Problem Set 8
Solutions
Economics 1051
Spring 2009
1. Bargaining. It is November 1st, and Angus and Bronwen are arguing
over how to divide the remainder of their left-over trick or treat candy. They
decide on the f
...
Problem Set 8
Solutions
Economics 1051
Spring 2009
1. Bargaining. It is November 1st, and Angus and Bronwen are arguing
over how to divide the remainder of their left-over trick or treat candy. They
decide on the following rules. On Nov. 1st, Angus will make Bronwen an offer
[sA(1); sB(1)] where sA(1) + sB(1) = 1. If Brownen accepts then Angus gets a
share sA(1) of the candy and Brownen gets a share sB(1). If Brownen refuses,
the noone gets any candy on Nov 1st. In this case, on Nov. 2nd, Brownen gets
to make Angus an offer [sA(2); sB(2)] where sA(2)+sB(2) = 1. If Angus accepts
then Angus gets a share sA(2) of the candy and Brownen gets a share sB(2).
If Angus refuses, then noone gets a candy on Nov 2nd. In this case, on Nov.
3rd, Angus will make Bronwen an offer [sA(3); sB(3)] where sA(3) + sB(3) = 1.
If Brownen accepts then Angus gets a share sA(3) of the candy and Brownen
gets a share sB(3). If Brownen refuses, the noone gets any candy on Nov 3rd.
In this case, on Nov. 4th, Brownen gets to make Angus an offer [sA(4); sB(4)]
where sA(4) + sB(4) = 1. If Angus accepts then Angus gets a share sA(4) of
the candy and Brownen gets a share sB(4). If Angus refuses, however, then all
the candy is thrown away by their parents.
There is a catch. Both Angus and Bronwen are hungry and impatient for
candy so they discount candy tomorrow. For Angus, each day, one candy tomorrow is worth only δA candy today where δA < 1. For Bronwen, each day, one
candy tomorrow is worth only δB candy today where δB < 1. It is commonly
known that Angus and Bronwen are rational and that they are not interested
in distributive justice. You may also assume that each of Angus and Bonwen
accepts an offer whenever he or she is indifferent.
a) What do you expect to happen, and what allocation of candy would arise
if δA = δB? Explain your reasoning carefully.
Solution. We use the backward induction. On day 4, Brownen will offer
[sA(4); sB(4)] = [0; 1]: indeed, Angus must accept any offer ["; 1 − "] for " > 0,
and is indifferent between accepting and rejecting offer [0; 1]; hence, equilibrium
requires that Brownen offers [0; 1] and Angus accepts. On day 3, Angus anticipates this, and knows that he must offer Brownen at least δB = δ: if he offers
less, then Brownen would prefer 1 on day 4 to this smaller amount on day 3.
Hence, [sA(3); sB(3)] = [1 − δ; δ].
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