Under restrictive assumptions, the invisible hand of the market, dependent on thou-
sands of individual decisions, will guide producers toward maximum social welfare.
In an economy that consists of perfectly competitive firms that (1) produce only final
goods, (2) render no external costs or benefits to other production units, (3) pro-
duce under conditions of constant (marginal opportunity) costs, and (4) pay market-
clearing prices for production factors, a private firm that maximizes its rate of return
also will maximize the increase in national product.
However, the social and private profitability of any investment are frequently dif-
ferent. When Equation 11-1 is used for the private firm rather than national-planning
authorities,
B
becomes benefits and
C
costs incurred by the firm from the project,
and
r
becomes the prevailing rate of interest that the firm pays on the capital market.
Private investors want to maximize the commercial profitability of the investment. By
contrast, the national planner is likely to consider not only the internal rate of return
to a given investment project but also its effect on the profitability of other production
units and on consumers. The following discussion examines the divergence between
private and social marginal productivity