war, slavery was often a better fate than the alternative, massacre, and in many societies slavery shaded into milder forms of servitude, employment, military service, and occupational guilds. But violence is inherent to the definition of slavery -- if a person did all the work of a slave but had the option of quitting at any time without being physically restrained or punished, we would not call him a slave -- and this violence was often a regular part of a slave's life. Exodus 21:20-21 decrees, "When a slave-owner strikes a male or female slave with a rod and the slave dies immediately, the owner shall be punished. But if the slave survives for a day or two, there is no punishment; for the slave is the owner's property." Slaves' lack of ownership of their own bodies left even the better-treated ones vulnerable to vicious exploitation. Women in harems were perpetual rape victims, and the men who guarded them, eunuchs, had their testicles -- or in the case of black eunuchs, their entire genitalia -- hacked off with a knife and cauterized with boiling butter so they would not bleed to death from the wound.

The African slave trade in particular was among the most brutal chapters in human history. Between the 16th and 19th centuries at least 1.5 million Africans died in transatlantic slave ships, chained together in stifling, filth-ridden holds, and as one observer noted, "those who remain to meet the shore present a picture of wretchedness language cannot express." Millions more perished in forced marches through jungles and deserts to slave markets on the coast or in the Middle East. Slave traders treated their cargo according to the business model of ice merchants, who accept that a certain proportion of their goods will be lost in transport. At least 17 million Africans, and perhaps as many as 65 million, died in the slave trade. The slave trade not only killed people in transit, but by providing a continuous stream of bodies, it encouraged slaveholders to work their slaves to death and replace them with new ones. But even the slaves who were kept in relatively good health lived in the shadow of flogging, rape, mutilation, forced separation from family members, and summary execution.

Why did people eventually forswear the ultimate labor-saving device? Historians have long debated the extent to which the abolition of slavery was driven by economics or by humanitarian concerns. At one time the economic explanation seemed compelling. In 1776 Adam Smith reasoned that slavery must be less efficient than paid employment because only the latter was a positive-sum game:

Slaveholders in many times have manumitted their slaves, often in their wills, as they became personally close to them. In some places, such as Europe in the Middle Ages, slavery gave way to serfdom and sharecropping when it became cheaper to tax people than to keep them in bondage, or when weak states could not enforce a slave owner's property rights. But a mass movement against chattel slavery as an institution arose for the first time in the 18th century and rapidly pushed it to near extinction.

The work done by slaves, though it appears to cost only their maintenance, is in the end the dearest of any. A person who can acquire no property, can have no other interest but to eat as much, and to labour as little as possible. Whatever work he does beyond what is sufficient to purchase his own maintenance can be squeezed out of him by violence only, and not by any interest of his own.

The political scientist John Mueller points out, "Smith's view garnered adherents, but not, as it happens, among slaveowners. That is, either Smith was wrong, or slaveholders were bad businessmen." Some economists, such as Robert Fogel and Stanley Engerman, have concluded that Smith was at least partly wrong in the case of the antebellum South, which had a reasonably efficient economy for the time. And southern slavery, of course, did not gradually give way to more cost-effective production techniques but had to be obliterated by war and by law.

It took guns and laws to end slavery in much of the rest of the world as well. Britain, once among the most exuberant slave-trading nations, outlawed the slave trade in 1807 and abolished slavery throughout the empire in 1833. By the 1840s it was jawboning other countries to end their participation in the slave trade, backed up by economic sanctions and by almost a quarter of the Royal Navy.

Most historians have concluded that Britain's policing of the abolition of slavery was driven by humanitarian motives. Locke undermined the moral basis for slavery in his 1689 work Two Treatises on Government, and though he and many of his intellectual descendants hypocritically profited from the institution, their advocacy of liberty, equality, and the universal rights of man let a genie out of the bottle and made it increasingly awkward for anyone to justify the practice. Many of the Enlightenment writers who inveighed against torture on humanitarian grounds, such as Jacques-Pierre Brisson in France, applied the same logic to oppose slavery. They were joined by Quakers, who founded the influential Society for the Abolition of the Slave Trade in 1787, and by preachers, scholars, free blacks, former slaves, and politicians.

At the same time, many politicians and preachers defended slavery, citing the Bible's approval of the practice, the inferiority of the African race, the value of preserving the southern way of life, and a paternalistic concern that freed slaves could not survive on their own. But these rationalizations withered under intellectual and moral scrutiny. The intellectual argument held that it was indefensible to allow one person to own another, arbitrarily excluding him from the community of decision-makers whose interests were negotiated in the social contract. As Jefferson put it, "The mass of mankind has not been born with saddles on their backs, nor a favored few booted and spurred, ready to ride them legitimately." The moral revulsion was stimulated by first-person accounts of what it was like to be a slave. Some were autobiographies, like The Interesting Narrative of the Life of Olaudah Equiano, the African, Written by Himself (1789) and Narrative of the Life of Frederick Douglass, an American Slave (1845). Even more influential was a work of fiction, Harriet Beecher Stowe's Uncle Tom's Cabin, or Life Among the Lowly (1852). The novel depicted a wrenching episode in which mothers were separated from their children, and another in which the kindly Tom was beaten to death for refusing to flog other slaves. The book sold three hundred thousand copies and was a catalyst for the abolitionist movement. According to legend, when Abraham Lincoln met Stowe in 1862, he said, "So you're the little woman who started this great war."

Closely related to slavery is the practice of debt bondage. Beginning in biblical and classical times, people who defaulted on their loans could be enslaved, imprisoned, or executed. The word draconian comes from the Greek lawgiver Draco, who in 621 BCE codified laws governing the enslavement of debtors. Shylock's right to cut a pound of flesh from Antonio in The Merchant of Venice is another reminder of the practice. By the 16th century defaulters were no longer enslaved or executed, but they filled up debtors' prisons by the thousands. Sometimes they were charged for food, despite being broke, and had to survive on what they could beg from passersby through the windows of the jail. In early-19th-century America, thousands of people, including many women, languished in debtors' prisons, half of them for debts of less than ten dollars. In the 1830s a reform movement sprang up which, like the antislavery movement, appealed to both reason and emotion. A congressional committee argued that it ran contrary to the principles of justice "to give the creditor, in any case whatever, power over the body of his debtor." The committee also noted that "if all the victims of oppression were presented to our view in one congregated mass, with all the train of wives, children, and friends, involved in the same ruin, they would exhibit a spectacle at which humanity would shudder." Debt bondage was abolished by almost every American state between 1820 and 1840, and by most European governments in the 1860s and 1870s.

In 1865, after the most destructive war in American history, slavery was abolished by the Thirteenth Amendment to the Constitution. Many countries had abolished it before that time, and France had the dubious distinction of abolishing it twice, first in the wake of the French Revolution in 1794 and again, after Napoleon had restored it in 1802, during the Second Republic in 1848. The rest of the world quickly followed suit. Many encyclopedias provide time lines of the abolition of slavery, which differ slightly in how they delineate territories and what they count as "abolition," but they all show the same pattern: an explosion of abolition proclamations beginning in the late 18th century. Figure 4-6 shows the cumulative number of nations and colonies that have formally abolished slavery since 1575.

FIGURE 4-6: Time line for the abolition of slavery

Source: The most comprehensive list of abolitions I have found is "Abolition of slavery timeline," Wikipedia, http://en. wikipedia. org/wiki/Abolition_of_slavery_timeline, retrieved Aug. 18, 2009. Included are all entries from "Modern Timeline" that mention formal abolition of slavery in a political jurisdiction.

The history of our treatment of debtors, Payne notes, illustrates the mysterious process in which violence has declined in every sphere of life. Western societies have gone from enslaving and executing debtors to imprisoning them and then to seizing their assets to repay the debt. Even the seizure of assets, he points out, is a kind of violence: "When John buys groceries on credit and later refuses to pay for them, he has not used force. If the grocer goes to court and gets the police to seize John's car or bank account, the grocer and police are the ones who are initiating the use of force." And because it is a form of violence, even if people don't usually think of it that way, this practice too has been in decline. The trend in bankruptcy law has been away from punishing debtors or squeezing assets out of them and toward giving them the opportunity of a fresh start. In many states a debtor's house, car, retirement accounts, and spouse's assets are protected, and when a person or company declares bankruptcy, they can write off many debts with impunity. In the old days of debtors' prisons, people might have predicted that this lenience would spell the demise of capitalism, which depends on the repayment of loans. But the commercial ecosystem evolved workarounds for this loss of leverage. Credit checks, credit ratings, loan insurance, and credit cards are just some of the ways that economic life continued after borrowers could no longer be deterred by the threat of legal coercion. An entire category of violence evaporated, and mechanisms that carried out the same function materialized, without anyone realizing that that was what was happening.

Slavery and other forms of bondage, of course, have not been obliterated from the face of the earth. As a result of recent publicity about the trafficking of people for labor and prostitution, one sometimes hears the statistically illiterate and morally obtuse claim that nothing has changed since the 18th century, as if there were no difference between a clandestine practice in a few parts of the world and an authorized practice everywhere in the world. Moreover, modern human trafficking, as heinous as it is, cannot be equated with the horrors of the African slave trade. As David Feingold, who initiated the UNESCO Trafficking Statistics Project in 2003, notes of today's hotbeds of trafficking:

The identification of trafficking with chattel slavery -- in particular, the transatlantic slave trade -- is tenuous at best. In the 18th and 19th centuries, African slaves were kidnapped or captured in war. They were shipped to the New World into life-long servitude, from which they or their children could rarely escape. In contrast, although some trafficking victims are kidnapped, for most…, trafficking is migration gone terribly wrong. Most leave their homes voluntarily -- though sometimes coerced by circumstance -- in search of a materially better or more exciting life. Along the way, they become enmeshed in a coercive and exploitative situation. However, this situation rarely persists for life; nor… do the trafficked become a permanent or hereditary caste.

Feingold also notes that the numbers of trafficking victims reported by activist groups and repeated by journalists and nongovernmental organizations are usually pulled out of thin air and inflated for their advocacy value. Nonetheless, even the activists recognize the fantastic progress that has been made. A statement by Kevin Bales, president of Free the Slaves, though it begins with a dubious statistic, puts the issue in perspective: "While the real number of slaves is the largest there has ever been, it is also probably the smallest proportion of the world population ever in slavery. Today, we don't have to win the legal battle; there's a law against it in every country. We don't have to win the economic argument; no economy is dependent on slavery (unlike in the 19th century, when whole industries could have collapsed). And we don't have to win the moral argument; no one is trying to justify it any more."

The Age of Reason and the Enlightenment brought many violent institutions to a sudden end. Two others had more staying power, and were indulged in large parts of the world for another two centuries: tyranny, and war between major states. Though the first systematic movements to undermine these institutions were nearly strangled in the crib and began to predominate only in our lifetimes, they originated in the grand change in thoughts and sensibilities that make up the Humanitarian Revolution, so I will introduce them here.