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May 24, 2024

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Economics 11 Week 5 Sungwoo Cho * April 2024 I Duality ¯ U = V ( p x , p y , I ) and E ( p x , p y , ¯ U ) = I Marshallian to Hicksian g x ( p x , p y , E ( p x ,p y , ¯ U ) ) = h x ( p x ,p y , ¯ U ) and g y ( p x , p y , E ( p x ,p y , ¯ U ) ) = h y ( p x ,p y , ¯ U ) Hicksian to Marshallian h x ( p x ,p y , V ( p x , p y , I ) ) = g x ( p x , p y ,I ) and h y ( p x ,p y , V ( p x , p y , I ) ) = g y ( p x , p y ,I ) * Contact: chosw13@ucla.edu . If you find any typo or error please send me an email. 1
II Income and Substitution Effects Remark : A change in price causes two effects known as the substitution and income effect. As the price of x decreases the consumer becomes richer, this is known as the income effect. Also as the price of x decreases x becomes relatively cheaper than y, this is known as the substitution effect. Large price changes Total effect The difference between the new consumption bundle and the old consumption bundle is called the total effect. The next graph shows that the budget constraint rotates to the right. The new consumption bundle is marked by point B. Total effect x = g x ( p 0 x ,p y ,I ) - g x ( p x ,p y ,I ) p 0 x - p x Total effect y = g y ( p 0 x ,p y ,I ) - g y ( p x ,p y ,I ) p 0 x - p x Substitution effect The substitution effect measures how much the consumer would substitute x for y given the price change holding utility constant. How do we compute demand while holding utility constant? The substitution effect is the movement from point A to point C. 2
Note that point A is the initial optimal consumption bundle. To get point C, we shift the new budget constraint (which slope is the new price ratio) towards the indifference curve with the initial level of utility. That is, towards the indifference curve where point A is located. Note that A and C belong to the same indifference curve, thus we are holding constant the utility level. The Hicksian demand functions do precisely this! Substitution effect x = h x ( p 0 x ,p y , ¯ U ) - h x ( p x ,p y , ¯ U ) p 0 x - p x Substitution effect y = h y ( p 0 x ,p y , ¯ U ) - h y ( p x ,p y , ¯ U ) p 0 x - p x Income effect The income effect measures the change in demand given an increase in income holding the price ratio constant. How do we compute demand holding the price ratio constant? The income effect is the movement from point C to point B. Note that point B is determined by the tangency of the new budget constraint (slope p 0 x p y ) to the indifference curve U 2 . The point C is determined by the tangency of the auxiliary budget constraint that has slope p 0 x p y and is tangent to the indifference curve U 1 . That is, we are holding constant the price ratio, by comparing two points were the slope is equal to p 0 x p y . 3
Income effect x = Total effect x - Substitution effect x Income effect x = g x ( p 0 x ,p y ,I ) - g x ( p x ,p y ,I ) p 0 x - p x - h x ( p 0 x ,p y , ¯ U ) - h x ( p x ,p y , ¯ U ) p 0 x - p x Income effect y = Total effect y - Substitution effect y Income effect y = g y ( p 0 x ,p y ,I ) - g y ( p x ,p y ,I ) p 0 x - p x - h y ( p 0 x ,p y , ¯ U ) - h y ( p x ,p y , ¯ U ) p 0 x - p x Small price changes Slutsky Equation Start from the duality result: g x ( p x , p y , E ( p x ,p y , ¯ U )) = h x ( p x ,p y , ¯ U ) Let’s take the partial derivative with respect to p x and use the fact : I = E ( p x ,p y , ¯ U ) ∂g x ( p x , p y , I ) ∂p x + ∂g x ( p x , p y , I ) ∂I ∂E ( p x ,p y , ¯ U ) ∂p x = ∂h x ( p x ,p y , ¯ U ) ∂p x By Shephard’s Lemma: ∂g x ( p x , p y , I ) ∂p x + ∂g x ( p x , p y , I ) ∂I h x ( p x ,p y , ¯ U ) = ∂h x ( p x ,p y , ¯ U ) ∂p x By duality: ∂g x ( p x , p y , I ) ∂p x + ∂g x ( p x , p y , I ) ∂I g x ( p x ,p y ,I ) = ∂h x ( p x ,p y , ¯ U ) ∂p x Re-arrange terms: ∂g x ( p x , p y , I ) ∂p x | {z } Total Effect = ∂h x ( p x ,p y , ¯ U ) ∂p x | ¯ U = V | {z } Substitution Effect - ∂g x ( p x , p y , I ) ∂I g x ( p x ,p y ,I ) | {z } Income Effect 4
Note: We evaluate the derivative of the Hicksian demand at the indirect utility function to make the equation consistent and to express everything in terms of prices and income. Derivation: cross-price Slutsky Equation Start from the duality result: g x ( p x , p y , E ( p x ,p y , ¯ U )) = h x ( p x ,p y , ¯ U ) Let’s take the partial derivative with respect to p y and use the fact : I = E ( p x ,p y , ¯ U ) ∂g x ( p x , p y , I ) ∂p y + ∂g x ( p x , p y , I ) ∂I ∂E ( p x ,p y , ¯ U ) ∂p y = ∂h x ( p x ,p y , ¯ U ) ∂p y By Shephard’s Lemma: ∂g x ( p x , p y , I ) ∂p y + ∂g x ( p x , p y , I ) ∂I h y ( p x ,p y , ¯ U ) = ∂h x ( p x ,p y , ¯ U ) ∂p y By duality: ∂g x ( p x , p y , I ) ∂p y + ∂g x ( p x , p y , I ) ∂I g y ( p x ,p y ,I ) = ∂h x ( p x ,p y , ¯ U ) ∂p x y Re-arrange terms: ∂g x ( p x , p y , I ) ∂p y | {z } Total Effect = ∂h x ( p x ,p y , ¯ U ) ∂p y | ¯ U = V | {z } Substitution Effect - ∂g x ( p x , p y , I ) ∂I g y ( p x ,p y ,I ) | {z } Income Effect Price changes for normal goods: If a good is normal, substitution and income effects reinforce one another: when the price falls, both effects lead to a rise in quantity demanded. Price changes for inferior goods: If a good is inferior, substitution and income effects move in opposite directions: the combined effect is indeterminate. When the price falls, the substitution effect leads to a rise in quantity demanded, but the income effect moves in the opposite direction Giffen’s paradox For an inferior good, if the income effect dominates the substitution effect an increase in price generates an increase in demand: An increase in price leads to a drop in real income. Since the good is inferior, a drop in income causes the quantity demanded to rise. If the income effect is greater than the substitution effect consumption increases. 5
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