Engerman referenced Adam Smith's understanding of slavery which he described as being that slaves had no incentive towards greater productivity, with the result that using slave labor rather than free labor was inefficient. Smith thus attributed the fact that people use slavery despite it's inefficiency to the will to domineer over others:
But if great improvements are seldom to be expected from great proprietors, they are least of all to be expected when they employ slaves for their workmen. The experience of all ages and nations, I believe, demonstrates that 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. In ancient Italy, how much the cultivation of corn degenerated, how unprofitable it became to the master when it fell under the management of slaves, is remarked by both Pliny and Columella. In the time of Aristotle it had not been much better in ancient Greece. Speaking of the ideal republic described in the laws of Plato, to maintain five thousand idle men (the number of warriors supposed necessary for its defence) together with their women and servants, would require, he says, a territory of boundless extent and fertility, like the plains of Babylon.
The pride of man makes him love to domineer, and nothing mortifies him so much as to be obliged to condescend to persuade his inferiors. Wherever the law allows it, and the nature of the work can afford it, therefore, he will generally prefer the service of slaves to that of freemen. The planting of sugar and tobacco can afford the expence of slave-cultivation. The raising of corn, it seems, in the present times, cannot. In the English colonies, of which the principal produce is corn, the far greater part of the work is done by freemen. The late resolution of the Quakers in Pennsylvania to set at liberty all their negro slaves, may satisfy us that their number cannot be very great. Had they made any considerable part of their property, such a resolution could never have been agreed to. In our sugar colonies, on the contrary, the whole work is done by slaves, and in our tobacco colonies a very great part of it. The profits of a sugar-plantation in any of our West Indian colonies are generally much greater than those of any other cultivation that is known either in Europe or America; and the profits of a tobacco plantation, though inferior to those of sugar, are superior to those of corn, as has already been observed. Both can afford the expence of slave-cultivation, but sugar can afford it still better than tobacco. The number of negroes accordingly is much greater, in proportion to that of whites, in our sugar than in our tobacco colonies.
(Wealth of Nations, Book III, Ch2, Para 9-10)
By contrast, David Hume is described as having, if possible, a more cynical view: Free workers are more efficient than slaves because they are always on the brink of penury and starvation and thus will work harder, while slaves at least have the assurance that it is in their master's interest to keep them alive. However, no specific Hume citation is given, and running searches on Hume on slavery, I'm not finding anything along these lines. Is this characterization of his views accurate? Perhaps we have among our readers a Hume scholar who can shed some light on this?
Regardless, all this leads to some interesting thoughts.
First, reading Smith's quote, it strikes me that while he's doubtless right about people having a drive to domineer, he may also be overlooking an economic incentive towards slavery in the production of high manual labor, high value crops such as sugar, tobacco and later cotton: While it's true that free, individual farmers working small plots for their own betterment would doubtless be more productive than slaves because it was in their interest to do so, this higher productivity might well not result in increased wealth for the small number of larger planters who were the most direct beneficiaries of the slave system in the US and West Indies. The planters might well have recognized that free farmers would be more productive overall (say, on a square mile of cultivated land basis, and on a per laborer basis) but have believed that they themselves benefited more from taking all of the profits generated by slaves rather than collecting some smaller percentage of the profits generated by free farmers via rents. For them to conclude otherwise, they would have to accept a sort of variant on "voodoo" supply-side economics, predicting that the increase in productivity of free tenant farmers would more than compensate them for getting only a percentage of the profits rather than all of them. Given how hard a sell that has been in modern politics, one can imagine why it wouldn't go over well with the 18th and 19th century planter class.
Second, both the Smith quote and the more cynical thinking on the inefficiency of slavery attributed to Hume suggest a certain attractive element to slavery: "A person who can acquire no property, can have no other interest but to eat as much, and to labour as little as possible." Clearly, no one wants to be a plantation slave, but if the minimum level of substance which one's master owed one could be defined at a higher level (say, an apartment, car, food, flat panel TV and 300 cable channels) the attraction becomes obvious. We naturally do not like insecurity, and so being assured a level of security in return for a lack of material advancement beyond a certain minimum and the chance to "eat as much and to labour as little as possible" is always going to have attractions to some people. Indeed, one might also see a certain non-materialist virtue to it.
Thus points to a trade off which is, I think, always a temptation to societies -- for those who see themselves as having less opportunity to accept a state of semi-servility in order to assure security against material want, and for the most wealthy and powerful segment of society to be eager to give them such guarantees in order to assure that the lowest reaches of society do not become too restive while the elites maximize their profits.