Let me say from the outset that this is a contentious issue among historians. It is one that has been fiercely debated and which regularly goes through periods of renewed conflict and tension. In particular, the notion that the wealth and economic prosperity of the western world are owed to slavery has been given new life by a book recently published by Edward Baptist titled The Half Has Never Been Told: Slavery and the Making of American Capitalism. I am very firmly situated in opposition to that perspective.

As a whole, historians do agree that slavery has profound significance to the economic history of the United States, the British Caribbean, Brazil and so on. That much is undeniable; slavery made a lot of money for a lot of people. The profits and resources generated by slavery were certainly significant to the story of American economic development. Unfree labour had an important role to play in making early colonies in particular economically and socially viable. Where there is disagreement is on the crux of your question: does the United States owe its present-day success and vast wealth to racial slavery? That is to say, it is an indispensable aspect of America's economic success in the 21st Century and without it, would the world be radically different?

In short, the answer is 'probably not' - especially when we frame your question in terms of wealth. Johnston and Williamson's work on estimating US GDP (the measure of how much an economy is worth) back to 1790 shows that an estimated 86% of all US economic growth as measured by GDP took place post-1945. The abolition of slavery in the antebellum South correlates closely with a net reduction of real-term GDP of just 4.6% according to their work; the US economy returns to pre-abolition levels of overall prosperity on a per capita (per person) basis by 1872, and net growth is achieved as early as 1868. Whilst a 5% retraction in GDP in a single year is a harsh recession for any economy, it is nothing compared to the Great Depression; between 1930 and 1933, the US economy loses more than 25% of its 1929 GDP.

Whilst GDP has its obvious flaws as a historical comparator - even today national GDP is only an estimate - there is little evidence to suggest that slavery was the backbone of the US economy in the 19th Century. The US economy simply did not experience the kind of catastrophic loss we should expect from abolition in that case. For one, as we know, the Northern United States, where the bulk of industrialisation and commercialisation took place, abolished slavery a long time before the South. Whilst money and resources generated by the economy of the antebellum South did feed into the free North, most of the wealth created in the South remained trapped within the slave system; the evidence that slavery was essential to industrialisation is weak. Whilst there certainly was a contribution it is not on the scale that makes it essential to economic prosperity. The US economy was simply too diverse already to be characterised as a state dependent upon Southern slavery.

Crucially to your question about wealth though, one of the features of slavery is that it reduces Human beings not only to property but also to capital. Slaves aren't just useful for the products of their labour; they're useful because they have value as property and, crucially, self-reproducing property. If you have two slaves who have four children who survive to adulthood, not only have you doubled your labour force at a minimal cost, but you now also have four slaves you can sell - for an initial investment that only cost you two. So there's a huge perceived incentive in the antebellum South to invest in slaves; not only because they represent a cheap labour force but because they're useful as a capital investment.

So during slavery, the system does produce huge quantities of wealth. But a significant portion of it is false wealth - it doesn't actually exist until you sell a slave, just like most of the wealth tied up in a house you own today doesn't actually exist until you sell your house. With the abolition of slavery in its totality by 1865, all of this wealth tied up in slaves disappears virtually overnight - with profound and devastating implications for the Southern economy. A comparison with Britain is worth noting here. When the British abolished slavery in the 1830s, the British government authorised a compensation scheme of £20million for slave-owners. £20million might not sound like a lot today, but in relative terms, that's three times the size of the 2008 US bailout, and more than double what the UK spends on defence today.

That's significant for two reasons: first, it reflects a keen awareness on the part of the British that abolition would wipe out vast swathes of wealth. Second, it shows that Britain was clearly in a strong enough economic position to finance a huge bailout of the slaveholding class after abolition (of course, it can be argued that in the long-term it would have been much wiser economically and socially to invest in the ex-slaves). Even so, this compensation proved insufficient at staving off the decline and consolidation of many of the Caribbean's plantations.

Historians have attributed this tying up of capital in the slave system as a key reason why the South failed to invest in urban development and infrastructure on the same scale as the rest of the United States. Many contemporary critics of slavery argued that it was dragging down the nation's economic health - and in the modern world, unfree labour appears to be a detriment, not a boon, to prosperity. In some ways, it can be argued slavery has been profoundly harmful to long-term economic development. I don't think there's much of an argument in claiming America would be significantly worse-off if slavery had been abolished uniformly across the United States at the end of 18th Century. It would be a very different nation with a very different history, certainly - but I don't imagine it would be particularly less well-off.

So essentially, I would say that there are three key things to keep in mind:

1. Slavery had a role to play in our modern-day economic prosperity but it was by no means definitive; it is illegitimate in my view to look back and say that the United States, or the west at large, owes its present success to slavery and slavery alone. It is far from being the backbone of American wealth, in my view, most of which has been created long after slavery's end.
2. If slavery is the essential and defining ingredient of the economic success of the US, we should expect a much more profound economic shock with its abolition. Yet the post-abolition recession pales compared to the Great Depression; industrialisation was aided by, but did not depend by any means on, the profits of slavery.
3. Whilst slavery did create enormous wealth within slave economies, it also tended to be trapped within the slave system, and abolition also destroyed much of that wealth. This is not to deny that there certainly are people who do very much own their present-day status to slaveholding ancestry, of course.

However, it is important to stress that slavery does play a decisive role in explaining many of the economic conditions in the present United States. The obscenely and disproportinately high poverty rate of black families, and the profoundly inequitable distribution of income along racial and ethnic lines, stems undeniably from the economic order created by slavery and the failure of successive governments to meaningfully ameliorate this legacy in the post-abolition period. Much of the poverty found in the rural South can be attributed to the legacy of plantation economy, which created a privileged class of wealthy white landowners who went on to develop new systems of exploitation in the postbellum period. There are descendants of those who navigated the abolition period successfully who very much owe their own wealth and prosperity to a slave-holding heritage. And, of course, it is in slavery that we find the roots of modern-day racism that still has ramifications for American society socially and economically.

So there is an important distinction to be made. Slavery does not, in my view, adequately explain how the United States was able to achieve such economic prominence and success, nor does it adequately explain the vast wealth that exists in America today. That is an over-simplification. Unfree labour certainly had an enormous role to play in making colonial development possible, but slavery was not the first or only system used to this end. The historical record does not support the contention that slavery, or its profits, were an indispensable ingredient in western industrialisation and progress.

What slavery does explain is a lot of the inequities in how wealth is distributed in America. Its legacy permeates American society and economy in a pretty profound and fundamental way. So whilst slavery might not be the backbone of modern American wealth, it very much is the framework around which the historic socio-economic order in the United States has been built; it has massively distorted the access to and distribution of wealth in the United States. In that sense, it can be said the US owes its present economic order to slavery if not the actual volume of its wealth, and that depending on where you are within modern society's racial and class systems, you are either a beneficiary or a victim of the legacy of racial slavery.

You might find this discussion (in which I also highlight the profound flaws in Baptist's book and cite some useful alternative analyses) from a month ago interesting.

If slavery was the difference between wealthy nations and poor nations, then South America and Brazil in particular would be the richest nations on earth since Brazil had 10x as many slaves as US had as its peak.