Full Idea
In the beginning slaves probably offered themselves of their own free will; their motive was poverty, or a sense of their own lack of intelligence.
Gist of Idea
Originally slavery was probably voluntary, driven by necessities
Source
Samuel Pufendorf (On the Duty of Man and Citizen [1673], II.04)
A Reaction
He goes on to say that the commonest type of slave is a prisoner of war (who is lucky to be alive). There are still people who are trapped in poverty or illiterate, and are facing such a dilemma.
Book Reference
Pufendorf,Samuel: 'On the Duty of Man and Citizen' [CUP 1991], p.129