4. _Custom and national temperament affect the efficiency of labor by determining the normal period of labor time._ After the bare necessities of life are provided for, the worker has a wide or narrow margin of productive energy to use as he pleases. If four hours' work a day would enable him to live, will he work longer or will he stop? The answer is determined by the balance of utility and disutility. Will additional hours of labor yield more gratification than idleness yields? Does the pain of toil repel more than its fruits attract? The use made of spare time differs according to climate, race, and temperament. In the tropics the margin is converted usually into loafing, in the temperate zones largely into objective forms of enjoyment. Individual differences are plainly seen when each man labors on his own field. The prudent man, in the old maxims, makes hay while the sun shines and ploughs deep while sluggards sleep. In the modern larger organization of industry, working hours are much the same for all workers in the establishment. Individual preferences are still expressed, however, in irregularity of employment. In the South some manufacturers have found that on an average the negroes will work in a factory not more than five or six hours a day, working ten hours for four days and lying off two days a week. Such a standard of working hours is the mark of the primitive stage of wants and industrial qualities, although a shortening of the hours of manual labor, as incomes increase above bare subsistence, is in accord with a rational valuation of leisure. A moderate change in that direction cannot but increase rather than diminish the efficiency of labor. § III. DIVISION OF LABOR [Sidenote: Division and exchange of labor] 1. _Division of labor is a term expressing that complex arrangement of industrial society whereby individual workers are enabled to apply themselves to the production of certain kinds of goods, securing others by exchange._ The term "division of labor" is simple, but the thought is a complex one. Its full discussion would cover the whole field of political economy, but only its most essential aspects can here be touched upon. Division of labor and exchange are counterparts and mutually determine each other. Division of labor depends on the extent of the market, and in turn widens its limits. The number of articles that any one would care to produce at one time and place depends upon the opportunity to exchange them. These two aspects of industry thus are inseparable in thought and practice. The worker finds division of labor existing as a social institution and, according as he adapts himself to it wisely or foolishly, it increases more or less his efficiency. [Sidenote: Division of labor between trades and territories]