About 1870 a new school developed, sometimes called the Austrian school from the fact that many of its principal members taught in Vienna, but perhaps better called the Marginalist school. The movement itself was thoroughly international, and included such figures as William Stanley Jevons in England and Léon Walras in France. The so-called Austrian theory of capital is mainly based on the work of Eugen Böhm-Bawerk. His Positive Theory of Capital (1889) set off a controversy that has not yet subsided. In the Austrian view the economic process consisted of the embodiment of “original factors of production” in capital goods of greater or lesser length of life that then yielded value or utility as they were consumed. Between the original embodiment of the factor and the final fruition in consumption lay an interval of time known as the period of production. In an equilibrium population it can easily be shown that the total population (capital stock) equals the annual number of births or deaths (income) multiplied by the average length of life (period of production). The longer the period of production, therefore, the more capital goods there will be per unit of income. If the period of production is constant, income depends directly on the amount of capital previously accumulated. Here is the wages fund in a new form. Unfortunately, the usefulness of Böhm-Bawerk’s theory is much impaired by the fact that it is confined to equilibrium states. The great problems of capital theory are dynamic in character, and comparative statics throws only a dim light on them.

The Marginalist school culminated in the work of three men—P.H. Wickstead in England, Knut Wicksell in Sweden, and Irving Fisher in the United States. The last two especially gave the Austrian theory clear mathematical expression. Perhaps the greatest contribution of the Austrian theory was its recognition of the importance of the valuation problem in the relation of capital to interest. From the mere fact that physical capital produces an income stream, there is no explanation of the phenomenon of interest, for the question is why the value of a piece of physical capital should be less than the total of future values that are expected to accrue from it. The theory also makes a contribution to the problem of rational choice in situations involving waiting or maturing. The best example is that of slowly maturing goods such as wines or timber. There is a problem here of the best time to draw wine or to cut down a tree. According to the marginal theory this is at the time when the rate of net value growth of the item is just equal to the rate of interest, or the rate of return in alternative investments. Thus, if a tree or a wine is increasing in value at the rate of 7 percent per annum when the rate of interest is 6 percent it still pays to be patient and let it grow or mature. The longer it grows, however, the less the rate of value growth, and when the rate of value growth has fallen to the rate of interest, then is the time to reap the fruits of patience.

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John Maynard Keynes, detail of a watercolour by Gwen Raverat, c. 1908; in the National Portrait Gallery, London.

Courtesy of the National Portrait Gallery, London

The contributions of John Maynard (Lord) Keynes to capital theory are incidental rather than fundamental. Nevertheless, the “Keynesian revolution” had an impact on this area of economic thought as on most others. It overthrew the traditional assumption of most economists that savings were automatically invested. The great contribution of Keynes, then, is the recognition that the attempt to save does not automatically result in the accumulation of capital. A decision to restrict consumption is only a decision to accumulate capital if the volume of production is constant. If abstention from consumption itself results in a diminution of production, then accumulation (production minus consumption) is correspondingly reduced.

The theory of capital was not a matter of primary concern to economists in the late 20th century, though some revival of interest occurred in the late 1950s. Nevertheless, certain problems remain of perennial interest. They may be grouped as follows.

First are the problems involved in measuring aggregates of goods. Real capital includes everything from screwdrivers to continuous strip-rolling mills. A single measure of total real capital can be achieved only if each item can be expressed in a common denominator such as a given monetary unit (e.g., dollars, sterling, francs, pesos, etc.). The problem becomes particularly complicated in periods of rapid technical change when there is change not only in the relative values of products but in the nature of the list itself. Only approximate solutions can be found to this problem, and no completely satisfactory measure is ever possible.

A related problem that has aroused considerable interest among accountants is how to value capital assets that have no fixed price. In the conventional balance sheet the value of some items is based on their cost at an earlier period than that of others. When the general level of prices is changing this means that different items are valued in monetary units of different purchasing power. The problem is particularly acute in the valuation of inventory. Under the more conventional “FIFO” (First In, First Out) system, inventory is valued at the cost (purchase price) of the latest purchases. This leads to an inflation of inventory values, and therefore of accounting profits, in time of rising prices (and a corresponding deflation under falling prices), which may be an exaggeration of the long-run position of the firm. This may be partially avoided by a competing system of valuation known as LIFO (Last In, First Out), in which inventory is valued at the purchase price of the earliest purchases. This avoids the fluctuations caused by short-run price-level changes, but it fails to record changes in real long-run values. There seems to be no completely satisfactory solution to this problem, and it is wise to recognize the fact that any single figure of capital value that purports to represent a complex, many-dimensional reality will need careful interpretation.