Big question: Why did legal slavery end in the 19th century?

Humans started enslaving other humans sometime in the dim mists of prehistory. Until the late 18th century, few thought this practice odd or morally objectionable. People who were enslaved were unhappy about their condition, but the existence of slavery itself went largely unchallenged.

Then, in a very short period of time (compared with the very long history of the institution) legal slavery disappeared from the earth. Compelled labor didn't vanish, but it went underground. In 1800, slave owners were pillars of their communities and proud to boast of the numbers of men and women they owned. In 1900, they were criminals engaged in traffic that was almost universally illegal.

How did this happen? Why did it happen when it happened?

A view commonly held today is that humanity became more enlightened and awoke to the evil of slavery. In this version of history, the question isn't why the change, but why the change was so late in coming.

Yet this is simply the original question in different form. What happened in the 19th century to make people more enlightened? Was humanity seized by a fit of morality? Did sinners become saints? Or were other factors at work?

Events of the 20th century, conspicuously including two world wars, argue against a rise in humanity’s overall morality quotient. There must have been other causes. What were they? 

Although slavery was long tolerated, it was rarely celebrated. It was treated as a necessary evil, something like war. Perhaps events of the 19th century made slavery less necessary. This would have allowed its evilness to stand out and might have prompted people to say: Enough is enough.

Consider the timing of slavery’s demise in various countries and empires. France abolished slavery in France itself in the 14th century but continued to practice it in French colonies. In 1794 the revolutionary government of France freed slaves in the colonies as part of a general assault on property and privilege. After overthrowing the revolution, Napoleon Bonaparte reinstituted slavery in the colonies in 1802, and it persisted until 1848.

Britain abolished slavery on British soil in 1772 by judicial decree in the Somerset case. In 1807 Parliament ended the overseas slave trade. In 1833 Parliament passed the Slavery Abolition Act, to end slavery throughout the British empire. It took effect the following year.

In the United States, northern states began abolishing slavery as soon as they were out from under British imperial law, which had prevented colonial action against slavery. Pennsylvania went first, others followed, and by the first decade of the 19th century, slavery was dead or dying in all the northern states.

Strikingly, slavery had never been an important part of the economy in France itself, Britain itself, or the northern states in America. Ending it there was relatively inexpensive. It was a necessary evil that, on reflection, didn't seem so necessary, and was relinquished without much fuss.

Ending slavery in the French and British empires was a bigger lift. The West Indies were important to the economies of both, and slavery was woven into the economy of the West Indies. But the West Indian planters didn't form a critical mass in French or British politics, where a new force, representing emerging industry, was taking hold.

Things were different in America. Conventional reading of the Constitution left the question of slavery to the states to decide for themselves. It was under this interpretation that northern states passed their abolition laws. No appreciable interest groups in those states had a large stake in maintaining the institution. But in the South, slaveholders dominated state legislatures. The planters weren't a majority in any state, but they were wealthy and determined, two characteristics that often allow minorities to have their way.

At the same time, the southern planters were losing ground nationally to the merchants and industrialists of the North. They lost the House of Representatives in the early 19th century, and the Senate in the 1850s. When Abraham Lincoln won the presidency in 1860 at the head of a new antislavery party, the planters panicked and led a bolt from the Union. It was a fatal miscalculation. Lincoln hadn't intended to tamper with slavery in the southern states, but his war to save the Union propelled him to abolition. Designed to preserve slavery, secession doomed it.

What if the southern planters had not been so foolish? How long would slavery in their states have persisted?

The short answer is: Until the southern states decided to give it up. Absent the Civil War, the southern states would have been able to block an emancipation amendment to the Constitution. 

The southern states wouldn’t have clung to slavery forever. Southern slavery was buoyed in the first half of the 19th century by the rising price of cotton. But cotton prices plunged in the late 19th century as a result of global oversupply. Planters would have discovered the downside of treating labor as a fixed cost, which slavery did. Hired labor can be shed at no cost to the employer. Slaves cannot. Just as the price of slaves had risen with the price of cotton, so the price of slaves would have fallen with the falling price of cotton. Slave owners would have been hit with a double whammy, having to feed and clothe workers they had no use for, and unable to sell them except at a loss.

The historical fact is that southern slavery was built on an unsustainable bubble of demand for cotton. When the bubble burst, as bubbles do, slavery would have become economically unattractive.

Meanwhile the moral stigma of slavery would have continued to intensify. When every country practiced slavery, none pointed fingers at the others. But as abolition spread, the abolitionists targeted the holdouts. Political pressure accompanied moral disapproval. Britain refused to recognize the Confederacy in no small part because British leaders and voters didn't want to associate with slave owners. That refusal crippled the Confederacy’s chances of sustaining secession.

At some point the balance between necessity and evil would have tipped against slavery in the American South, as it did elsewhere. When would this have happened? Brazil abolished slavery in 1888, becoming the last country in the Americas to do so. Would American states have held on longer? It seems unlikely. 

Virginia, the most populous of the southern states, likely would have been the first to terminate slavery. Virginia had been slow to join the Confederacy, waiting until Lincoln made clear he was going to march an army through the state to get at South Carolina, where hostilities had begun. Virginia had strong commercial ties to the North, and its merchants were reluctant to abandon those. Moreover, long before 1860, plantation agriculture per se had become unprofitable on the worn-out soil of the Old Dominion. What kept the planters in business was the export of their slaves to the newer states of the southwest. Lincoln wasn’t going to touch slavery in the states directly, but he was adamant about preventing the spread of slavery farther west. When Virginia’s export market disappeared, the part of its economy that rested on slavery would have collapsed. The Virginia legislature would have followed the lead of the state’s northern-looking merchants rather than the southern-looking planters. Slavery would have been tossed. Where Virginia led, other southern states would have followed.

None of this can be proved. Counterfactuals never can. But it seems likelier than the alternatives. Slavery was more than an economic system, but it wasn’t so much more that once it lost its economic rationale it could withstand the increasing moral disapproval.

Big changes in history have multiple causes. But the answer to the question of why slavery was outlawed around the world in the 19th century is relatively simple. Changes in moral attitudes combined with changes in economics to shift the perception balance from necessity to evil on this ancient practice. The shift occurred sooner in some places than in others. But by the end of the 19th century, the moral costs of slavery outweighed the economic benefits almost everywhere.