The basic idea in neoclassical distribution theory is that incomes are earned in the production of goods and services and that the value of the productive factor reflects its contribution to the total product. Though this fundamental truth was already recognized at the beginning of the 19th century (by the French economist J.B. Say, for instance), its development was impeded by the difficulty of separating the contributions of the various inputs. To a degree they are all necessary for the final result: without labour there will be no product at all, and without capital total output will be minimal. This difficulty was solved by J.B. Clark (c. 1900) with his theory of marginal products. The marginal product of an input, say labour, is defined as the extra output that results from adding one unit of the input to the existing combination of productive factors. Clark pointed out that in an optimum situation the wage rate would equal the marginal product of labour, while the rate of interest would equal the marginal product of capital. The mechanism tending to produce this optimum begins with the profit-maximizing businessman, who will hire more labour when the wage rate is less than the marginal product of additional workers and who will employ more capital when the rate of interest is lower than the marginal product of capital. In this view, the value of the final output is separated (imputed) by the marginal products, which can also be interpreted as the productive contributions of the various inputs. The prices of the factors of production are determined by supply and demand, while the demand for a factor is derived from the demand of the final good it helps to produce. The word derived has a special significance since in mathematics the term refers to the curvature of a function, and indeed the marginal product is the (partial) derivative of the production function.

One of the great advantages of the neoclassical, or marginalist, theory of distribution is that it treats wages, interest, and land rents in the same way, unlike the older theories that gave diverging explanations. (Profits, however, do not fit so smoothly into the neoclassical system.) A second advantage of the neoclassical theory is its integration with the theory of production. A third advantage lies in its elegance: the neoclassical theory of distributive shares lends itself to a relatively simple mathematical statement.

An illustration of the mathematics is as follows. Suppose that the production function (the relation between all hypothetical combinations of land, labour, and capital on the one hand and total output on the other) is given as Q="f" (L,K) in which Q stands for total output, L for the amount of labour employed, and K for the stock of capital goods. Land is subsumed under capital, to keep things as simple as possible. According to the marginal productivity theory, the wage rate is equal to the partial derivative of the production function, or ∂Q/∂L. The total wage bill is (∂Q/∂L) · L. The distributive share of wages equals (L/Q) · (∂Q/∂L). In the same way the share of capital equals (K/Q) · (∂Q/∂K). Thus the distribution of the national income among labour and capital is fully determined by three sets of data: the amount of capital, the amount of labour, and the production function. On closer inspection the magnitude (L/Q) · (∂Q/∂L), which can also be written (∂Q/Q)/(∂L/L), reflects the percentage increase in production resulting from the addition of 1 percent to the amount of labour employed. This magnitude is called the elasticity of production with respect to labour. In the same way the share of capital equals the elasticity of production with respect to capital. Distributive shares are, in this view, uniquely determined by technical data. If an additional 1 percent of labour adds 0.75 percent to total output, labour’s share will be 75 percent of the national income. This proposition is very challenging, if only because it looks upon income distribution as independent of trade union action, labour legislation, collective bargaining, and the social system in general. Obviously such a theory cannot explain all of the real economic world. Yet its logical structure is admirable. What remains to be seen is the degree to which it can be used as an instrument for understanding the real economic world.

Neoclassical theory assumes that the total product Q is exactly exhausted when the factors of production have received their marginal products; this is written symbolically as Q = (∂Q/∂L) · L + (∂Q/∂K) · K. This relationship is only true if the production function satisfies the condition that when L and K are multiplied by a given constant then Q will increase correspondingly. In economics this is known as constant returns to scale. If an increase in the scale of production were to increase overall productivity, there would be too little product to remunerate all factors according to their marginal productivities; likewise, under diminishing returns to scale, the product would be more than enough to remunerate all factors according to their marginal productivities.

Research has indicated that for countries as a whole the assumption of constant returns to scale is not unrealistic. For particular industries, however, it does not hold; in some cases increasing returns can be expected, and in others decreasing returns. This situation means that the neoclassical theory furnishes at best only a rough explanation of reality.

One difficulty in assessing the realism of the neoclassical theory lies in the definition and measurement of labour, capital, and land, more specifically in the problem of assessing differences in quality. In macroeconomic reasoning one usually deals with the labour force as a whole, irrespective of the skills of the workers, and to do so leaves enormous statistical discrepancies. The ideal solution is to take every kind and quality of labour as a separate productive factor, and likewise with capital. When the historical development of production is analyzed it must be concluded that by far the greater part of the growth in output is attributable not to the growth of labour and capital as such but to improvements in their quality. The stock of capital goods is now often seen as consisting, like wine, of vintages, each with its own productivity. The fact that a good deal of production growth stems from improvements in the quality of the productive inputs leads to considerable flexibility in the distribution of the national income. It also helps to explain the existence of profits.