Much of economic analysis has concentrated on single-product processes of production. This preoccupation with single production seems to be based on either or both of the following premises. First, single production is empirically far more important than joint production. Since from this point of view joint production is a curiosum, its study implies a search for generality for its own sake. Second, the results derived within a single-product framework essentially carry over to the joint products case. Hence the analysis of joint production systems is unrewarding since it does not substantially improve our understanding of economic phenomena.

While empirical investigations have undermined the first premise, theoretical analysis has raised doubts about the correctness of the second. The view that these cases of joint production, far from being “some peculiar cases,” as John Stuart Mill tended to assume, “form the general rule, to which it is difficult to point out any clear or important exception” had been already advocated a century ago by William Stanley Jevons ([1871] 1965, p. 198); see also Steedman (1984a). However, from the point of view of economic theory, whether or not joint production is predominant in the “real world” does not appear to be that important. This follows from the fact that the presence of even one multiple-product process of production in an otherwise single-product system can qualitatively alter some of the characteristic features of the system as a whole (cf. exercises 8.10 and 8.11).