The case just examined is far from unique. On the contrary, the benefits offered by such voluntary exchanges are far more widespread than generally appreciated. Indeed, the law of association, also known as Ricardo's law of association (after classical economist David Ricardo) or the law of comparative advantage, shows that individuals or organizations in the same marketplace will almost always be able to reap mutual benefits from trading goods and services. Before stating the law in its general form and proving it, let us first see how it might apply to Crusoe and Friday.

Before they met, both Crusoe and Friday produced both bamboo poles and berries for their individual use. Because each individual has his own characteristic skills, they vary somewhat in their productivity at these two tasks. Of course, each individual's marginal product in each task depends in part on the available quantities of other factors of production. To avoid unnecessary complications, let us assume that each man expends quantities of other factors proportional to the hours he allots to each task, so that we can examine his productivity simply in terms of output per hour of labor. We also assume that for each man, there is no significant difference in the disutility of one hour's labor between the two tasks.