What it means
Economists split any reaction to a price change into two parts. The substitution effect covers switching towards the item that has become relatively cheaper, while the income effect covers the fact that your money now stretches further or less far than before.
The two usually work together on a price fall, which is why demand normally rises. The size of the income effect depends on how much of your budget the item takes up.
A 10% fall in the price of chewing gum changes nobody's spending power in any meaningful way, but a 10% fall in rent or fuel frees a real amount of money that gets spent elsewhere. This is why housing and energy prices have such wide effects across the rest of an economy.
Direction depends on the type of good. For normal goods, extra real income means buying more, so a price fall reinforces the substitution effect; for inferior goods, extra real income means buying less of the cheap substitute and trading up instead.
In rare cases the income effect on an inferior staple is strong enough to reverse the usual relationship altogether, which is the Giffen good case. For businesses the practical use is in forecasting demand after shocks that are not about your own pricing.
When mortgage rates jump or energy bills spike, households lose real income and cut discretionary categories first, regardless of whether your prices changed at all. Reading that correctly stops a firm from blaming its own marketing for a fall driven entirely by customers' budgets.
The same logic runs in reverse during pay rises, tax cuts or falling inflation. Real income improves, discretionary categories recover faster than staples, and firms selling premium versions of everyday items usually see the swing first.
Tracking real disposable income is therefore a better leading indicator for many businesses than tracking nominal wages.
In practice
Real-world examples.
Example
Petrol prices fall by 20% over a quarter and a family that drives a fixed commute saves roughly $60 a month. They do not drive more, but they eat out twice more each month, so the local restaurant sees a demand increase caused entirely by the income effect from a different market.
Example
A national insurance rise reduces take-home pay for most employees. A mid-market clothing retailer sees volumes drop even though it has not changed a single price, because customers have less real income to allocate to discretionary categories.
Example
A supermarket cuts the price of its own-brand pasta. Some low-income shoppers buy less of it rather than more, because the saving lets them afford the branded version they actually prefer, a textbook income effect on an inferior good.
Formula
Calculation
Income effect in units = Money freed by the price change / New price, holding the original consumption basket constant.
A regular customer buys 20 coffees a month at $5 each, spending 20 x $5 = $100. The cafe cuts the price to $4. Buying the same 20 coffees now costs 20 x $4 = $80, so $100 - $80 = $20 of purchasing power has been freed.
If that freed $20 is spent on more coffee, it buys $20 / $4 = 5 additional cups, which is the income effect measured in units. Any further increase beyond those 5 cups, caused by coffee now being cheap relative to tea or soft drinks, is the substitution effect rather than the income effect.Case study
Seen in the real world.
Halverton Leisure is a fictional operator of local gyms, used here as an illustrative example. Over two quarters it lost 9% of its members despite holding prices flat and running its usual promotions, and the marketing team concluded the campaigns had stopped working.
A closer look at the timing told a different story. The losses clustered in postcodes where a large share of households had come off fixed-rate mortgages onto much higher payments, losing several hundred dollars a month in real income. Membership was cancelled as part of a general squeeze, not because a competitor had won those customers.
Halverton responded with a lower-priced off-peak tier rather than more advertising, and in this illustrative scenario recovered about half the lost members within six months. The lesson was that the demand shift came from customers' budgets, not from the product.
Watch out
Common mistakes.
- Treating every demand change after a price cut as the income effect. Most of the response usually comes from substitution towards the now cheaper option.
- Assuming the income effect always increases purchases. For inferior goods, more real income means people trade up and buy less of the cheaper item.
- Ignoring price changes in other markets. Rent, energy and interest costs move households' real income and change demand for products whose prices never moved.
Questions
People also ask.
How is the income effect different from the substitution effect?
The income effect is about how much your money buys overall, while the substitution effect is about switching between items whose relative prices have changed.
Does the income effect matter for small purchases?
Barely, because items that take up a tiny share of a budget do not move purchasing power in any noticeable way.
Can the income effect ever outweigh the substitution effect?
Yes, for strongly inferior staples this produces the Giffen good case where demand rises as price rises, though genuine examples are very rare.
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