The quantity theory of money

The second model in this standard

• The quantity theory of money explains inflation in terms of the amount of money in the economy and how fast it is spent.
• It is written as:

$$M \times V = P \times Q$$

• The left side is total spending in the economy. The right side is the total value of everything bought. They must be equal, because every dollar spent is a dollar received.

The four variables — learn these by name

$M$ — the money supply

• The total amount of money in the economy: notes, coins and deposits.
• Controlled largely by the central bank and the banking system.

$V$ — the velocity of circulation

• The average number of times each dollar is spent in a year.
• It measures how fast money moves, not how much there is.
• $V$ rises when people spend confidently and quickly; it falls when people are uncertain and hold on to their money.

$P$ — the price level

• The average price of goods and services — the same $PL$ as on the AS/AD model.

$Q$ — real output

• The quantity of goods and services actually produced — the same as real GDP.

How to use it to explain inflation

• The equation must always balance. So if one side rises, the other must rise too.
• The standard assumption at Level 2: hold two variables constant and see what must happen to the rest.

If…	…and these are constant	Then
$M$ rises 5%	$V$ and $Q$	$P$ rises 5%
$V$ rises 3%	$M$ and $Q$	$P$ rises 3%
$M$ falls 2%	$V$ and $Q$	$P$ falls 2%
$M$ rises 6%, $Q$ rises 6%	$V$	$P$ unchanged

• The key insight: inflation happens when total spending ($MV$) grows faster than real output ($Q$). If the extra money buys extra goods, prices need not rise at all.

The role of real output $Q$

• If $M$ rises and $Q$ rises by the same proportion, there is more money and more to buy, so $P$ does not change.
• If $M$ rises while $Q$ is fixed — because the economy is at capacity — the whole increase must show up in $P$. This is the same idea as an AD shift when the economy is near full capacity.
• If $Q$ falls while $MV$ is unchanged, $P$ must rise. Less to buy, same spending.

Velocity and the business cycle

• $V$ is not fixed, and the exam has tested this directly.
• In a recession, households and firms are uncertain about jobs and demand. They delay purchases and hold money, so each dollar changes hands less often and $V$ falls.
• A falling $V$ reduces total spending $MV$, which puts downward pressure on $P$ — one reason inflation typically falls during a downturn.
• In a boom, confidence is high, money moves quickly, $V$ rises and inflationary pressure builds.