Nieboer starts out by defining slavery, and then through a systematic examination of the available ethnographic reports of the life of indigenous people he looked for any correlations between the holding of slaves and ways of securing subsistence.
I am not going to exposit or explain his conclusions, except to provide you with the following chart which he made of his conclusions.
|Furthering the growth of slavery.
|Hindering the growth of slavery.
|I. Internal causes.
|1. Open resources and subsistence easy to acquire.
|1. Closed resources.
2. Subsistence difficult to acquire.
|B. Secondary, economic:
|1. A high position of women.
3. Preserving of food.
|1. Female labour serving as a substitute for slave labour.
2. Subjection of tribes as such.
|C. Secondary, non-economic:
|1. Militarism (where slaves are employed in warfare).
2. Slaves kept as a luxury.
|1. Militarism (especially where foreigners are adopted).
|II. External causes:
|1. Fixed habitations.
2. Living in large groups.
3. Preserving of food.
4. The slave-trade.
5. The neighbourhood of inferior races.
“The most important result of our investigation seems to us the division, not only of all savage tribes, but of all peoples of the earth, into peoples with open, and with closed resources.” [p. 418] [Comment: I keep talking about this in terms of a free access to subsistence land.]
In the following paragraphs, he provides some support for Oppenheimer’s theory of State formation.
“In North-East Africa, however, there is one more cause at work, making slavery superfluous. This is the existence of a kind of substitute for slavery, viz. subjection of tribes as such. Pastoral tribes often levy tributes on agricultural tribes, to which they are superior in military strength; the latter cannot easily leave the lands they cultivate and seek a new country; if not too heavily oppressed, they will prefer paying a tribute. And to pastoral nomads the levying of a tax on agricultural tribes brings far more profit than the enslaving of individuals belonging to such tribes, whom they would have to employ either in pastoral labour, which they do not want, or in tilling the soil, which work the nomads would be unable to supervise. There are also pastoral tribes subjected by other pastoral nomads, the latter forming the nobility and the military part of society. Finally we find subjected tribes of hunters, smiths,  etc.; here we have sometimes rather to deal with a voluntary division of labour.”
“We see that the difference between the slave-keeping and the other pastoral tribes consists in external circumstances. Pastoral tribes have no strong motives for making slaves, for the use of slave labour is small. On the other hand, there are no causes absolutely preventing them from keeping slaves. These tribes are, so to speak, in a state of equilibrium; a small additional cause on either side turns the balance. One such additional cause is the slave-trade; another is the neighbourhood of inferior races. There may be other small additional causes, peculiar to single tribes. We shall not inquire whether there are, but content ourselves with the foregoing conclusions, of which the principal are these, that the taming of animals does not naturally lead to the taming of men, and that the relation between capital and labour among pastoral tribes renders the economic use of slavery very small. [ 290]
Recapitulating, we may remark that our general theory, that there is no great use for slave labour where subsistence depends on capital, is fully verified by our investigation of economic life among pastoral tribes.
Two secondary internal causes found in the second chapter have been also met with among pastoral tribes: slaves are sometimes employed in warfare, and sometimes for domestic labour to relieve the women of their task. Two new secondary factors have been found in this chapter: slaves are kept as a luxury; and sometimes the subjection of tribes as such, serving as a substitute for slavery, makes slavery proper superfluous.
With regard to the external causes it has been shown that the coercive power of pastoral tribes is not very strong, as they are nomadic and live in rather small groups; but this want is sometimes compensated for by the slave-trade and the neighbourhood of inferior races. The two latter circumstances may therefore rank as new external causes, the slave-trade taking the place of the existence of a homogeneous group. On the Pacific Coast of N. America it is the trade between tribes of the same culture, among pastoral nomads it is the trade with Arabia, etc. ; but in either case it is the slave-trade that furthers the growth of slavery.”