Chapter 8

UTILITY AND DEMAND 8 After studying this chapter, you will be able to: Explain the limits to consumption and describe preferences using the concept of utility Explain the marginal utility theory of consumer choice Use marginal utility theory to predict the effects of changes in prices and incomes and to explain the paradox of value Describe some new ways of explaining consumer choices The choices you make as a buyer of goods and services are influenced by many factors, which economists summarize as Consumption possibilities Preferences Consumption Possibilities Consumption possibilities are all the things that a consumer can afford to buy. We’ll study the consumption possibilities of Lisa, who buys only two goods: movies and soda. Consumption Choices A Consumer’s Budget Line Consumption possibilities are limited by income, the price of a movie, and the price of soda. When Lisa spends all of her income, she reaches the limits of her consumption possibilities. Lisa’s budget line shows the limits of her consumption possibilities. Consumption Choices Lisa has $40 to spend, the price of a movie is $8 and the price of soda is $4 a case. Consumption Choices Lisa can afford any of the combinations at the points A to F. Some goods are indivisible and must be bought in whole units at the points marked. Other goods are divisible goods and can be bought in any quantity. The line through points A to F is Lisa’s budget line. Consumption Choices The budget line is a constraint on Lisa’s consumption choices. Lisa can afford any point on her budget line or inside it. Lisa cannot afford any point outside her budget line. Consumption Choices Changes in Consumption Possibilities Consumption possibilities change when income or prices change. A rise in income shifts the budget line outward and leaves its slope unchanged. Consumption possibilities expand. A rise in a price changes the slope of the line and shrinks consumption possibilities. The budget line shows what is possible; the consumer's preferences determine which possibility is chosen. Consumption Choices Preferences The choice that Lisa makes depends on her preferences—her likes and dislikes. Her benefit or satisfaction from consuming a good or service is called utility. Total Utility Total utility is the total benefit a person gets from the consumption of goods. Generally, more consumption gives more total utility. Consumption Choices Table 8.1 shows Lisa’s total utility schedule. Total utility from a good increases as the quantity of the good increases. For example, as Lisa sees more movies in a month, her total utility from movies increases. Consumption Choices Marginal Utility Marginal utility from a good is the change in total utility that results from a unit-increase in the quantity of the good consumed. As the quantity consumed of a good increases, the marginal utility from it decreases. We call this decrease in marginal utility as the quantity of the good consumed increases the principle of diminishing marginal utility. Consumption Choices Table 8.1 shows how to calculate Lisa’s marginal utility from her total utility. Marginal utility from a good decreases as the quantity of the good increases. For example, as the number of movies seen in a month increases, marginal utility from movies decreases. Consumption Choices Figure 8.2(a) shows Lisa’s total utility and marginal utility from soda. Total utility from soda increases as more soda is consumed. The bars along the total utility curve show the extra total utility (marginal utility) from each additional case of soda. Consumption Choices Graphing Liz’s Utility Schedule Figure 8.2(b) illustrates diminishing marginal utility. As Lisa increases the quantity of soda she drinks, her marginal utility from soda diminishes. Consumption Choices The key assumption is that the household chooses the consumption possibility that maximizes total utility. A Spreadsheet Solution The direct way to find the utility-maximizing choice is to make a table in a spreadsheet and do the calculations. Find the just-affordable combinations Find the total utility for each just-affordable combination The utility-maximizing combination is the consumer’s choice Utility-Maximizing Choice Find the Just-Affordable Combinations Lisa has $40 a month to spend on movies and soda. The price of a movie is $8 and the price of soda is $4 a case. Each row of Table 8.2 shows a combination of movies and soda that exhausts Lisa’s $40. Utility-Maximizing Choice Find the Total Utility for Each Just-Affordable Combination When Lisa sees 1 movie and drinks 8 cases of soda a month, … she gets 50 units of utility from the 1 movie and 248 units of utility from the 8 cases of soda. Her total utility is 298 units. Utility-Maximizing Choice Consumer Equilibrium Lisa chooses the combination that gives her the highest total utility. Lisa maximizes her total utility when she sees 2 movies and drinks 6 cases of soda a month. Lisa gets 90 units of utility from the 2 movies and 315 units of utility from the 6 cases of soda. Utility-Maximizing Choice Consumer equilibrium is the situation in which Lisa has allocated all of her available income in the way that maximizes her total utility, given the prices of movies and soda. Lisa’s consumer equilibrium is 2 movies and 6 cases of soda a month. Utility-Maximizing Choice A more natural way of finding the consumer equilibrium is to use the idea of choices made at the margin. Choosing at the Margin Having made a choice, would spending a dollar more or a dollar less on a good bring more total utility? Marginal utility is the increase in total utility that results from consuming one more unit of the good. The marginal utility per dollar is the marginal utility from a good that results from spending one more dollar on it. Utility-Maximizing Choice Marginal Utility per Dollar The marginal utility per dollar equals the marginal utility from a good divided by its price. Calling the marginal utility from movies MUM and the price of a movie PM, then the marginal utility per dollar from movies is MUM/PM . Calling the marginal utility of soda MUS and the price of soda PS , then the marginal utility per dollar from soda is MUS/PS. By comparing MUM/PM and MUS/PS , we can determine whether Lisa has allocated her budget in the way that maximizes her total utility. Utility-Maximizing Choice Utility-Maximizing Rule A consumer’s total utility is maximized by following the rule: Spend all available income Equalize the marginal utility per dollar for all goods Utility-Maximizing Choice Lisa’s Marginal Calculation Figure 8.3 shows why the utility-maximizing rule works. Each row of the table (on the next slide) shows a just-affordable combination. Start by choosing a row—a point on the budget line. Utility-Maximizing Choice In row B, MUS/PS < MUM/PM. Utility-Maximizing Choice Lisa spends too much on soda and too little on movies. Utility-Maximizing Choice If Lisa spends less on soda and more on movies, MUS increases and MUM decreases. Utility-Maximizing Choice In row D, MUS/PS > MUM/PM. Lisa spends too little on soda and too much on movies. Utility-Maximizing Choice If Lisa spends more on soda and less on movies, MUS decreases and MUM increases. Utility-Maximizing Choice In row C, MUS/PS = MUM/PM. Lisa is maximizing utility. We can use marginal utility theory to make some predictions. A Fall in the Price of a Movie When the price of a good falls the quantity demanded of that good increases—the demand curve slopes downward. For example, if the price of a movie falls, we know that MUM/PM rises, so before the consumer changes the quantities bought, MUM/PM > MUS/PS. To restore consumer equilibrium (maximum total utility), the consumer increases the movies seen to drive down the MUM and restore MUM/PM = MUS/PS. Predictions of Marginal Utility Theory A change in the price of one good changes the demand for another good. If the price of a movie falls, MUM/PM rises, … so before the consumer changes the quantities consumed, MUM/PM > MUS/PS. To restore consumer equilibrium (maximum total utility), the consumer decreases the quantity of soda consumed to drive up the MUS and restore MUM/PM = MUS/PS. Predictions of Marginal Utility Theory Table 8.3 shows Lisa’s just-affordable combinations when the price of a movie is $4. Before Lisa changes what she buys: MUM/PM > MUS/PS. To maximize total utility, Lisa sees more movies and drinks less soda. Predictions of Marginal Utility Theory Figure 8.4 illustrates these predictions. A fall in the price of a movie increases the quantity of movies demanded—a movement along the demand curve for movies, ... and decreases the demand for soda—a shift of the demand curve for soda. Predictions of … A Rise in the Price of Soda Now suppose the price of soda rises. We know that MUS/PS falls, so before the consumer changes the quantities bought, MUS/PS < MUM/PM. To restore consumer equilibrium (maximum total utility), the consumer decreases the quantity of soda consumed to drive up the MUS and increases the quantity of movies seen to drive down MUM. These changes restore MUM/PM = MUS/PS. Predictions of Marginal Utility Theory Table 8.4 shows Lisa’s just-affordable combinations when the price of soda is $8 and the price of a movie is $4. Before Lisa changes what she buys: MUS/PS < MUM/PM. To maximize her total utility, Lisa drinks less soda. Predictions of Marginal Utility Theory Figure 8.5 illustrates these predictions. A rise in the price of soda decreases the quantity of soda demanded—a movement along the demand curve for soda. Predictions of Marginal Utility Theory A Rise in Income When income increases, the demand for a normal good increases. Given the prices of movies and soda, when Lisa’s income increases from $40 to $56 a month, she buys more movies and more soda. Movies and soda are normal goods. Table 8.5 shows these predictions. Predictions of Marginal Utility Theory Table 8.5 shows Lisa’s just-affordable combinations when she has $56 to spend. With $40 to spend, Lisa sees 6 movies and drinks 4 cases of soda a month. With $56 to spend, Lisa spends the extra $16, so she buys more of both goods. She sees 8 movies and drinks 6 cases of soda a month. Predictions of Marginal Utility Theory Figure 8.6 illustrates these predictions. Predictions of Marginal Utility Theory The Paradox of Value Water, which is essential to life, is far cheaper than diamonds, which are not essential. The paradox of value of why water is far cheaper than diamonds is resolved by distinguishing between total utility and marginal utility. We use so much water that the marginal utility from water consumed is small, but the total utility is large. We buy few diamonds, so the marginal utility from diamonds is large, but the total utility is small. Predictions of Marginal Utility Theory The Paradox Resolved The paradox is resolved by distinguishing between total utility and marginal utility. For water, the price is low, total utility is large, and marginal utility is small. For diamonds, the price is high, total utility is small, and marginal utility is high. But marginal utility per dollar is equal for water and diamonds. Predictions of Marginal Utility Theory Value and Consumer Surplus The supply of water is perfectly elastic, so the quantity of water consumed is large and the consumer surplus from water is large. In contrast, the supply of diamonds is perfectly inelastic, so the price is high and the consumer surplus from diamonds is small. Predictions … Temperature: An Analogy Utility is similar to temperature. Both are abstract concepts, and both have units of measurement that are arbitrary. The concept of utility helps us to predict consumption choices in the same way that the concept of temperature enables us to predict when water will turn to ice or steam. The concept of utility helps us understand why People buy more of a good when its price falls. People buy more of most goods when income increases. Predictions of Marginal Utility Theory Behavioral Economics Behavioral economics studies the ways in which limits on the human brain’s ability to compute and implement rational decisions influence economic behavior—both the decisions that people make and the consequences of those decisions for the way markets work. There are three impediments to rational choice: Bounded rationality Bounded willpower Bounded self-interest New Ways of Explaining Consumer Choices Bounded Rationality Bounded rationality is rationality that is bounded by the computing power of the human brain. Faced with uncertainty, consumers cannot rationally make choices and instead rely on other decision-making methods such as rules of thumb, listening to the views of others, or gut instinct. New Ways of Explaining Consumer Choices Bounded Willpower Bounded will-power is the less-than-perfect willpower that prevents us from making a decision that we know, at the time of implementing the decision, we will later regret. Bounded Self-Interest Bounded self-interest is the limited self-interest that sometimes results in suppressing our own interests to help others. Main applications are in finance where uncertainty is the key factor and savings where future is the key factor. New Ways of Explaining Consumer Choices One behavior observed by behavioral economists is more general and might affect your choices. The Endowment Effect The endowment effect is the tendency for people to value something more highly simply because they own it. New Ways of Explaining Consumer Choices Neuroeconomics Neuroeconomics is the study of the activity of the human brain when a person makes an economic decision. Different decisions appear to activate different areas of the brain. Some decisions are made In the pre-frontal cortex where memories are stored and data analyzed and might be deemed rational. In the hippocampus where memories of anxiety and fear are stored and might be deemed irrational. New Ways of Explaining Consumer Choices Controversy Should economics focus on explaining the decisions we observe or should it focus on what goes on inside people’s heads? This is the controversy. For most economists, the goal of economics is to explain the decisions that we observe people make, and not to explain what goes on inside people’s heads. New Ways of Explaining Consumer Choices