In the history of human society slavery has two types - intra-tribal and extra-tribal. Intra-tribal slavery occurs when members of a community lose their status and fall into a subservient position. An example would be someone who cannot repay his debts or commits a serious crime. Extra-tribal slavery occurs when one group is defeated in war by another and is absorbed into the victor’s population.
Often, the existence of a slave class has an adverse effect on the society. As unpaid laborers, the slaves contribute to high productivity, but they also displace the lower classes from the job market. During the Roman Republic, the accumulation of wealth among a few patricians and their use of slave labor to work their estates led to the death of the small farm and the unemployment of the small farmer. Eventually, there was an exodus of displaced persons to Rome where they became restless urban poor.