Econ 201Lecture 1.5

Consumer Demand Theory

1-9-2009

Overview

• Basic behavioral assumptions• Marginal Value or Marginal Willingness-to- Pay• First Law of Demand• Total WTP, Total Amount Paid, Consumer

Surplus• Individual and Market Demand Curves• Factors that Shift Demand Curves• More useful websites

Basic Behavioral Postulates1. For each person some goods are scarce

- have to make choices (core of the economic problem)2. Each person desires many goods and services

- tradeoffs3. Each person is willing to forsake some of an economic good to

get more of other economic goods- Opportunity costs – cost of foregone good to acquire another one

4. More one has of any good, the lower it personal marginal value- Diminishing marginal value

5. Not all people have the same tastes and preferences- Not all people get the same value from consuming a good- Provides a basis for exchange/trade even if everyone had same amount of

goods

6. People are innovative and rational- People learn and behave in their own best self-interest (or at least are not

systematically irrational – Becker)

First Law of Demand

• First law of demand, – The lower a good’s price it, the greater the

quantity demanded (by an individual or the market)

• Demand– Entire schedule: quantity demanded at various prices

• Quantity demanded– The amount demanded at a given price

From the Demand Side

• First Law of Demand– What Does Law Of Demand

Mean?– all other factors being equal, as

the price of a good or service increases, consumer demand for the good will decrease and vice versa. 

– http://www.investopedia.com/

terms/l/lawofdemand.asp

Consumer’s Marginal Value

• Some basic definitions– Total Willingness-to-pay: “value in use”

• Maximum total amount you would be willing to pay for x units of the good than go without?

– Equals the area under the demand curve up to x units

Individual's Demand Curve

$0.00

$2.00

$4.00

$6.00

$8.00

$10.00

$12.00

1 2 3 4 5 6 7 8 9 10

Quantity Demanded

Pri

ce

pe

r u

nit

Price

Total Value of 4 units

All the things a demand curve tells you about value of the good

Demand Curve

$0$2

$4$6

$8$10

$12

1 2 3 4 5 6 7 8 9 10

Quantity Demanded

Av

era

ge

Pric

e (

pric

e

pe

r u

nit

)Demand Curve is

Also Marginal Valueand Avg Revenue

Amount Paid

CS

Total WTP =CS + Amt Paid

In Class Example

Avg Pric Qty Dem Tot Amt Paid Tot Value (WTP) Marg Val Cons Surp$10 1 $10 $10 $10 $0$9 2 $18 $19 $9 $1$8 3 $24 $27 $8 $3$7 4 $28 $34 $7 $6$6 5 $30 $40 $6 $10$5 6 $30 $45 $5 $15$4 7 $28 $49 $4 $21$3 8 $24 $52 $3 $28$2 9 $18 $54 $2 $36$1 10 $10 $55 $1 $45

Avg P*Qd TV(Q-1)+MV(Q)

Also = MV(Q) TV(Q)-TV(Q-1)

Tot Val- Tot Paid

Also = Avg Rev

Buy Rules

• Consumer will buy a good as long as:– Total Willingness-to-Pay > Amount Paid

• There is always some consumer surplus, or incentive for consumer

• Consumer Surplus ≡ Difference () between maximum amount that you are willing-to-pay and what you have to pay

– CS ≡ Total WTP – Average Price x Qty Purhased

• Consumer will choose how much to buy (quantity demanded):– Marginal Value >= price paid for the last unit

• For Perfect Competition: price same for all units -> price paid for last unit = average price

What Does a Demand Curve Tell You?

• A Demand Curve is also– A Marginal Value Curve

• Tells you what the consumer’s marginal value of the last (incremental/additional) unit is

– An Average Revenue Curve• Tells you what the average price needs to be in

order to sell x units

Individual Demand versus Market Demand

• Individual Demand curve– One individual’s willingness-to-pay or demand

• Market Demand curve– The sum of all individuals demand curves in

the market

An Example

Price Tom Harry Dick Market

$10 1 1 2 4

$9 1 2 3 6

$8 2 3 4 9

$7 2 4 5 11

$6 3 5 6 14

$5 3 6 7 16

$4 4 7 8 19

$3 4 8 9 21

$2 5 9 10 24

$1 5 10 11 26

Individual and Market Demand

Quantity Demanded

Tom

Harry

Dick

Market

Market Demand = Sum of Tom + Dick + Harry

Market Demand: Maybe a little bit different

• Market Demand may be kinked as new buyers enter at different price points

Shift of the Entire Curve

• Shift out/right of the Demand Curve– WTP increases for all Qd

• Shift in/left of the Demand Curve– WTP decreases for all levels of Qd

Shift out/right

Shift in/left

D2

Factors That Can Shift Demand

• Individual and Market Demand Curves– Income– Price of Substitutes– Price of Compliments– Product Quality– Future Prices– Taste and Preferences

• Market Demand Curves– Population (market)

Factors That Can Shift Demand

• Income– Normal good

• As income rises, quantity demanded increases at a given price -> Demand curve shifts out

• Superior good – percent of budget spent on good increases more than percent increase in income

– Demand curve shift is very large

– Inferior good• As income rises, quantity demanded decreases at

a given price– Demand curve shifts in

Factors That Can Shift Demand

• Substitutes– Goods that are similar to the “good in question” (or

being analyzed)• E.g., Pepsi/Coke

• Shifts in Demand Curve– If price of substitute increases, then demand for “GIQ”

shifts out as it becomes relatively less expensive– If price of substitute decreases, then demand for

“GIQ” shifts in as it becomes relatively more expensive

– “closer” the substitute -> more demand curve shifts (i.e., greater cross-price elasticity)

– For substitutes: relative price matters!

Factors That Can Shift Demand

• Complements– Goods that are “jointly consumed” with “GIQ”, e.g.

coffee & cream

• Demand Curve shifts– Price of complement increases, then demand for

“GIQ” shifts in as total cost of consumption has increased

– Price of complement decreases, then demand for “GIQ” shifts out as total cost of consumption has decreased

– For complements: total cost of consumption matters!

Factors That Can Shift Demand

• Product Quality– Better or improved product quality increases (relative) demand

• Demand curve shifts out

• Future price (hedge market) of the good– If the price is expected to go up in the future -> increases current

demand (shift out/right)• Cheaper to consume today and stockpile

– If the price is expected to decrease in the future -> decrease current demand (shift in/left)

• Wait to buy (christmas/after christmas sales)

• Taste and Preferences– Always assumed constant unless you have empirical proof

(new MRI imaging)

Factor that Affects the ONLY Aggregate Market Demand Curve

• As the population/number of buyers (Nb) increases -> Market Demand curve shifts outward

Key Assumptions Demand Curve

• For a given (individual’s) demand curve– These factors are held constant (ceteris paribus):

• Price of: – substitutes, – complements, – future price of the good

• income, • quality, and • taste and preferences

• And for a market demand curve: – number of buyers (Nb)

• Only price and quantity demanded are allowed to vary

A Short QuizSeattle Times Oct 3, 2007

Olympic National Park officials are suggesting raising the price of an entrance pass for motorists — good for seven days — from $15 to $25 starting in 2009, with the fee for individuals such as cyclists climbing from $5 to $12. Season passes would increase from $30 to $50

But public response, particularly from tourist-dependent local businesses has been generally negative said a spokeswoman for Olympic National Park.

1. Illustrate the effect of the increase of the price for park passes on the demand for trips to the park

2. Illustrate how the park fee increase would affect the demand for other tourist-related businesses, e.g., hotels, restaurants.

A Take Home Problem

Price Indiv 1 Indiv 2 Indiv 3 Indiv 4 Total Rev Total WTP Cons Surp$10 1 0 0 0$9 2 0 1 1$8 3 1 2 1$7 4 1 3 2$6 5 2 4 2$5 6 2 5 3$4 7 3 6 3$3 8 3 6 4$2 9 4 6 4$1 10 4 6 5

Useful Websites

– Understanding differences between factors that cause shifts in demand or supply

• http://hspm.sph.sc.edu/COURSES/ECON/SD/SD.html

– Basics of demand and supply• http://www.investopedia.com/university/economics/

economics3.asp

– Cobweb theorem• http://www.bized.co.uk/current/mind/2004_5/25100

4.ppt

The Cobweb Theorem

D

S

7

Price (£)

Quantity Bought and Sold (millions)

9

D1

In a ‘divergent cobweb’ -also termed an unstable cobweb - the price tends to move away from equilibrium.

Assume the initial equilibrium price is £7 and the quantity 9. If demand rises, the shortage pushes the price up to £11 per turkey.

11

15

Farmers respond by planning to increase supply, ten months later, the supply of turkeys is 15 million. At this level, there will be a surplus of turkeys and the price drops.

8

The price falls to £5 and farmers react by cutting plans for turkey production. Ten months later, supply on the market will be 8 million.

5

This creates a massive shortage of 9 million turkeys and the price is forced up – and so the process continues!

A divergent cobweb leads to price instability over time.

17

Cobweb Theorem

• http://www.bized.co.uk/current/mind/2004_5/251004.ppt• Hungarian-born economist Nicholas Kaldor (1908-1986)• Simple dynamic model of cyclical demand with time lags

between the response of production and a change in price (most often seen in agricultural sectors).

• Cobweb theory is the process of adjustment in markets • Traces the path of prices and outputs in different

equilibrium situations. Path resembles a cobweb with the equilibrium point at the center of the cobweb.

• Sometimes referred to as the hog-cycle (after the phenomenon observed in American pig prices during the 1930s).

What We’ve Learned

• Sell rule for firms (Qs: P=MC)– Firms will supply y units up to the point where the MC

of producing the next/last unit (yth) is just equal to the price it receives for the good

– First law of supply: supply curves will be upward sloping

• Buy rule for consumers (Qd: P=MV)– Consumers will buy x units up to the point that price

equals MV for the last (xth) unit• First law of demand: demand curves are downward sloping• Negative slope diminishing marginal value of consuming

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