How Similar were traditional slavery to European slavery?

Forced marriages or early marriages are often considered types of slavery. Forced marriage continues to be practiced in parts of the world including some parts of Asia and Africa and in immigrant communities in the West. Sacred prostitution is where girls and women are pledged to priests or those of higher castes, such as the practice of Devadasi in South Asia or fetish slaves in West Africa. Marriage by abduction occurs in many places in the world today, with a national average of 69% of marriages in Ethiopia being through abduction. Economists have attempted to model the circumstances under which slavery (and variants such as serfdom) appear and disappear. One observation is that slavery becomes more desirable for landowners where land is abundant but labour is scarce, such that rent is depressed and paid workers can demand high wages. If the opposite holds true, then it becomes more costly for landowners to have guards for the slaves than to employ paid workers who can only demand low wages because of the amount of competition. Thus, first slavery and then serfdom gradually decreased in Europe as the population grew but were reintroduced in the Americas and in Russia as large areas of new land with few people became available.Slavery is more common when the labour done is relatively simple and thus easy to supervise, such as large-scale growing of a single crop, like sugar and cotton, in which output was based on economies of scale. This enables such systems of labour, such as the gang system in the United States, to become prominent on large plantations where field hands were monitored and worked with factory-like precision. For example, each work gang was based on an internal division of labour that assigned every member of the gang to a precise task and simultaneously made their own performance dependent on the actions of the others. The hoe hands chopped out the weeds that surrounded the cotton plants as well as excessive sprouts. The plow gangs followed behind, stirring the soil near the rows of cotton plants and tossing it back around the plants. Thus, the gang system worked like an assembly line.Since the 18th century, critics have argued that slavery tends to retard technological advancement because the focus is on increasing the number of slaves doing simple tasks rather than upgrading the efficiency of labour. For example, it is sometime argued that, because of this narrow focus, theoretical knowledge and learning in Greece – and later in Rome – was not applied to ease physical labour or improve manufacturing. Scottish economist Adam Smith states that free labour was economically better than slave labour, and that it is nearly impossible to end slavery in a free, democratic, or republican form of government since many of its legislators or political figures were slave owners, and would not punish themselves. He further states that slaves would be better able to gain their freedom when there was centralized government, or a central authority like a king or the church. Similar arguments appear later in the works of Auguste Comte, especially when it comes to Smith's belief in the separation of powers, or what Comte called the "separation of the spiritual and the temporal" during the Middle Ages and the end of slavery, and Smith's criticism of masters, past and present. As Smith states in the Lectures on Jurisprudence, "The great power of the clergy thus concurring with that of the king set the slaves at liberty. But it was absolutely necessary both that the authority of the king and of the clergy should be great. Where ever any one of these was wanting, slavery still continues..." Worldwide, slavery is a criminal offense, but slave owners can get very high returns for their risk. According to researcher Siddharth Kara, the profits generated worldwide by all forms of slavery in 2007 were $91.2 billion