. © 1999-2003
Douglas A.Ruby

Aggregate Demand

The Accelerator Model

Wealth Creation

Capital Accumulation

Macroeconomic Theory

Microeconomic Theory
Investment -- The Profit-Maximizing Approach

Investment is the act of acquiring income-producing assets, known as physical capital, either as additions to existing assets or to replace assets that have worn out (depreciated). These assets may be in the form of fixed nonresidential plant and equipment, housing (fixed residential) or business inventories. Decisions about the appropriate quantity of assets or capital are often based on profit-maximizing behavior of a private business firm producing goods and services or providing housing services.

The investment relationship may be written as follows:

Jt = K*t - Kt-1 + dKt-1


  • Jt = Gross Investment (measured in units of new or replacement capital)
  • K*t = Desired (profit-maximizing level) Capital Stock
  • Kt-1 = Existing Capital Stock
  • d = rate of depreciation.

The difference in the first two terms (K*t - Kt-1 ) represents net or new investment and the last term (dKt-1) represents replacement investment. Gross investment is just the sum of the two expressions.

By multiplying both sides of the equation by Pk (the price of capital) and factoring out 'Kt-1', we can write an expression for Investment expenditure(It):

It = Pk[K*t - (1-d)Kt-1]

where It = PkJt.

The key to understanding investment decisions is the determination of K*, the desired capital stock. This desired level is based, as stated above, on profit-maximizing behavior of the firm. There are several methods by which we can approach the profit-maximizing level of capital stock. First, we can analyze the net present value of different amounts of capital to determine which quantity gives the greatest discounted stream of profits. For example, given the following:

  • Pk = $1000/unit
  • d = 10%/year which implies that a unit of capital has a life of 10 years.
    We assume that the capital has no salvage value.
  • r = 5%/annually -- This is the real market rate of interest.
  • Px = $5 -- the market price of a unit of output.

With this data we will use the following annuity factor computation (see the "Yields" web page for details):

PVn = (Revenue)[1 - (1+r)-n] / r
       = (Revenue)[1 - (1.05)-10] / 0.05 = (Revenue)7.722

K Output
Revenue PV
less Costs
Net Present Value
1 50 $250 $1930.50 $1000 $930.50
2 90 $450 $3474.90 $2000 $1474.90
3 120 $600 $4632.00 $3000 $1632.00
4 140 $700 $5405.40 $4000 $1405.40
5 150 $750 $5791.50 $5000 $791.50

In the above example (assuming diminishing marginal productivity) we find that three units of capital would generate the greatest net present value and thus the greatest profits.

A second approach is the marginal approach where we compare the contribution to revenue by using one more unit of capital with the costs of acquiring that unit of capital. The contribution to revenue is known as the Marginal Revenue Product of capital and is calculated by multiplying the marginal product with the market price of the output produced:


The contribution to costs is known as the Rental Cost of Capital which includes borrowing costs (or opportunity costs of using internal funds for investment expenditure) and depreciation costs:

RCC =PK(r) + PK(d) = PK(r + d)

We will use similar data and a table for an example:

K Output MPk MRPk RCC (r = 5%, d=10%)
1 50 units 50 $250 $150
2 90 40 $200 $150
3 120 30 $150 $150
4 140 20 $100 $150
5 150 10 $50 $150

If the MRP exceeds RCC then profits will increase by acquiring additional units of capital (contribution to revenue exceed contribution to costs). If the opposite is true then the additional costs associated with one more unit of capital exceed the revenue generated and profits will decline.

Similar to the Net Present Value approach, we find that with three units of capital the contribution to revenue of the third unit is just equal to the costs of acquiring and using that third unit--profits will be a maximum at this level of input as shown in the diagram below-left.

Figure 1 -- The Optimal Capital Stock 'K*"

A change in interest rates would increase the rental cost of capital 'RCC' such that this third unit is no longer profitable. The firm would react to this cost increase (an increase in the price of capital would have similar effect), by using less capital in production. This is shown in the above diagram-right.

On your own -- Click Here: The Optimal Capital Stock

A third approach is using a calculation similar to the present value of a perpetuity (again, see the "Yields" web page). We begin by using the marginal conditions above:

Marginal Revenue Product = Rental Cost of Capital

MPKPx = PK(r + d)

And we rearrange the terms:

[MRPK - PK(d)] / PK = 'y'

This term y represents the yield on the last unit of capital employed in the production process:

K Output MRPK - PK(d) PK Yield
1 50 units $250-$100 $1000 15%
2 90 $200-$100 $1000 10%
3 120 $150-$100 $1000 5%
4 140 $100-$100 $1000 0%
5 150 $50-$100 $1000 NA

In this case, we find that the yield on the first two units of capital is greater than the market interest rate and thus may be acquired to earn profits over-and-above the borrowing costs. It is the third unit of capital where the yield is just equal to the market rate of interest -- no additional profits may be earned by hiring additional capital.

In all three approaches we find that the desired capital stock 'K*' is equal to three units based on the productivity of capital, the rate of depreciation, the price of capital and the output being produced and, of greatest importance, market interest rates.

We can use a Cobb-Douglas form of the production function to solve for K* and combine this with the expression for investment defined above:

Xoutput = ALaKb and MPk = bX/K

given MRPk = RCC,

b(X/K)Px = Pk(r+d)


K* = [b(Px)X] / [Pk(r+d)]

Inserting this expression for K* into the investment expenditure equation gives us:

It = Pk{ [b(Px)X] [Pk(r+d)] - (1-d)Kt-1}


It = [b(Px)X] / (r+d) - (1-d)PkKt-1

where we find that Investment is inversely related to market interest rates and the price of a unit of capital, positively related to output prices and the productivity of capital (as measured by the parameter 'b'), and undetermined with respect to the rate of depreciation.

A different approach to understanding investment decisions is in using an expression known as Tobin's-q. This q-value represents a ratio between the market value of existing capital and its replacement cost:

q = Market Value / Replacement Cost
= {(Revenue)[1 - (1+r)-n] / r} / [PK K]

If this value is greater than one, then the value of capital is greater than replacement costs and it would make sense to add or invest in more capital. It the q-value is less than one then the market value is less than replacement costs and business firms will allow existing capital to depreciate resulting in negative levels of investment. Given the inverse relationship between asset prices and market interest rates we can note that when these interest rates rise the market value of installed capital falls relative to replacement costs -- disinvestment will occur.