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1
Chapter 1
Budget Constraint
2
CONSUMER THEORY
Consumer theory: consumers choose the best bundle of goods they can afford.
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CONSUMER THEORY
Two parts to theory
1. “can afford”___ budget constraint
2. “best” ___ according to consumers’ preferences
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Budget Constraints
consumption bundle :
(x1, x2, …,xn) __ how much of each good is consumed
Commodity prices are p1, p2, … , pn.
m __ money the consumer has to spend
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Budget Constraints
A bundle (x1, … , xn) is affordable at prices p1, … , pn :
When p1x1 + … + pnxn mwhere m is the consumer’s (disposable) income.
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Budget Constraints
The consumer’s budget set is the set of all affordable bundles
The bundles that are only just affordable form the consumer’s budget line.
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Budget Set and Constraint for Two Commoditiesx2
x1
p1x1 + p2x2 = m is x2 = -(p1/p2)x1 + m/p2
so slope is -p1/p2.
m /p1
BudgetSet
m /p2
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Budget Constraints For n = 2 and x1 on the horizontal
axis, the constraint’s slope is -p1/p2. What does it mean?
xpp
xmp2
1
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2
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Budget Sets & Constraints; Income and Price Changes
The budget line and budget set depend upon prices and income. What happens as prices or income change?
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How do the budget set and budget constraint change as income m
increases?
Originalbudget set
x2
x1
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Higher income gives more choice
Originalbudget set
New affordable consumptionchoices
x2
x1
Original andnew budgetconstraints areparallel (sameslope).
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How do the budget set and budget constraint change as income m
decreases?
Originalbudget set
x2
x1
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How do the budget set and budget constraint change as income m
decreases?x2
x1
New, smallerbudget set
Consumption bundlesthat are no longeraffordable.Old and new
constraintsare parallel.
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Budget Constraints - Price Changes
What happens if just one price decreases?
Suppose p1 decreases.
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How do the budget set and budget constraint change as p1 decreases
from p1’ to p1”?
Originalbudget set
x2
x1
m/p2
m/p1’ m/p1”
-p1’/p2
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How do the budget set and budget constraint change as p1 decreases
from p1’ to p1”?
Originalbudget set
x2
x1
m/p2
m/p1’ m/p1”
New affordable choices
-p1’/p2
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How do the budget set and budget constraint change as p1 decreases
from p1’ to p1”?
Originalbudget set
x2
x1
m/p2
m/p1’ m/p1”
New affordable choices
Budget constraint pivots; slope flattens from -p1’/p2 to -p1”/p2
-p1’/p2
-p1”/p2
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Taxes, subsidies and rationing
1. Quantity tax ___ tax levied on units bought : p1 + t
2. Value tax ___ tax levied on rupees spent : p1 + αp1. Also known as ad valorem tax
3. Subsidies ___ opposite of a tax
a) p1 - s
b) p1 - µp1
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Taxes, subsidies and rationing
4. lump sum tax or subsidy __ amount of tax or subsidy independent of the consumer’s choices. Also called a head tax or a poll tax.
5. Rationing ___ can’t consume more than a certain amount of some good
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The Food Stamp Program
Food stamps are coupons that can be legally exchanged only for food.
How does a commodity-specific gift such as a food stamp alter a family’s budget constraint?
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The Food Stamp Program
Suppose m = $100, pF = $1 and the price of “other goods” is pG = $1.
The budget constraint is then F + G =100.
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The Food Stamp ProgramG
F100
100
F + G = 100; before stamps.
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The Food Stamp ProgramG
F100
100
F + G = 100: before stamps.
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The Food Stamp ProgramG
F100
100
F + G = 100: before stamps.
Budget set after 40 foodstamps issued.
14040
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The Food Stamp ProgramG
F100
100
F + G = 100: before stamps.
Budget set after 40 foodstamps issued.
140
The family’s budgetset is enlarged.
40
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The Food Stamp Program
What if food stamps can be traded on a black market for $0.50 each?
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Shapes of Budget Constraints - Quantity Discounts
Suppose p2 is constant at $1 but that p1=$2 for 0 x1 20 and p1=$1 for x1>20.
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Shapes of Budget Constraints with a Quantity Discount
m = $100
50
100
20 80
x2
x1
Budget Set
Budget Constraint
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Shapes of Budget Constraints with a Quantity Penalty
x2
x1
Budget Set
Budget Constraint
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Shapes of Budget Constraints - One Price Negative
Commodity 1 is stinky garbage. You are paid $2 per unit to accept it; i.e. p1 = - $2. p2 = $1. Income, other than from accepting commodity 1, is m = $10.
constraint ?
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More General Choice Sets
Choices are usually constrained by more than a budget; e.g. time constraints and other resources constraints.
A bundle is available only if it meets every constraint.
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More General Choice Sets
Food
Other Stuff
10
At least 10 units of foodmust be eaten to survive
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More General Choice Sets
Food
Other Stuff
10
Budget Set
Choice is also budgetChoice is also budgetconstrained.constrained.
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More General Choice Sets
Food
Other Stuff
10
Choice is further restricted by a time constraint.
35
More General Choice Sets
So what is the choice set?So what is the choice set?
36
More General Choice Sets
Food
Other Stuff
10
37
More General Choice Sets
Food
Other Stuff
10
38
More General Choice Sets
Food
Other Stuff
10
The choice set is theintersection of all ofthe constraint sets.