It has become a common trope that slavery and the slave trade is responsible for the industrial revolution, if not our entire modern prosperity. Slavery is often called capitalism’s “dark side.” A recent column in the Guardian claimed the slave trade “heralded the age of capitalism” and Guardian columnist George Monbiot said on Twitter: “The more we discover about our own history, the less the ‘trade’ on which Britain built its wealth looks like exchange, and the more it looks like looting. It meant extracting stolen resources and the products of slavery, debt bondage and land theft from other nations.” The same line has been taken by London Mayor Sadiq Khan, who tweeted: “It’s a sad truth that much of our wealth was derived from the slave trade.”

But what did the “father of modern economics,” Adam Smith, actually think about slavery? And is it responsible for our modern prosperity?

Adam Smith argued not only that slavery was morally reprehensible, but that it causes economic self-harm. He provided economic and moral ammunition for the abolitionist movement that came to fruition after his death in 1790. Smith was pessimistic about the potential for full abolition, but he was on the side of the angels.

Smith’s The Wealth of Nations, published in 1776, contains perhaps the best known economic critique of slavery. Smith argued that free individuals work harder and invest in the improvement of land, motivated by their interest in earning a higher income, than slaves. Smith refers to ancient Italy, where the cultivation of corn degraded under slavery. The cost of slavery is “in the end the dearest of any,” Smith writes.

His thinking about slavery can be traced further back. In the Lectures on Justice, Police, Revenue and Arms, delivered in 1763 long before Britain’s abolitionist movement was formalised, Smith writes:

Slaves cultivate only for themselves; the surplus goes to the master, and therefore they are careless about cultivating the ground to the best advantage. A free man keeps as his own whatever is above his rent, and therefore has a motive to industry.

Smith describes how serfs in Western Europe—in feudal relationships with lords—were progressively transformed into free tenants as they acquired cattle and tools. Harvests were more evenly divided between landlord and tenant to encourage better use of land, and tenants eventually progressed to simply giving the landlord a sum for lease. As government became more established, the influence of lords over the lives of tenants was also loosened.

Capitalism was, as Marx described, the next stage in human development after feudal slave relations. Smith’s commercial society is in direct opposition to a slave society. Smith, at his core, is an advocate for individuals being free to specialize and trade, including to trade their labor. Everyone acting with regard to their “own interest,” not because of coercion, creates general prosperity.

Smith’s case against slavery is proven by history: The huge uptick in human prosperity came largely after the end of feudal relations and the abolition of slavery and the slave trade. We are many magnitudes richer than when lords held slaves, or even chattel slavery proliferated in the Americas. The setting free of humanity led to extraordinary innovation and entrepreneurialism. This is only possible, as Smith argued, when individuals can benefit from the fruits of their own labor (slaves cannot hold property in their own name, and hence cannot trade or choose to specialise).

We didn’t become rich because a few hundred years ago people toiled on farms in awful conditions. In fact, the opposite. “It was precisely the replacement of human muscle power with that of steam and machines which did away with the vileness of chattel slavery and forced labor,” Tim Worstall has explained.

Nor did the slave trade fund the industrial revolution. Leading economic historian Deirdre McCloskey explains that the slave trade, and the goods produced by slaves, were a tiny portion of foreign trade in Britain. Additionally, slaves were not passive: From Jamaica to St Dominique they rebelled against their masters. Quashing these rebellions was not cheap. More broadly, McCloskey argues that the industrial revolution was spurred by domestic innovations and not trade or minuscule imperialist returns.

Put differently, if all that it took for a country to be rich was historic slavery then countries would be rich in proportion to their historic level of slavery, but this is not so. Just because America had grotesque slavery and got rich does not mean that America got rich because of slavery. Many countries that had extensive slavery in the past, such as former Spanish colonies in South America, are not particularly rich today. There are many alternative, more plausible explanations for human prosperity.

Additionally, just because some individuals made personal fortunes from slavery, does not mean that nations overall benefited very much. In fact, the average person not only got little or nothing materially out of the likes of the British Empire—they also had to pay huge expenses for its defence in various wars, up to and including their own lives. In any case, the claim that imperialism spurred the industrial revolution gets the timeline wrong: Empire required steam and steel ships, and hence came after the industrial revolution had already begun.

So in the end, slavery, the slave trade and imperialism were not only morally disgusting but also of dubious economic value. A small number of people profited from the trade—and they campaigned against abolition. But this should not be confused with a broader claim that our modern prosperity is built on benefits that went to a small number a few hundred years ago.

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That slaves were able to develop beneficial skills while in bondage is a tribute to the human ability to wrest value and create meaning even under conditions of almost unfathomable duress.

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The moral case

Slavery wasn’t just bad economics. For Smith, slavery was inhumane and evil. In the aforementioned lectures, Smith discusses the brutal treatment of slaves in ancient Rome, where, in the night time “nothing was to be heard but the cries of slaves whom their masters were punishing”:

Ovid tells us that the slave who kept the gate was chained to it, and the slaves who manured the ground were chained together lest they should run away, and what was more cruel, when an old slave was incapable for work he was turned out to die on an island near the city kept for that purpose.

Smith also observed that:

[W]e may see what a miserable life the slaves must have led; their life and their property entirely at the mercy of another, and their liberty, if they could be said to have any, at his disposal also.

Smith’s revulsion at the idea of slavery may indicate some motivated reasoning in the economic arguments; he may have wanted to show that an alternative world without slavery would lead to prosperity in order to bolster the abolitionist case.

Smith was, nevertheless, notably pessimistic about the broader chances of abolition:

[S]lavery takes place in all societies at their beginning, and proceeds from that tyrannic disposition which may almost be said to be natural to mankind… It is indeed all-most impossible that is should ever be totally or generally abolished.

He even thought that as societies got richer they would be able to afford more slaves. Smith downplayed the likelihood that free or monarchical societies, or religion, would lead to abolition. At the time, the British Empire, and many others, were transporting millions of people from Africa across to the Americas in the extraordinarily violent and barbaric practice of chattel slavery—often justified by extreme racism and involving extensive torture and sexual exploitation.

Smith was, thankfully, wrong. This practice would come to an end. Over the coming decades Britain’s anti-slavery movement abolished slavery throughout the British Empire and helped spur the global abolitionist movement. (Chattel slavery never existed under English or Scottish law—though there were some imported slaves under the guise of domestic work.) Parliamentarian William Wilberforce worked, with researchers Thomas Clarkson and Zachary Macaulay, and Quaker and Anglican campaigners, on a life-long crusade against the barbaric practice from the late 1780s. This group became known as the “Clapham Sect,” residing in south west London. They brought attention to the issue, attained wide support from William Pitt to Edmund Burke, travelled the country, pioneered lobbying techniques such as parliamentary petitions, wrote pamphlets, printed badges, and held public meetings.

Smith has been widely credited with influencing the antislavery movement. His work has been described as a “generative site of abolitionist ideology.” Smith provided the economic case against slavery in both the United Kingdom and, later, the United States. His arguments against slavery were quoted in early antislavery material. Wilberforce, who met Adam Smith in 1787, quoted Smith often. Quaker abolitionist James Cropper quoted Smith’s ideas about slavery’s economic inefficiency. Additionally, Smith’s ideas about ethics and empathy, developed in the Theory of Moral Sentiments (where he describes the “levity, brutality, and baseness” of the slave traders), also came to significantly influence the rhetorical strategy of the abolitionist movement. Smith, of course, was not alone. A wide array of liberal thinkers made the case for the supremacy of the individual and against slavery.

These campaigns managed to achieve an extraordinary “collective change of heart,” historian Niall Ferguson has written, in the face of the organised and powerful beneficiaries of slavery. In 1807, Parliament abolished the slave trade across the British Empire, which came to cover hundreds of millions of people. But this was not all. The Royal Navy used its dominance of the sea to suppress the slave trade by foreigners, both seizing slave ships and coercing other countries such as Spain and Portugal into signing treaties committing to end their slave trade.

By 1860, the Royal Navy’s West Africa Squadron had seized approximately 1,600 ships involved in slavery and freed 150,000 Africans aboard those vessels. About 1,587 members of the squadron are estimated to have died from both disease and being killed in action. Freetown in Sierra Leone takes its name from its first settlers, returned slaves by Brits. According to Ferguson: “The freed slaves walked through a Freedom Arch bearing the inscription—now almost obscured by weeds: ‘Freed from slavery by British valor and philanthropy.’” In 1833, slavery was abolished across the British Empire. The British and Foreign Anti-Slavery Society, now known as Anti-Slavery International, was formed in 1839 to stamp out slavery across the globe. It is the world’s oldest human rights organization.

This took far too long. Slavery is, was, and continues to be (in its modern forms) totally repugnant. But what makes the British Empire unique is not its entanglement with the slave trade, which was true of nearly every empire, as Smith explains, but rather its moral crusade against the trade from the 19th century.

It is no coincidence that the birthplace of the industrial revolution is also the birthplace of the global antislavery movement. Slavery is the antitheses to market economies which depend on the voluntary exchange of labor.

Slavery has been a consistent feature of human history; the Enlightenment liberal and Christian thinkers contributed substantially to the case for its abolition. Adam Smith was one of those early thinkers. In 1764, an anonymous American published an anti-slavery pamphlet based on The Theory of Moral Sentiments. The pamphlet concludes on Smith:

How had he bless’d mankind, and rescu’d me!