A causal chain explains an effect in steps, from the event through the mechanism to the result. A limiting condition names what must hold for the chain to work.
This lesson belongs to evidence-based economic answers. If you want practice at single cause and effect links, read writing cause, mechanism and effect explanations first.
What does a good chain look like?
Write it as arrows between short statements, then turn it into sentences. Every arrow needs a reason, so you can say “because” at each one.
Finish with a clause beginning “provided that”. That clause is the limiting condition.
Worked example: a storm in a fictional fishing village
A fictional village market for fish has Qd = 800 − 100P and Qs = 100P, with P in RM per kg and quantity in kg. Setting them equal gives 800 − 100P = 100P, so P = 4 and Q = 400.
A storm keeps boats in harbour and cuts supply by 200 kg at every price, so Qs = 100P − 200.
The chain.
- The storm reduces the number of fishing trips, so supply shifts left.
- At RM4, buyers still want 400 kg but only 200 kg is offered, so there is a shortage.
- The shortage raises the price until quantity demanded matches quantity supplied.
- The higher price moves buyers up the unchanged demand curve, so quantity falls.
The numbers. Set 800 − 100P = 100P − 200, so 200P = 1 000 and P = 5.
Quantity is 800 − 500 = 300. Check in supply: 100 × 5 − 200 = 300. Price rose from RM4 to RM5 and quantity fell from 400 to 300.
The limiting condition. The chain holds provided that demand does not also change and that fish cannot be brought in from elsewhere at no extra cost. If cheap fish arrived from another market, supply would partly recover and the price rise would be smaller.
Writing it as sentences
“The storm reduces fishing trips, so supply shifts left. At the old price there is a shortage, which pushes the price up. The higher price reduces the quantity demanded along the same demand curve. This holds provided that demand is unchanged and that no extra supply arrives from elsewhere.”
Four sentences carry four links and one condition. Each sentence can be checked against the numbers.
The mistake that costs marks
The common slip is to jump: “The storm makes the price go up.” The claim may be right, but the link from storm to price is missing.
| Step | Wrong | Right |
|---|---|---|
| Event to curve | Skipped | Fewer trips, supply shifts left |
| Shortage | Not mentioned | 400 wanted against 200 offered at RM4 |
| Price to quantity | Skipped | Movement up the demand curve to 300 |
| Condition | None | Demand unchanged, no cheap imports |
A second slip is a condition that is too vague, such as “if things stay the same”. Name the specific thing that must stay the same.
Check yourself
A fictional roti stall market has Qd = 600 − 50P and Qs = 50P. The price of cooking gas rises, cutting supply by 100 at every price. Find the old and new equilibrium, write a four-link chain and state one limiting condition.
Answer
Old: 600 − 50P = 50P, so P = 6 and Q = 300.
New supply is Qs = 50P − 100. Set 600 − 50P = 50P − 100, so 100P = 700 and P = 7. Quantity is 600 − 350 = 250. Check in supply: 350 − 100 = 250.
Chain: higher gas cost raises the cost of making roti, so supply shifts left. At RM6, buyers want 300 but sellers offer 200, a shortage. The shortage pushes the price to RM7. The higher price reduces quantity demanded to 250.
Condition: provided that demand does not change, for example because incomes or tastes stay the same.
What to study next
Go on to comparing two explanations without inventing motives or declaring a policy winner, or check your understanding with the cluster practice set. The supply-demand diagram reasoning explorer shows the shifts used here.
If you want a teacher to listen to your chain and question each link, see online one-to-one Economics tuition.