The third model brings a crucially important—but hitherto generally neglected—element into the picture of the economic system; namely, financial markets. For simplicity, the model has only one financial market; there is only one class of financial instruments (referred to as “securities”) and only one yield (a single interest rate). The standard security may be thought of as a bond promising to pay annually a fixed number of dollars. The interest rate is the value of the coupon expressed as a percentage of the market price of the bond. Consequently, if excess demand for bonds brings their price up, the interest rate falls; if excess supply sends the bond price down, the interest rate rises.

The working of the financial market is depicted in the model as follows. Investment by the business sector is assumed to be financed through the issue of securities. The higher the interest rate that firms must pay on their securities, the smaller will be the investment program that they see as promising to be profitable. Thus, investment will be discouraged by a rise and encouraged by a fall in the interest rate. Households, in deciding how to divide their income between consumption and saving, will consider the amount of future consumption that can be gained by abstaining from consumption now (i.e., by saving). The higher the rate of interest, the larger the amount that can be spent on future consumption per dollar not spent in the present. Thus, saving is encouraged by a rise and discouraged by a fall in the interest rate. Coins, notes, and some checking deposits are assets on which interest is not paid. An individual who holds them has the alternative of converting some part of his money holdings into interest-bearing form. Thus, the amount of money demanded will tend to diminish when the interest rate rises and to increase when it falls. The banking system creates money by buying assets from the public, paying for the assets through the issuance of additional monetary liabilities (e.g., checking deposits). Banks must decide whether turning part of their cash reserves to an income-earning use is worth the risks of decreased “liquidity” entailed by lower bank reserves. Hence there is a tendency for the money supply to increase when the interest rate rises and to decrease when it falls.

In this model, then, the interest rate acts as a price in controlling the behaviour of the individual agents whose activities are to be coordinated. The interest rate itself is determined by the demand for and supply of money and securities. An increase in planned investment will be associated with the issuance of a large volume of securities. It will tend, therefore, to create an excess supply of securities, to lower securities prices, and to raise the rate of interest. Similarly, an increase in planned saving will tend to create an excess demand for securities, to raise their prices, and to lower the rate of interest. An increased demand for money will, in part, reduce the demand for and increase the supply of securities; it tends to create an excess supply of securities and to raise the interest rate. An increase in the supply of money will tend to reduce the rate of interest.

These qualitative propositions are the framework of the new model, integrating the two previous models as follows: (1) I = I(r); (2) C = C(Y,r); (3) S="Y" - C; (4) S = I; (5) M^d = M^d(Y,r); (6) M^s = M^s(r); and (7) M^d = M^s. Here, Equations 1 through 4 restate the income model with the modification that investment is no longer simply “autonomous” but depends on the current level of the interest rate (r). Equations 5 through 7 restate the money model with the modification that the demand for money and the supply of money also depend on the interest rate. Two conditions now have to be simultaneously fulfilled for the system to be in equilibrium: desired saving must equal desired investment (Equation 4), and the amount of money that individuals and firms desire to hold must equal the amount that the banking sector desires to supply (Equation 7).

Only a partial account of the ways in which this model works can be given here. The following illustrative examples begin with the system in equilibrium at full employment. The first illustration adopts the view of someone who has learned the income model and hence is thoroughly imbued with the idea that rising income results from an excess of planned investment over planned saving. Faced with the proposition, drawn from the money model, that an increase in the money supply will also cause income to rise, he will ask how such a change in the money supply can cause a discrepancy between saving and investment when there was none to begin with. The answer is that an increase in M^s will mean that there is an excess supply of money and a corresponding excess demand for commodities and securities, but the immediate impact of excess demand will be felt almost exclusively in the securities market. The excess demand for securities drives the rate of interest down—and this encourages investment and discourages saving. At that point, consequently, a “gap” opens up between desired saving and investment.

For the second illustration, consider instead someone who has learned the money model and who, consequently, knows that income falls when the amount of money demanded exceeds the supply. In Keynes’s work the “disturbance” given the most play is some unspecified event that makes business firms take a darker view of the returns to be expected from new investment. Hence, the amount of investment that they will want to undertake at the prevailing interest rate declines. The question is how such a change in planned investment can cause a discrepancy between money demand and money supply when there was none to begin with. The simplest answer is that a decline in planned investment will be associated with a reduction in the amount of securities floated on the market and thus with the emergence of an excess demand for securities. This drives securities prices up, which is to say that the interest rate falls. At a lower rate of interest, individuals will desire larger money balances than before; in addition, the banks will tend to reduce the money stock somewhat. At that point, consequently, a gap will open between the amount of money demanded and the amount supplied.

The analysis of the consequences of government fiscal action is somewhat more complicated. If the government tries to stimulate the economy through increased expenditures, the effects will be felt in at least two ways. First, the increased spending is an “injection” added to commodity demand and may be treated, therefore, from the Model A standpoint in the same way as an increase in private investment. Second, however, this spending may be financed through increased taxes, through government borrowing, through creation of new money, or through some combination of the three. The strongest effects are gained by following the third alternative, the creation of new money. The excess demand for goods and services created by the increase in spending will then be matched by an excess supply of money, which, as seen above, will drive down the interest rate and cause increased investment, etc. To the direct stimulus of the spending program, this method of paying for it adds the indirectly achieved stimulus of increased private investment. (Needless to say, the double effect on money income is not always desirable. The fact that this method of financing government spending has almost always been heavily resorted to in wartime accounts for the historical association of large inflations with wars.) The method of the second alternative, government borrowing, consists of financing the increase in spending through the issue of government bonds. This creates an excess supply of securities, driving up the interest rate. At the higher interest rate, money demand is lessened and money supply somewhat increased, but the consequent excess supply of money will be of smaller magnitude than that entailed by creating new money. The higher interest rate will also discourage private investment. Thus, the indirect effects of government borrowing are seen to involve a decrease in private investment partially offsetting the initial increase in government spending. The size of this offset has become one of the major issues between “monetarist” and “income–expenditure” economists. The monetarists argue that the offset is so nearly complete that fiscal action will be largely ineffectual unless it is accompanied by an increase in the money supply, but an increase in the money supply will have almost as powerful effects without any simultaneous fiscal action. The other side concedes that fiscal action will be more powerful when financed through changes in the money supply but maintains that countercyclical variations in government spending financed through borrowing must still be regarded as an important stabilization method.

Around the turn of the century, the Swedish economist Knut Wicksell contributed greatly to the understanding of the function of the rate of interest in the mechanism determining income and price-level movements. Assuming an economy initially in full-employment equilibrium, Wicksell analyzed the various ways in which the system might depart from that position because of discrepancies between the prevailing market rate of interest and what he termed the “natural rate.” The latter rate, hypothetical rather than directly observable, may be thought of as the interest rate level that would have to prevail for the system to remain at full employment with stable prices. In illustrating the use made of this concept, one should distinguish between processes initiated by “real” disturbances (the first two examples below) and those initiated by “monetary” disturbances (the third example).

The first example is one in which business firms see increased opportunities for profitable investment. The system is already at full employment, and hence an increase in spending on investment without a corresponding decrease in spending for consumption would spell inflation. What kind of adjustment will maintain stable prices? A rise in the interest rate will (1) moderate the increase in investment spending and (2) cause households to divert some of their income from consumption into increased saving. The hypothetical level of the interest rate that will exactly match the net increase in investment with the decrease in consumption (increase in saving) is the new value of Wicksell’s “natural rate.” But the adjustment of the market rate may, for several reasons, come to a halt after going only part of the way to the new natural rate level. At some level of the market rate below natural rate, where planned investment still exceeds the savings that households provide for its financing, the banks may step in and finance the difference through expansion of the money supply. Thus, inflation results. In Wicksell’s theory there is inflationary pressure on the system associated with a market rate below the natural level and, in the version of it given here, with an increase in the money supply.

The second example involves a change in public behaviour in that households desire to save more and consume less, out of any given level of income. The decreased demand for consumption goods threatens to cause deflation (or unemployment). To prevent this it is necessary to switch resources over to investment goods production, which requires a lowering of the interest rate. Thus, an increase in saving means that the natural rate of interest declines. The adjustment of the market rate of interest may again be incomplete if falling rates induce banks, say, to reduce their new lending below scheduled loan repayments, thus reducing the money supply. Part of the saving done by households then goes, directly or indirectly, into reducing the private sector’s indebtedness to banks rather than into financing investment. Thus, deflationary pressure on the system is, in Wicksell’s theory, associated with a market rate of interest above the natural rate and, in this example, with a decreased supply of money.

The third example is one in which banks desire to expand their loans and, thereby, their monetary liabilities—creating a “monetary” disturbance. Since “real” incentives to save and to invest have not changed, the natural rate of interest has not changed. The increased supply of bank credit will, however, drive the market rate down. It goes below the natural rate, the money supply is increased in the process, and inflation is the result.