Economics 9708/12 — February/March 2020

Background Concept

A production possibility curve (PPC, also called a production possibility frontier, PPF) is a model that shows the maximum possible output combinations of two goods an economy can produce when all resources are fully and efficiently employed, given the current state of technology. The shape of the PPC reveals the nature of opportunity cost in the economy:

• A straight, downward-sloping PPC indicates constant opportunity cost: each additional unit of one good always requires the same sacrifice of the other good. This occurs when resources are perfectly substitutable between the production of the two goods (for example, all workers are equally skilled at catching fish and picking coconuts).
• A bowed-out (concave) PPC indicates increasing opportunity cost: each additional unit of one good requires a larger sacrifice of the other good. This is the more common real-world case, as resources are not perfectly adaptable to producing both goods (for example, workers specialised in fishing are less productive if moved to coconut picking, so more and more fishing output is given up to produce each extra unit of coconuts).

Understanding the Question

The question provides a diagram of a desert island economy's PPC, with coconuts on the vertical axis and fish on the horizontal axis. The curve is a straight downward-sloping line. The question asks what explains this specific shape. The four options test whether you can link PPC shape to the underlying economic concept of opportunity cost, and distinguish PPC analysis from other microeconomic models like indifference curves.

Approach

To answer this, first recall the link between PPC shape and opportunity cost. Then evaluate each option against this knowledge, eliminating any that refer to unrelated concepts or misstate the relationship. The correct option must directly explain why the PPC is straight.

Step-by-Step Reasoning

1. First, identify the key feature of the given PPC: it is a straight line, not bowed out.
2. Evaluate Option A: "Coconuts and fish are perfect complements." Perfect complements describe a consumption relationship where two goods are always consumed together in fixed proportions, which is a feature of indifference curves, not PPCs. This is irrelevant to the shape of the production possibility curve, so A is incorrect.
3. Evaluate Option B: "Coconuts and fish are perfect substitutes." Perfect substitutes also describe a consumer preference (where a consumer is willing to swap one good for the other at a constant rate), again a feature of indifference curves, not PPCs. This does not explain the PPC's shape, so B is incorrect.
4. Evaluate Option C: "The inhabitants consume more fish than coconuts." Consumption patterns are not shown on a PPC, which only illustrates production trade-offs. The shape of the PPC is determined by production technology and resource adaptability, not how much the population chooses to consume. So C is incorrect.
5. Evaluate Option D: "The opportunity cost of increasing fish production is constant." A straight-line PPC has a constant gradient, meaning the trade-off between the two goods is the same at every production point. This directly matches the definition of constant opportunity cost, so D is the correct explanation.

Key Takeaways

• The shape of a PPC is determined by the nature of opportunity cost in the economy: straight = constant opportunity cost, bowed out = increasing opportunity cost.
• PPC analysis is about production trade-offs, not consumer preferences or consumption patterns. Do not confuse PPCs with indifference curves, which describe consumer choice.
• When answering questions about PPC shape, always link the shape directly to the opportunity cost concept.

Common Mistakes

• Confusing PPCs with indifference curves: options A and B are common distractors that test this confusion. Remember that PPCs relate to production, while indifference curves relate to consumption preferences.
• Assuming consumption levels affect PPC shape: the PPC shows what can be produced, not what is consumed. Consumption choices are shown by points inside or on the PPC, not the curve's shape.
• Forgetting that a straight PPC is a special case: most real-world PPCs are bowed out, but a straight line is used in models to represent constant opportunity cost, usually when resources are perfectly adaptable between the two goods.

Things to Be Careful About

• Always check what the model in the question is: if it is a PPC, the answer must relate to production and opportunity cost, not consumer behaviour.
• The gradient of the PPC is the opportunity cost: a constant gradient (straight line) means constant opportunity cost, a rising gradient (bowed out) means increasing opportunity cost.
• For 1-mark multiple choice questions, you do not need to write an extended explanation, just identify the correct link between the curve's shape and the economic concept.