1. Background to Demand: The Theory of Consumer Choice5
Background to Demand: The Theory of Consumer Choice
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2. I. The Standard Economic Model
SEM or classical theory of consumer behaviour assumes rational behaviour:
People face trade offs in their role as consumers.
Assumptions made about consumers.
Buyers are rational.
More is preferred to less.
Buyers seek to maximize their utility.
Consumers act in self-interest and do not consider the utility of others.
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3. Value
Utility is the satisfaction derived from the consumption of a product.
Utility tells us about ranking.
The amount buyers are prepared to pay for a good tells us something about the value they place on it.
The opportunity costs is what was given up to make a purchase.
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4. II. The Budget Constraint: What The Consumer Can Afford
The budget constraint depicts the limit on the consumption “bundles” that a consumer can afford.
People consume less than they desire because their spending is constrained, or limited, by their income.
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5. Budget Constraint: An example
The budget constraint shows the various combinations of goods the consumer can afford given his or her income and the prices of the two goods.
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6. The Consumer’s Budget Constraint
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7. Budget Constraint: An example
The Consumer’s Budget Constraint
Any point on the budget constraint line indicates the consumer’s combination or trade-off between two goods.
For example, if the consumer buys no pizzas, he can afford 500 litres of cola (point B). If he buys no cola, he can afford 100 pizzas (point A).
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8. Figure 1a The Consumer’s Budget Constraint
Quantity
of cola
500 B
Consumer’s
budget constraint
100 A
Quantity
of Pizza

9. Budget Constraint: An example
The Consumer’s Budget Constraint
Alternately, the consumer can buy 100 pizzas and 500 litres of cola.
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10. Figure 1b The Consumer’s Budget Constraint
Quantity
of cola
500 B
Consumer’s
budget constraint
250 50 C
100 A
Quantity
of Pizza
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11. Budget Constraint: An Example
The slope of the budget constraint line equals the relative price of the two goods, that is, the price of one good compared to the price of the other.
It measures the rate at which the consumer can trade one good for the other.
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12. A Change In Income A change in income shifts the budget constraint.
A rise in income causes it to shift to the right.
A fall in income causes it to shift to the left.
Because the relative price of the two goods has not changed in both examples, the slope of the budget constraint remains the same.
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13. Figure 2. A Change In Income
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14. A Change In Price
A change in price of one good changes the budget constraint.
The budget constraint will pivot on the axis of the good that does not change in price.
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15. Figure 3. A Change In Price
Cola price rises from €2 to €5 or cola price falls from €2 to €1.60 for someone with an income of €1 000
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16. Figure 3. A Change In Price
If the price of only one good changes, the slope of the budget constraint will change.
The budget constraint would pivot.
A change in the price of both goods.
The change in slope is dependent on the relative change in the prices of the two goods.
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17. III. Preferences: What The Consumer Wants
A consumer’s preference among consumption bundles may be illustrated with indifference curves.
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18. Representing Preferences With Indifference Curves
An indifference curve is a curve that shows consumption bundles that give the consumer the same level of satisfaction.
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19. Figure 4a The Consumer’s Preferences
Quantity I2
Indifference
curve, I1
of cola C B D A
Quantity
of Pizza

20. Representing Preferences With Indifference Curves
The Consumer’s Preferences
The consumer is indifferent, or equally happy, with the combinations shown at points A, B, and C because they are all on the same curve.
The Marginal Rate of Substitution
The slope at any point on an indifference curve is the marginal rate of substitution. It is:
The rate at which a consumer is willing to trade one good for another.
The amount of one good that a consumer requires as compensation to give up one unit of the other good.
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21. Figure 4b The Consumer’s Preferences
Quantity I2
Indifference
curve, I1
of cola C B D 1
MRS A
Quantity
of Pizza

22. Four Properties Of Indifference Curves
Higher indifference curves are preferred to lower ones.
Indifference curves are downward sloping.
Indifference curves do not cross.
Indifference curves are bowed inward.
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23. Four Properties Of Indifference Curves
Property 1: Higher indifference curves are preferred to lower ones.
Consumers usually prefer more of something to less of it.
Higher indifference curves represent larger quantities of goods than do lower indifference curves.
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24. Figure 4c The Consumer’s Preferences
Quantity I2
Indifference
curve, I1
of cola C B D A
Quantity
of Pizza

25. Four Properties Of Indifference Curves
Property 2: Indifference curves are downward sloping.
A consumer is willing to give up one good only if he or she gets more of the other good in order to remain equally happy.
If the quantity of one good is reduced, the quantity of the other good must increase.
For this reason, most indifference curves slope downward.
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26. Figure 5 The Consumer’s Preferences
Quantity
Indifference
curve, I1
of cola
Quantity
of Pizza

27. Four Properties Of Indifference Curves
Property 3: Indifference curves do not cross.
Points A and B should make the consumer equally happy.
Points B and C should make the consumer equally happy.
This implies that A and C would make the consumer equally happy.
But C has more of both goods compared to A.
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28. Figure 6 The Impossibility Of Intersecting Indifference Curves
Quantity
of cola C A B
Quantity
of Pizza

29. Four Properties Of Indifference Curves
Property 4: Indifference curves are bowed inward.
People are more willing to trade away goods that they have in abundance and less willing to trade away goods of which they have little.
These differences in a consumer’s marginal substitution rates cause his or her indifference curve to bow inward.
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30. Figure 8 Bowed Indifference Curves
Quantity
of cola
Indifference
curve 14 2 1
MRS
= 6 8 3 A 4 6 3 7 B 1
MRS
= 1
Quantity
of Pizza

31. Total And Marginal Utility
Total utility is the satisfaction that consumers gain from consuming a product
Marginal utility of consumption is the increase in utility that the consumer gets from an additional unit of that good.
Diminishing marginal utility refers to the tendency for the additional satisfaction from consuming extra units of a good to fall.
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32. The Marginal Rate Of Substitution
Marginal rate of substitution is the rate at which a consumer is willing to trade one good for another.
The marginal rate of substitution is equal to the slope of the indifference curve at any point.
The marginal rate of substitution (also the slope of the indifference curve) equals the marginal utility of one good divided by the marginal utility of the other good.
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33. Two Extreme Examples Of Indifference Curves
Perfect substitutes
Perfect complements
1. Perfect Substitutes
Two goods with straight-line indifference curves are perfect substitutes.
The marginal rate of substitution is a fixed number.
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34. Figure 9 Perfect Substitutes And Perfect Complements
(a) Perfect Substitutes
10 cent coins 15 I3 10 I2 5 I1 3 1 2
50 cent coins

35. Two Extreme Examples Of Indifference Curves
2. Perfect Complements
Two goods with right-angle indifference curves are perfect complements.
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36. Figure 10 Perfect Substitutes And Perfect Complements
(b) Perfect Complements
Left
Shoes I1 I2 7 5
Right Shoes

37. Optimization: What The Consumer Chooses
Consumers want to get the combination of goods on the highest possible indifference curve.
However, the consumer must also end up on or below his budget constraint.
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38. The Consumer’s Optimal Choices
Combining the indifference curve and the budget constraint determines the consumer’s optimal choice.
Consumer optimum occurs at the point where the highest indifference curve and the budget constraint are tangent.
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39. The Consumer’s Optimal Choice
The consumer chooses consumption of the two goods so that the marginal rate of substitution equals the relative price.
At the consumer’s optimum, the consumer’s valuation of the two goods equals the market’s valuation.
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40. Figure 11 The Consumer’s Optimum
Quantity I3
of cola I2
Budget constraint I1
Optimum B A
Quantity
of Pizza

41. How Changes In Income Affect The Consumer’s Choices
An increase in income shifts the budget constraint outward.
The consumer is able to choose a better combination of goods on a higher indifference curve.
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42. Figure 12 An Increase in Income
Quantity
of cola
New budget constraint I2
1. An increase in income shifts the
budget constraint outward . . . I1
New optimum
and
cola
consumption.
Initial
optimum
Initial
budget
constraint
Quantity
raising pizza consumption . . .
of Pizza

43. How Changes In Income Affect The Consumer’s Choices
Normal versus Inferior Goods
If a consumer buys more of a good when his or her income rises, the good is called a normal good.
If a consumer buys less of a good when his or her income rises, the good is called an inferior good.
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44. Figure 13 An Inferior Good
Quantity
of cola
New budget constraint I2 I1
1. When an increase in income shifts the
budget constraint outward . . .
but
cola
consumption
falls, making
cola an
inferior good.
Initial
optimum
New optimum
Initial
budget
constraint
Quantity
pizza consumption rises, making pizza a normal good . . .
of Pizza

45. How Changes In Prices Affect Consumer’s Choices
A fall in the price of any good rotates the budget constraint outward and changes the slope of the budget constraint.
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46. Figure 14 A Change In Price
Quantity
of cola
New budget constraint
1,000 D I1 I2
New optimum
1. A fall in the price of Pepsi rotates
the budget constraint outward . . .
500 B
100 A
and
raising cola
consumption.
Initial optimum
Initial
budget
constraint
Quantity
reducing pizza consumption . . .
of Pizza

47. Income And Substitution Effects
A price change has two effects on consumption.
An income effect
A substitution effect
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48. Income And Substitution Effects
The Income Effect
The income effect is the change in consumption that results when a price change moves the consumer to a higher or lower indifference curve.
The Substitution Effect
The substitution effect is the change in consumption that results when a price change moves the consumer along an indifference curve to a point with a different marginal rate of substitution.
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49. Income And Substitution Effects
A Change in Price: Substitution Effect
A price change first causes the consumer to move from one point on an indifference curve to another on the same curve.
Illustrated by movement from point A to point B.
A Change in Price: Income Effect
After moving from one point to another on the same curve, the consumer will move to another indifference curve.
Illustrated by movement from point B to point C.
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50. Figure 15 Income And Substitution Effects
Quantity
of cola I2 I1
New budget constraint C
New optimum
Income
effect
Income effect B A
Initial optimum
Initial
budget
constraint
Substitution
effect
Substitution effect
Quantity
of Pizza

51. Table 1 Income And Substitution Effects When The Price Of Pepsi Falls

52. Deriving The Demand Curve
A consumer’s demand curve can be viewed as a summary of the optimal decisions that arise from his or her budget constraint and indifference curves.
The price-consumption curve is a line showing the consumer optimum for two goods as the price of only one of the goods changes.
The price-quantity relationship is plotted on the lower graph in figure 5.16b to give the familiar demand curve.
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53. Figure 16a Deriving the Demand Curve
A change in price
Connects to figure 16b
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54. Figure 16b. Deriving the Demand Curve
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55. Do all demand curves slope downward?
Demand curves can sometimes slope upward.
This happens when a consumer buys more of a good when its price rises.
Giffen goods
Economists use the term Giffen good to describe a good that violates the law of demand.
Giffen goods are goods for which an increase in the price raises the quantity demanded.
The income effect dominates the substitution effect.
They have demand curves that slope upwards.
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56. Figure 17 A Giffen Good Quantity of Potatoes Initial budget constraint
Optimum with high
price of potatoes I2
Optimum with low
price of potatoes D E
which
increases
potato
consumption
if potatoes
are a Giffen
good.
1. An increase in the price of
potatoes rotates the budget
constraint inward . . . C
New budget
constraint
Quantity
of Meat
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57. Income Expansion Path
For normal goods, the consumer optimums are represented by points A, B and C in figure 5.18.
Inferior goods in figure 5.19 and 5.20.
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58. Figure 5.18 Income Expansion Path
A change in price
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59. Figure 19 . Where Pizza is an Inferior Good
A change in price
Figure 19 . Where Pizza is an Inferior Good
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60. Figure 20 . Where Cola is an Inferior Good
A change in price
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61. Income Expansion Path The Engel Curve
Engel observed that as income rises the proportion of income spent of food decreases whereas the proportion of income devoted to other goods such as leisure, increases.
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62. Figure 21a Engle’s Curve A change in price
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63. Figure 21b . Engle’s Curve A change in price
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64. Behavoiural Approaches To Consumer Behaviour
Bounded rationality is the idea that humans make decisions under the constraints of limited, and sometimes unreliable, information.
People are overconfident.
Give too much weight to a small number of observations.
Are reluctant to change their minds.
Have a tendency to look for examples which confirm their existing view.
Consumers use rules of thumb – heuristics.
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65. Types of Heuristics Anchoring – using familiarity to make decisions.
Availability – assessing risks of the likelihood of something happening.
Representativeness – decisions made based on how representative something is to a stereotype.
Persuasion – attributes a consumer attaches to a product or brand.
Simulation – visualising or simulating the outcome of a decision.
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66. Summary
A consumer’s budget constraint shows the possible combinations of different goods he can buy given his income and the prices of the goods.
The slope of the budget constraint equals the relative price of the goods.
The consumer’s indifference curves represent his preferences.
Points on higher indifference curves are preferred to points on lower indifference curves.
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67. Summary
The slope of an indifference curve at any point is the consumer’s marginal rate of substitution.
The consumer optimizes by choosing the point on his budget constraint that lies on the highest indifference curve.
When the price of a good falls, the impact on the consumer’s choices can be broken down into an income effect and a substitution effect.
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68. Summary
The income effect is the change in consumption that arises because a lower price makes the consumer better off.
The income effect is reflected by the movement from a lower to a higher indifference curve.
The substitution effect is the change in consumption that arises because a price change encourages greater consumption of the good that has become relatively cheaper.
The substitution effect is reflected by a movement along an indifference curve to a point with a different slope.
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69. Summary
The theory of consumer choice explains why demand curves slopes downwards in general, but can potentially slope upwards.
Consumers are not always rational, they use rules of thumb (heuristics) and are influenced by the way in which information is presented (framing effects) which may alter the outcomes suggested by expected utility theory.
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