The maintenance of the slave at the full rate required for the preservation of lusty physique was essential.  The master could not reduce it below that standard without impairing his property as well as lessening its immediate return; and as a rule he could shift none of the charge to other shoulders, for the public would grant his workmen no dole from its charity funds.  On the other hand, he was often induced to raise the scale above the minimum standard in order to increase the zeal and efficiency of his corps.  In any case, medical attendance and the like was necessarily included in the cost of maintenance.

The capital investment in a slave reared by his master would include charges for the insurance of the child’s mother at the time of his birth and for her deficit of routine work before and afterward; the food, clothing, nurse’s care and incidentals furnished in childhood; the surplus of supplies over earnings in the period of youth while the slave was not fully earning his own keep and his overhead charges; compound interest on all of these until the slave reached adolescence or early manhood; and a proportion of similar charges on behalf of other children in his original group who had died in youth.  In his teens the slave’s earnings would gradually increase until they covered all his current charges, including the cost of supervision; and shortly before the age of twenty he would perhaps begin to yield a net return to the owner.

A slave’s highest rate of earning would be reached of course when his physical maturity and his training became complete, and would normally continue until his bodily powers began to flag.  This period would extend in the case of male field hands from perhaps twenty-five to possibly fifty years of age, and in the case of artizans from say thirty to fifty-five years.  The maximum valuation of the slave as property, however, would come earlier, at the point when the investment in his production was first complete and when his maximum earnings were about to begin; and his value would thereafter decline, first slowly and then more swiftly with every passing year, in anticipation of the decline and final cessation of his earning power.  Thus the ratio between the capital value of a slave and his annual net earnings, far from remaining constant, would steadily recede from the beginning to the end of his working life.  At the age of twenty it might well be as ten to one; at the age of fifty it would probably not exceed four to one; at sixty-five it might be less than a parity.

In the buying and selling of nearly all non-human commodities the cost of production, or of reproduction, bears a definite relation to the market price, in that it fixes a limit below which owners will not continue to produce and sell.  In the case of slaves, however, the cost of rearing had no practical bearing upon the market price, for the reason that the owners could not, or at least did not, increase or diminish the