Ceteris paribus
Definition
Ceteris paribus: a Latin phrase meaning other things equal, used to isolate the effect of one variable while every other influence is assumed to stay unchanged.
Movement along a curve: a response to a change in the variable on the axis (price), with all other influences held constant.
Shift of a curve: a response to a change in a non-price determinant, which occurs precisely because ceteris paribus no longer holds.
- It is an analytical device, not a claim that nothing else in the real world ever changes.
- By freezing all other influences, economists can trace the effect of a single cause on its own.
- Almost every economic model, diagram and prediction rests on this assumption.
Key Idea
- Ceteris paribus lets economists study one cause at a time.
- When the assumption is dropped, the other influences are allowed to move as well.
Why economists need it
- In a real economy many variables change at once, which hides individual cause and effect.
- Because a whole economy cannot be placed in a laboratory, economists cannot run fully controlled experiments.
- Holding other influences constant is the next best thing, letting one relationship be examined cleanly.
Example
- Suppose the price of coffee falls and we want the effect on quantity demanded.
- We assume incomes, tastes and the prices of tea and sugar all stay the same.
- Then any rise in quantity demanded can be attributed to the lower price alone.
- Without this assumption, a change in income could mask the effect of price.
In demand and supply
- When we draw a demand curve, we vary price and hold the other determinants of demand constant.
- A price change therefore causes a movement along the curve, ceteris paribus.
- If a non-price determinant such as income changes instead, the whole curve shifts, because ceteris paribus no longer holds.
Example
- Compare two events in the market for bicycles.
- If the price rises, quantity demanded falls: a movement up along the demand curve, ceteris paribus.
- If instead incomes rise while price is unchanged, the whole demand curve shifts to the right.
- The second case breaks ceteris paribus, because a non-price determinant has changed.
How realistic is it?
- In reality other things rarely stay equal, so a prediction made under the assumption may not hold exactly.
- Even so, isolating one cause is essential for clear analysis and remains the standard method.
- Good practice is to state the assumption, then ask whether it is reasonable for the case in hand.
Exam technique
- Say ceteris paribus whenever you isolate one variable, to show other things are held constant.
- Use it correctly with diagrams: a price change moves along the curve, a determinant change shifts it.
- In evaluation, question whether the assumption is realistic for the case in front of you.
Common Mistake
- Do not treat ceteris paribus as a claim that nothing else ever changes, because it is only an analytical device.
- Do not forget it either, because ignoring it makes cause and effect impossible to identify.
Self review
- What does the term ceteris paribus mean?
- Why do economists need the ceteris paribus assumption?
- Under ceteris paribus, does a price change shift the demand curve or move along it?
- Give one example of ceteris paribus used in demand analysis.
- Is ceteris paribus a description of the real world?
