4. Demand Curves and Income/Substitution Effects

MIT OpenCourseWare・2 minutes read

Jonathan Gruber discusses deriving demand curves based on consumer choice, elasticity of demand, and income effects. The lecture touches on perfectly inelastic and elastic demand, as well as the distinction between luxury and necessity goods, using examples like fast food and jewelry.

Insights

  • The lecture emphasizes the importance of elasticity of demand in shaping the demand curve, showcasing how changes in price and income affect consumer choices.
  • Gruber introduces the concept of perfectly inelastic and elastic demand, highlighting extreme cases where quantity demanded remains constant or varies infinitely with price changes, shedding light on essential distinctions in consumer behavior.

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Recent questions

  • What is the relationship between price and quantity demanded?

    The relationship between price and quantity demanded is illustrated through the demand curve, which shows how the quantity of a good or service that consumers are willing to buy changes as the price changes. When the price of a product decreases, the quantity demanded typically increases, and vice versa. This relationship is crucial in understanding consumer behavior and market dynamics.

  • How does income affect demand for goods?

    Changes in income can significantly impact the demand for goods. Normal goods have a positive income elasticity, meaning that as income increases, the demand for these goods also increases. In contrast, inferior goods have a negative income elasticity, where demand decreases as income rises. Understanding how income influences consumer choices is essential for businesses and policymakers in predicting market trends and consumer behavior.

  • What is the concept of elasticity of demand?

    Elasticity of demand measures how the quantity demanded of a good or service changes in response to price changes. It is defined as the percentage change in quantity demanded for a percentage change in price. Different types of elasticity, such as perfectly inelastic and perfectly elastic demand, illustrate the extremes of consumer responsiveness to price fluctuations. Elasticity of demand is a fundamental concept in economics that helps analyze market dynamics and consumer preferences.

  • What are luxury and necessity goods?

    Luxury goods are products that individuals spend a larger share of their budget on as they get richer, such as watches or boats. Necessity goods, on the other hand, see rich individuals spending more but a smaller share of their budget on them compared to poorer individuals. Understanding the distinction between luxury and necessity goods is crucial in analyzing consumer behavior and market trends, as it sheds light on how consumer preferences evolve with changes in income.

  • How do price changes impact consumer choices?

    Price changes impact consumer choices through the substitution effect and income effect. The substitution effect refers to the change in quantity demanded when the price of a good changes while keeping utility constant. The income effect, on the other hand, reflects the change in quantity demanded due to changes in income. By analyzing these effects, economists and businesses can predict how consumers will respond to price fluctuations and adjust their strategies accordingly to meet consumer demand effectively.

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Summary

00:00

Deriving Demand Curves and Elasticity Explained

  • Jonathan Gruber introduces the completion of the discussion on consumer choice by deriving the demand curve using the tools provided in class.
  • The lecture delves into the elasticity of demand, explaining how it influences the shape of the demand curve.
  • Changes in income's effect on demand are discussed, along with the impact of price changes.
  • The process of deriving demand curves is explained, starting with a utility function and budget constraints.
  • Graphical representations are used to illustrate the relationship between price and quantity demanded.
  • Examples are provided where changes in the price of cookies affect the quantity demanded, showcasing the derivation of a demand curve.
  • The concept of indifference curves and their relationship to utility functions is clarified.
  • Gruber addresses a question regarding the constant demand for pizza slices despite changes in cookie prices.
  • The elasticity of demand is defined as the percentage change in quantity for a percentage change in price.
  • Perfectly inelastic demand is discussed as an extreme case, where the quantity demanded remains constant regardless of price changes.

13:05

Essential Insulin, Elastic Demand, Income Elasticity

  • Water and insulin are essential for life, with insulin being crucial for diabetics to control blood sugar.
  • Perfectly inelastic demand occurs when there is no substitute for a product, like insulin for diabetics.
  • Perfectly elastic demand happens when there are perfect substitutes for a product, like different brands of gum.
  • The demand curve for perfectly elastic products is horizontal, with price remaining constant.
  • Income elasticity of demand measures how quantity demanded changes with income shifts.
  • Normal goods have a positive income elasticity, meaning demand increases with income.
  • Inferior goods have a negative income elasticity, where demand decreases as income rises.
  • Examples of inferior goods include fast food, where richer individuals consume less.
  • Linear Engel curves show the relationship between income and quantity demanded.
  • Constant elasticity curves are often drawn linearly for simplicity, although technically incorrect.

25:20

Wealth and Consumer Choices: A Summary

  • Rich individuals are more likely to have multiple refrigerators, indicating a discreteness issue in using this as an example.
  • Fast food serves as a better example, with potatoes historically being a cheap, filling food choice that people move away from when they have more money.
  • A distinction is made between luxuries (gamma > 1) and necessities (gamma < 1) in consumer goods.
  • Luxury goods, like watches or boats, see individuals spending a larger share of their budget on them as they get richer.
  • Necessities, such as food, see rich individuals spending more but a smaller share of their budget on them compared to poorer individuals.
  • The concept of luxury and necessity brands is introduced, with jewelry representing luxury and food representing necessity.
  • Elasticities of demand are used to estimate goods near the border between luxuries and necessities.
  • Consumer demand theory tools are explained, including how to determine desired quantities and derive demand curves.
  • The effects of price changes are revisited, focusing on the substitution effect (change in quantity with utility constant) and income effect (change in quantity as income changes).
  • Graphical representations are used to illustrate how price changes impact consumer choices through the substitution and income effects.

39:23

Effect of Price Increase on Inferior Goods

  • Price of steak is $5, price of potatoes is $1, and income is $25 in the example.
  • Choosing point a involves 7 and 1/2 potatoes and 3 and 1/2 steaks.
  • If the price of potatoes increases from $1 to $3, the compensated demand shifts from point a to point b.
  • Substitution effect decreases demand for potatoes from 7 and 1/2 to 4, while income effect raises demand from 4 to 5.
  • Income effect causes a desire for more potatoes due to the price increase making one effectively poorer.
  • Potatoes are considered an inferior good, leading to the opposite effects of substitution and income.
  • The income effect can go against the substitution effect, making the exercise interesting.
  • Giffen goods can result in an upward-sloping demand curve, where a higher price leads to increased demand, as seen in a study in China with rice coupons for very poor households.
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