Proposition 2: In cases (i), (iv) and (v),
∂2 Pr ∂ x*i
( a ) If i = 0, then Sign ⅛") = Sign (ηi ) .
∂xi ∂x ∂ I
ij
∂2Pr
( b ) If------,- ≠ 0
ij
, then (1) Sign (— ) = Sign (η)i ⇔ Sign (
∂I
∂2 Pr
д ʌ/ η j )= Sign (η i )
∂xi ∂x
ij
and
∂x >
(2) -> - 0 ⇔
∂I <
∂2 Pri -
∂2Prj
--------η i n,
∂xj∂xi
. η j n 7
∂xi <
By Proposition 2 (a), if the contestants are symmetric in equilibrium in terms of their
abilities , then the strategic rival’-stake (“substitution”) effects vanish (efforts are
independent) and the positive strategic own-stake (“income”) effect solely determines
the direct effect of a change in I on a contestant’s effort. In the perfectly symmetric
case where, ∀x and x , Pr (x ,x ) = 1 -Pr (x ,x ) and n = n = n , there
H L HLH HHL H L
∂2 PrL
exists a symmetric pure strategy Nash equilibrium, x*H = x *L, and -----—
∂x ∂x
LH
∂2 Pr
-----— = 0 , see Dixit (1987). Hence, by Proposition 2 (a),
∂x ∂x
HL
Corollary 2.1: If, ∀xH and xL, PrH (xL,xH) = 1 - PrH (xH,xL) and nH = nL = n , then
∂**
xh ∂xL
-------= ------- 0 .
∂n ∂n
This corollary generalizes a similar result established by Nti (1999), assuming a
particular contest success function of the logit form.
Proposition 2(b) can be used to determine the sensitivity of the contestants’
efforts in all possible situations corresponding to the three types of policy reforms
∂2 Pri
affecting both stakes and------≠ 0. Consider for example a type (i) policy reform
∂xi ∂x
ij
∂2 PrH
and suppose that -----— < 0, that is, the effort of the HB player is a strategic
∂x ∂x
HL
substitute of the effort of the LB player. By Proposition 2 (b), in such a case,
15
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