The investment function is a summary of the variables that influence the levels of aggregate investments. It can be formalized as follows: I=f(r,ΔY,q) - + + where r is the real interest rate, Y the GDP and q is Tobin's q. The signs under the variables simply tell us if the variable influences investment in a positive or negative way (for instance, if real interest rates were to rise, investments would correspondingly fall). The reason for

investment Investment is traditionally defined as the "commitment of resources into something expected to gain value over time". If an investment involves money, then it can be defined as a "commitment of money to receive more money later". From a broade ...

being inversely related to the

Interest rate An interest rate is the amount of interest due per period, as a proportion of the amount lent, deposited, or borrowed (called the principal sum). The total interest on an amount lent or borrowed depends on the principal sum, the interest rate, ...

is simply because the interest rate is a measure of the

opportunity cost In microeconomic theory, the opportunity cost of a choice is the value of the best alternative forgone where, given limited resources, a choice needs to be made between several mutually exclusive alternatives. Assuming the best choice is made, ...

of those resources. If the resources instead of financing the investment could be invested in financial assets, there is an opportunity cost of (1+r), where r is the interest rate. This implies higher investment spending with a lower interest rate. When

GDP Gross domestic product (GDP) is a monetary measure of the total market value of all the final goods and services produced and rendered in a specific time period by a country or countries. GDP is often used to measure the economic performance o ...

increases, the output and the capacity utilization increases. This results in an increase of capital investment. At last, a higher Tobins q is represented when the market puts a high value of the installed capital and buys stocks in the firm for a higher price. The firm can then raise more resources per share issued and increase their investments.

References

*Burda, Wyplosz (2005): Macroeconomics: A European Text, Fourth Edition, Oxford University Press {{Authority control Investment Macroeconomic indicators