1. Constraints, Choices, and Demand
chapter 5
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2. Learning Objectives
Demonstrate how price and income affect a consumer’s budget line.
Determine a consumer’s best choice based on his preferences and budget line.
Understand how find a consumer’s best choice by maximizing a utility function.
Analyze the effects of changes in prices and income on a consumer’s demand.
Show how volume-sensitive prices affect a consumer’s budget line and choices.
Explain how economists determine consumers’ preferences based on based on their choices.
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3. Overview Prices and income determine a consumer’s budget constraint
Given her preferences and budget constraint, a consumer selects the optimal consumption bundle
Changes in price or income affect a consumer’s optimal choice
Several new tools will allow us to better represent consumer preferences and behavior
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4. Income, Prices, and the Budget Constraint
A consumer’s income consists of the money he receives during some fixed period of time
A consumer can afford to purchase a particular consumption bundle if its cost does not exceed his income for that period
Budget constraint: cost of consumption bundle ≤ income
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5. Example – Budget Constraint
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6. Example – Budget Constraint
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7. Changes in Income and the Budget Line
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8. Changes in Price and the Budget Line
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9. Properties of Budget Lines
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10. Choosing Among Affordable Bundles
B is preferred to A because it contains more yogurt and more pizza than bundle A
D is the best choice because no affordable bundle lies on a higher indifference curve
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11. Choosing Among Affordable Bundles
E is not a best choice because there are other affordable bundles that are better than E (for example, G)
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12. Special Case – Perfect Complements
Maria always buys the same number of left and right shoes, independently of the slope of the budget line
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13. Interior Solutions
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14. Boundary Solutions
A consumption bundle that does not contain at least a little bit of every good is called a boundary choice
Bundle B is the consumer’s best affordable bundle, and a boundary solution
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15. Properties of Best Choices
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16. Utility Maximization
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17. Example – Utility Maximization
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18. Example – Utility Maximization
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19. Prices and Demand Price-consumption curve Individual demand curve
Price changes and shifts in demand
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20. Price-Consumption Curve
The price-consumption curve shows how the best affordable consumption bundle changes as the price of a good changes, holding everything else fixed
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21. Individual Demand Curve
An individual demand curve describes the relationship between the price of a good and the amount of a particular consumer purchases, holding everything else fixed
The demand curve is similar to the price-consumption curve, with different axes
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22. Price Changes and Shifts in Demand
A change in the price of one good can shift the demand for a second good
For example, a decrease in the price of butter (L1 to L3) causes a decrease in the quantity demanded of margarine without a change in the price of margarine
Thus, in this example a decrease in the price of butter causes the demand for margarine to shift to the left
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23. Price Changes and Shifts in Demand
The effect of the change in the price of one good on the demand for a second good will depend on whether they are substitutes or complements (or neither)
Substitutes: a decrease in P1 leads to a leftward shift in the demand for P2
Complements: a decrease in P1 leads to a rightward shift in the demand for P2
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24. Income and Demand Income-consumption curve Normal vs. inferior goods
Engel curve
Changes in income and shifts in the demand curve
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25. Income-Consumption Curve
An income effect is the change in the consumption of a good that results from a change in income
The income-consumption curve shows how the best affordable consumption bundle changes as income changes, holding everything else fixed
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26. Normal vs. Inferior Goods
Potatoes are inferior in this region
Potatoes are normal in this region
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27. Normal vs. Inferior Goods
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28. Properties of Normal and Inferior Goods
The income elasticity of demand is positive for normal goods and negative for inferior goods.
We can tell whether goods are normal or inferior by examining the slope of the income-consumption curve.
At least one good must be normal starting from any particular income level.
No good can be inferior at all levels of income.
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29. Engel Curve
The Engel curve for a good describes the relationship between income and the amount consumed, holding everything else fixed
In this example, yogurt is a normal good at all levels of income (upward sloping Engel curve)
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30. Engel Curve
The Engel curve for a good describes the relationship between income and the amount consumed, holding everything else fixed
In this example, potatoes are normal when income is less than $36 and inferior when income is greater than $36
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31. Income Changes and Shifts in Demand
For a normal good, an increase in income shifts the demand curve to the right
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32. Volume-Sensitive Pricing
So far we have assumed that every good is available in unlimited quantities at a single price
In practice, the price paid for a good can depend on the volume purchased – volume-sensitive pricing
Volume penalty occurs when a good’s price per unit rises with the amount purchased
Volume discount occurs when a good’s price per units falls with the amount purchased
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33. Example – Volume Penalties
With a volume penalty on usage over 500 kwh of electricity, the portion of the budget constraint that runs between bundle C and the horizontal axis rotates toward the origin
When the government or a supplier limits the amount that each consumer can purchase, we say that the good is rationed
Rationing is similar to a very large volume penalty
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34. Example – Volume Discounts
With a volume discount on usage over 500 kwh of electricity, the portion of the budget constraint that runs between bundle C and the horizontal axis rotates away from the origin
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35. Determining a Consumer’s Preferences
Two ways to learn about consumer preferences
Ask consumers to tell us what they like and dislike
Revealed preference approach: infer a consumer’s preferences from her actual choices
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36. Principle of Revealed Preference
One consumption bundle is revealed preferred to another if the consumer chooses it when both are available
If bundle A is purchased, it is revealed preferred to each of the bundles in the yellow area
Each bundle in the blue area should be preferred to A
The indifference curve running through A must lie entirely in the two unshaded areas
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37. Principle of Revealed Preference
A is revealed preferred to B
B is revealed preferred to E and all other bundles in the yellow area
By transitivity, A must be preferred to the bundles in the yellow area
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38. Principle of Revealed Preference
G is revealed preferred to A
All bundles in the blue rectangle are preferred to G (by more-is-better principle). Thus, they are preferred to A too (by transitivity)
If the indifference curve has a declining MRS, all bundles in the blue triangle are also preferred to A
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39. Review The consumer’s best choice lies on the budget line
The affordable bundle that maximizes the utility function is the consumer’s best choice
Interior solutions always satisfy the tangency condition
When the price of one good changes, the demand curve for another good may shift
When income changes, the demand for a good will shift, with the direction of the shift determined by whether the good is normal or inferior at that level of income
Consumer choices reveal their preferences
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40. Looking forward
Next, we will learn how we can use demand curves to measure changes in consumer welfare
We will dissect the effects of a price change, and we will build a different type of demand curve that will allow us to better analyze consumer welfare
We will also apply what we already know about demand to build the supply for labor
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