Explain the liquidity preference theory of interest. What are its criticisms?

The liquidity preference theory of interest, propounded by J.M. Keynes, holds that interest is the reward for parting with liquidity for a period, and that the rate of interest is determined by the interaction of the demand for money (liquidity preference) and the supply of money.

The theory explained:

1. Basic idea: interest is not a reward for saving as such but a reward for surrendering liquidity, so the rate depends on how strongly people wish to hold money.
2. Demand for money (liquidity preference): people demand money for three motives — the transaction, precautionary and speculative motives.
3. Transaction motive: holding cash for everyday expenses, depending on income (LT = f(Y)).
4. Precautionary motive: holding cash for unforeseen needs, also depending on income (LP = f(Y)); together these form the active cash balance.
5. Speculative motive: holding money to exploit expected changes in interest and bond prices; since bond prices and interest move inversely, it is a decreasing function of interest (LS = f(r)).
6. Total money demand: the sum of these three, giving a downward-sloping liquidity preference curve.
7. Supply of money: the money supply is fixed exogenously by the central bank at a point in time, so its curve is perfectly inelastic (vertical).
8. Determination of interest: the equilibrium rate is set where the downward-sloping money-demand curve meets the vertical money-supply curve.
9. Effect of a change in money supply: a rise in money supply shifts the supply curve right and lowers interest, while a fall raises it — an inverse relation between money supply and interest.
10. Effect of a change in money demand and the liquidity trap: with supply fixed, a rise in liquidity preference raises interest and a fall lowers it (a direct relation); but at a very low rate the curve becomes perfectly elastic (the liquidity trap), where extra money supply cannot reduce interest further.

The criticisms:

11. Short-run only: the theory applies mainly to the short run and does not adequately explain the long-run rate of interest.
12. One-sided: it over-emphasises the demand for money and treats the money supply as fixed, ignoring the forces that change supply.
13. Neglects real factors: it treats interest as a purely monetary phenomenon, ignoring real factors such as productivity of capital and thrift.
14. Questionable assumption about liquidity preference: it assumes liquidity preference depends only on income, and the inverse money-supply/interest relation holds only if liquidity preference stays constant.
15. Indeterminacy point: critics argue that, like the loanable funds theory, it cannot fully determine the rate without knowing income, which itself depends on the interest rate.

Thus, the liquidity preference theory explains interest as the price of liquidity, set where money demand (transaction, precautionary and speculative) meets a fixed money supply, but it is criticised for being short-run, one-sided, purely monetary and indeterminate.