Wealth is a stock. Income is a flow.
One distinction quietly organises half of economics: amounts at a moment versus rates over a period. Debt and deficit, capital and investment, savings and saving. Get the pair straight and a third of the news starts reading differently.
Amounts and rates
First principlesHalf of all economic confusion is a stock mistaken for a flow. A stock is an amount at a point in time: the money in your account on 31 March, the gold in a vault, a country's debt, its factories and roads. A flow is a rate over a period: a salary per month, GDP per quarter, a deficit per year. The two carry different units, and swapping them produces sentences that sound sensible and are not.
Picture a bathtub. The water in it is a stock; the tap and the drain are flows. Saving raises the level of wealth; a deficit raises the level of debt. Every stock in economics is the accumulated residue of flows past, and every flow is a stock in the act of changing.
The confusable twins
Four pairs, one plain line each| The pair | Stock / flow | One plain line |
|---|---|---|
| Wealth / Incomehaving vs earning | stock / flow | Wealth is what you have; income is what you earn. A wealthy person can have a bad year. |
| Debt / Deficitthe pile vs the year's addition | stock / flow | The deficit is one year's overspend; the debt is every year's overspend added up. |
| Capital / Investmentthe machines vs the building of them | stock / flow | Capital is the stock of machines, buildings and roads; investment is the year's addition to it. |
| Savings / Savingthe balance vs the habit | stock / flow | Savings are the pile accumulated over years; saving is what one year added to it. |
Why the mix-up matters
Three confusions, clearedDebt versus deficit is the classic. A government that has halved its deficit has still increased its debt: more slowly, but increased it, because every deficit adds to the pile. Only a surplus, a negative flow, brings the stock down. The headline sounds like repayment. The arithmetic is a smaller addition.
GDP versus wealth is the national version. GDP tells you how much the country produced in a year, not how rich the country is. A country can run a high GDP and own little, or sit on old wealth and produce little. A flood can destroy wealth and raise GDP in the same year, because rebuilding is a flow while the loss was a stock. Rich is a stock word; productive is a flow word. They often travel together, and they are not the same.
Investment versus capital trips the markets pages. High investment in a year means the capital stock is growing quickly, not that it is large. A young economy can out-invest an old one for decades and still hold less capital per person. The flow is the speed. The stock is the distance covered.
How to misread this concept
- i.
“The deficit is the debt.”
No. The deficit is one year's shortfall; the debt is the total of every shortfall still unpaid. A government can shrink the deficit every single year and watch the debt grow every single year. Both things happen at once, regularly.
- ii.
“GDP tells you how rich a country is.”
It tells you how much the country produced in a year. Rich is about accumulated assets: factories, roads, homes, savings. GDP is the tap, not the tub, and a country is a balance sheet as well as a payslip.
- iii.
“High investment means high wealth.”
No: high investment is a fast rate of addition, not a large total. The capital stock is decades of investment minus decades of wear. One year's flow is a small entry against a large inheritance, whoever you are.