Domains of social choice functions on which coalition
strategy-proofness and Maskin monotonicity areequivalent∗
Koji TAKAMIYA†
Institute of Social and Economic Research,
Osaka University
29 June 2006

Abstract
It is known that on some social choice and economic domains, a social
choice function is coalition strategy-proof if and only if it is Maskin monotonic(e.g. Muller and Satterthwaite, 1977). This paper studies the foundation of
those results. I provide a set of conditions which is sufficient for the equivalence between coalition strategy-proofness and Maskin monotonicity. This
generalizes some known results.
JEL Classification— C71, C72, D71.
Keywords— social choice function, coalition strategy-proofness, Maskin
monotonicity.

0

Introduction

This paper examines logical relations between coalition strategy-proofness and
Maskin monotonicity of social choice functions. Coalition strategy-proofness isa strong requirement of incentive compatibility. A social choice function is said tobe coalition strategy-proof if no group of individuals can benefit from jointly
misrepresenting their preferences, in other words, cannot manipulate the final outcome. A social choice function is said to be Maskin monotonic if the outcome
to be chosen by the function does not vary whenever each individual switches his
preference keeping or improving the relative ranking of that outcome. This property is very important in implementation theory. For example, it is well-known asa necessary condition for Nash implementation (see Maskin, 1985, 1999).
It has been observed that these two properties are strongly related to eachother. The classical result by Muller and Satterthwaite (1977) asserts that on the∗Part of the research has been done while I visited to the Indian Statistical Institute, Delhi
Centre. I am very grateful to Arunava Sen for his hospitality and helpful comments. And I thankthe people of the institute for their hospitality. All errors are my own responsibility.
†
Institute of Social and Economic Research, Osaka University. 6-1 Mihogaoka Ibaraki Osaka
567-0047 JAPAN. tel: +81-6-6879-8556. E-mail: takamiya@iser.osaka-u.ac.jp.

1unrestricted strict preference domain, a social choice function is strategy-proof if
and only if it is Maskin monotonic.1 Since on this domain, strategy-proofness
is equivalent to coalition strategy-proofness, the theorem states the equivalence
between coalition strategy-proofness and Maskin monotonicity.
In more recent studies (e.g. Svensson, 1999; Takamiya 2001, 2003), it has
been pointed out that coalition strategy-proofness and Maskin monotonicity areequivalent for allocation rules in a certain broad class of economies with indivisible
goods, which includes some notable problems such as “marriage problems” (Galeand Shapley, 1962) and “housing markets” (Shapley and Scarf, 1974). Also insome other environments, for example the classical exchange economies, it is known
that coalition strategy-proofness implies Maskin monotonicity (e.g. Barberà andJackson, 1995).
The purpose of this paper is to study the foundation of these close relationships
between coalition strategy-proofness and Maskin monotonicity. I examine what
conditions the domain of the social choice function should satisfy in order to havethe property that coalition strategy-proofness implies Maskin monotonicity, andits converse.
The main results of this paper provide two sufficient conditions. The first
condition, which is referred to as Condition A, is a sufficient condition for thatcoalition strategy-proofness implies Maskin monotonicity. This condition requires
the domain of the social choice function to satisfy two properties. The first property says that for any individual, and any preference admissible to him, if any twoalternatives are indifferent under this preference, then these alternatives are indifferent under all the preferences admissible to him. In other words, this requiresthat every individual has a partition of the set of alternatives, and his admissible
preferences contains only (but not necessarily all of) such preferences that any twoalternatives are indifferent if, and only if, these alternatives are in the same cellof the partition. The second property that Condition A requires is that if for any
preference profile in the domain, there is no pair of alternatives such that all individuals are indifferent between them. That is to say, if the alternative to be chosen
shifts from one to another, then there is always someone who cares about thisshift. This is equivalent to the requirement that the “join” (the coarsest common
refinement) of the partitions of all the individuals that arise in the first half of thiscondition equals to the finest partition (i.e. the one in which each cell contains
exactly one element).
The second condition, referred to as Condition B, is a sufficient condition forthat Maskin monotonicity implies coalition strategy-proofness. This condition isdefined as follows: Let any coalition be given. And pick up any preference profile forthis coalition, which I call the first profile. Then let us fix any two alternatives, sayx and y, such that y (weakly) Pareto dominates x within this coalition under the
first profile. Further, pick up another arbitrary preference profile for this coalition,
the second profile. Then this domain satisfies Condition B if the domain contains
at least one preference profile for this coalition such that x keeps or improves its
1“Strategy-proofness” requires that the social choice function cannot be manipulated by any
single individual. That is, unlike coalition strategy-proofness, manipulations by groups are notnecessarily ruled out.

2

relative ranking from the first profile to this profile, and so does y from the second
profile to this profile. Speaking very roughly, the third profile is a mixture of
the first and the second profiles in the sense of the desirability of x and y. And
Condition B requires such a mixture always exists.
Given these two sufficient conditions, we present a class of domains which
satisfies both of these conditions. This class is the collection of those domains
in which (i) every individual has a partition of the set of alternatives, and his
admissible preferences are exactly such preferences that any two alternatives are
indifferent if, and only if, these alternatives are in the same cell of the partition,and (ii) the join of all these partitions equals to the finest partition. Paraphrasing,such a domain is the maximal domain among those satisfying Condition A, given
a list of partitions. I call these domains essentially strict domains.
I point out that essentially strict domains are assumed in some previous results. This observation unifies the the Muller-Satterthwaite theorem and the similar equivalence theorem by Takamiya (2003) in the context of the “generalized indivisible good allocation problems” (Sönmez, 1999), which cover various problems
including well-known “housing markets” (Shapley and Scarf, 1974) and “marriage
problems” (Gale and Shapley, 1962).

1

Preliminaries

Let N denote the set of individuals. Assume that N is a nonempty finite set.
Call any nonempty subset of N a coalition. Let X be the set of alternatives(social outcomes). X is nonempty and may be finite or infinite.
Let Q be a nonempty set. Then denote by W (Q) the set of weak orderings (i.e.
complete and transitive binary relations) on Q. And denote by L(Q) the set of
linear orderings (i.e. complete, tansitive and anti-symmetric binary relations) on
Q.
For i ∈ N , call Ri ∈ W (Q) a preference relation on Q of individual i. Anda list (Ri )i∈N is called a preference profile. For x, y ∈ Q, xRi y reads that toindividual i, x is at least as good as y. As usual, P i denotes the asymmetricpart, and I i denotes the symmetric part of Ri . Let Ri ∈ W (Q) and Q0 ⊂ Q. Then
max Ri (Q) denotes the set of Ri -maximal elements in Q0 , {x ∈ Q0 | ∀y ∈ Q0 , xRi y}.
For i ∈ N , Di denotes the set of admissible preferences of individual i.
Assume that Di ⊂ W (X) for any i ∈ N . For S ⊂ N , DS denotes the CartesianQ
product i∈S Di . A social choice function (SCF) is a single-valued function
f : DN → X. DN is called the domain (of f ).
Let f be a SCF. Let S be a coalition. Then we say that S manipulates f at apreference profile R ∈ DN if there exists some R0S ∈ DS such that[∀i ∈ S, f (R−S , R0S )Ri f (R)] & [∃j ∈ S : f (R−S , R0S )P j f (R)].

(1)

Call f coalition strategy-proof if no coalition manipulates f at any R ∈ DN .
For Ri ∈ W (X), and x ∈ X, denote by L(x, Ri ) the set {y ∈ X | xRi y}. That
is, L(x, Ri ) is the lower-contour set of x relative to Ri . Call f Maskin monotonic3

if for any R, R0 ∈ DN ,[f (R) = x & (∀i ∈ N, L(x, Ri ) ⊂ L(x, R0i ))] ⇒ f (R0 ) = x.

2(2)

Main results

This section presents the main results. These results provide sufficient conditions
that the domain of the social choice function should satisfy to have the property
that coalition strategy-proofness implies Maskin monotonicity, and its converse. I
introduce these two sufficient conditions. Let a SCF f : DN → X be given.
Let P i be a partition of X. For x ∈ X, let us denote the cell of P i that contains x by P i (x). Let (P i )i∈N be a profile of partitions of X. And let us denoteWby “ ” the operation of taking the “join” (the coarsest common refinement) of thepartitions.
Conditon A. There exists some profile of partitions (P i )i∈N such that for any
R ∈ DN , any i ∈ N and any x, y ∈ X,
x ∈ P i (y) ⇔ xI i y,(3)

P i = {{x} | x ∈ X}.

(4)

and
_
i∈N

In words, Condition A consists of two parts, which correspondence to the formulas (3) and (4), respectively:(i) For any individual, and for any preference admissible to him, any two alternatives are indifferent under this preferences if, and only if, these alternatives are
indifferent under all the preferences admissible to him; and(ii) If for any preference profile, for any pair of alternatives there is at least one
individual who is not indifferent between these alternatives.
Condition B. For any S ⊂ N with S 6= ∅, any R̃S , R̂S ∈ DS , and any x, y ∈ Xsuch that (∀i ∈ S, y R̃i x) and (∃i ∈ S : y P̃ i x), there exists R?S ∈ DS such that∀i ∈ S, L(x, R̃i ) ⊂ L(x, R?i ) & L(y, R̂i ) ⊂ L(y, R?i ).

(5)

In words, Condition B condition is defined as follows: Let any coalition be
given. And pick up any preference profile for this coalition, which I call the firstprofile. Then let us fix any two alternatives, say x and y, such that y (weakly)
Pareto dominates x within this coalition under the first profile. Further, pick up
another arbitrary preference profile for this coalition, the second profile. Then thedomain satisfies Condition B if the domain contains at least one preference profile
for this coalition such that x keeps or improves its relative ranking from the firstprofile to this profile, and so does y from the second profile to this profile. Roughly
speaking, the third profile is a mixture of the first and the second profiles in thesense of the desirability of x and y. And the condition requires such a mixture
always exists.
4

Theorem 1 Let DN satisfy Condition A. Then if f is coalition strategy-proof,
then f is Maskin monotonic.
Proof Suppose that DN satisfies Condition A and that f is not Maskin monotonic.
Then I will show that f is not coalition strategy-proof. Since f is not Maskin
monotonic, we have for some i ∈ N , some R ∈ DN , and some R̃i ∈ Di ,L(x, Ri ) ⊂ L(x, R̃i ) & f (R−i , R̃i ) 6= x,(6)where x denotes the alternative f (R). Let us denote f (R−i , R̃i ) by y. Since DNsatisfies Condition A, there must be at least one individual j such that xP j y or
yP j x. Thus the set T = {j ∈ N | ¬xI j y} is nonempty. Then there are two cases.
(i) Assume that i ∈ T . Then either xP i y or yP i x. Suppose that xP i y holds
true. Then L(x, Ri ) ⊂ L(x, R̃i ) and Condition A together imply xP̃ i y. Thatis, f (R−i , Ri )P̃ i f (R−i , R̃i ), which says i manipulate at (R−i , R̃i ) by reporting Ri .
Thus f is not coalition strategy-proof.
In turn, suppose that yP i x holds true. Then similarly, i manipulate at R byreporting R̃i , which violates coalition strategy-proofness again.
(ii) Assume that i 6∈ T . Then xI i y. Let j ∈ T , which means either xP j y orj
yP x. Suppose that xP j y. Then similarly to the case (i), {i, j} manipulates at(R−{i,j} , R̃i , Rj ) by reporting (Ri , Rj ). In turn suppose that yP j x. Then {i, j}
manipulates at (R−{i,j} , Ri , Rj ) by reporting (R̃i , Rj ). In either way, f is notcoalition strategy-proof. 2
Theorem 2 Let DN satisfy Condition B. Then if f is Maskin monotonic, then f
is coalition strategy-proof.
Proof Suppose that DN satisfies Condition B and that f is Maskin monotonic but
not coalition strategy-proof. Then there is some coalition S which manipulates atsome R̃ ∈ DN by reporting R̂S ∈ DS . Let us denote the alternative f (R̃) by x, andf (R̃−S , R̂S ) by y. Then clearly, ∀i ∈ S, y R̃i x and ∃i ∈ S : y P̃ i x. Thus Condition
B implies that there is R?S ∈ DS such that for all i ∈ S,
L(x, R̃i ) ⊂ L(x, R?i ),i?iL(y, R̂ ) ⊂ L(y, R ).

(7)
(8)

Since f is Maskin monotonic, (6) implies f (R̃−S , R?S ) = x. On the other hand,(7) implies f (R̃−S , R?S ) = y. These imply x = y, a contradiction. 2

3

Further results

This section provides a class of domains which satisfies both Conditions A and
B. Then applying the results presented in Section 2, some previous results will bederived.

5

Essentially strict domain. Let (P i )i∈N be a profile of partitions of X. Then
DN is said to be the essentially strict domain with respect to (P i )i∈N if DN
is the collection of all the preference profiles R that satisfy for any x, y ∈ X,
x ∈ P i (y) ⇔ xI i y,(9)

P i = {{x} | x ∈ X}.

(10)

and (P i )i∈N satisfies_i∈N

To paraphrase, the essentially strict domain with respect to (P i )i∈N is the(inclusion) maximal domain among those which satisfy Condition A given (P i )i∈N .
It is less obvious that such a domain satisfies Condition B.
Lemma 1 If DN is the essentially strict domain with respect to (P i )i∈N , then DN
satisfies Condition B.
Proof Let S ⊂ N with S 6= ∅, and x, y ∈ X. Let R̃S ∈ DS such that (∀i ∈S, y R̃i x) & (∃i ∈ S, y P̃ i x), and R̂S ∈ DS . To show that DN satisfies Condition
B, we will give R?S ∈ DS such that∀i ∈ S, L(x, R̃i ) ⊂ L(x, R?i ) & L(y, R̂i ) ⊂ L(y, R?i ).

(11)

Let S + = {i ∈ S | y P̃ i x}, and S 0 = {i ∈ S | y I˜i x}. Let R?S be such that for eachi ∈ S+,
max R?i (X) = P i (y), and
?i

?i

i

(12)

max R (X \ max R (X)) = P (x),(13)

max R?i (X) = P i (y).

(14)

and for each i ∈ S 0 ,Note that for i ∈ S 0 , P i (x) = P i (y). Then evidently, L(y, R?i ) = X for any i ∈ S;
L(x, R?i ) = X for any i ∈ S 0 ; and L(x, R?i ) = X \ P i (y) for any i ∈ S + . Notethat L(x, R̃i ) ∩ P i (y) = ∅. Thus R?S satisfies (11). 2
Now we obtain the following result applying Theorems 1 and 2.
Theorem 3 Let DN be an essentially strict domain. Then f is coalition strategyproof if, and only if, f is Maskin monotonic.
In the following, we will derive two known results as special cases of Theorem 3.
First, let the partition profile (P i )i∈N be such that for each i, P i is {{x} | x ∈ X}.
Then the essentially strict domain with respect to (P i )i∈N coincides with L(X)N .
This yields the well-known Muller-Satterthwaite theorem.
Corollary 1 (Muller and Satterthwaite, 1977) Let DN = L(X)N . Then f iscoalition strategy-proof if, and only if, it is Maskin monotonic.
6

The second application is for “generalized indivisible good allocation problems,”
as defined in Sönemz (1999). This class of allocation problems contains well-known“marriage problems” (Gale and Shapley, 1962) and “housing markets” (Shapley
and Scarf, 1974) as special cases. A generalized indivisible good allocation
problem (an allocation problem, henceforth) is a list (N, Ω, A, R).2 Here N is
the set of individuals, as we have defined in Section 1. Ω is the set of goods, whichis assumed to be a nonempty finite set. An “allocation” is a set-valued function
x : N →→ Ω such that {x(i) | i ∈ N } is a partition of Ω. A is the set of feasible
allocations. And R is a preference profile belonging to the domain DN defined asfollows:
DN := {R | ∀i ∈ N, Ri ∈ W (A) & (∀x, y ∈ A, xI i y ⇔ x(i) = y(i))}.

(15)

That is, every individual has preferences that exhibit no consumption externalities,
and are strict over their own assignments.
Obviously, in this case, DN is an essentially strict domain with respect toi(P )i∈N , where for each i ∈ N , P i is the partition such that for any x, y ∈ A,
x ∈ P(y) ⇔ x(i) = y(i).
In this setting, we consider the set of allocation problems {(N, Ω, A, R) | R ∈DN }, and SCFs f : DN → A. Then we have the following known result as acorollary to Theorem 3.
Corollary 2 (Takamiya, 2003) Let f be a SCF in a setting of allocation problems.
Then f is coalition strategy-proof if, and only if, it is Maskin monotonic.

References[1] Barberà S and Jackson MO (1995) Strategy-proof exchange. Econometrica 63: pp51–
87[2] Gale D and Shapley L (1962) College admissions and the stability of marriage. AmerMath Monthly 69: pp9–15.
[3] Ishikawa S and Nakamura K (1979) The strategy-proof social choice functions. J MathEcon 6: pp283–95.
[4] Maskin E (1985) The theory of implementation in Nash equilibrium: A survey. Hurwicz L, Schmeidler D and Sonnenschein H (eds) Social Goals and Social Organization.
Cambridge University Press, Cambridge, pp 173–04[5] Maskin E (1999) Nash equilibrium and welfare optimality. Rev Econ Stud 66: pp23–
38.
[6] Muller E and Satterthwaite M (1977) The equivalence of strong positive associationand strategy-proofness. J Econ Theory 14: pp412–18.
[7] Shapley L and Scarf H (1974) On cores and indivisibilities. J Math Econ 1: pp23–27.
[8] Sönmez T (1999) Strategy-proofness and essentially single-valued cores. Econometrica
67: pp677–89.
2

The complete definition of the problem includes initial endowments, which are superfluous
for the present purpose thus dropped here.

7[9] Svensson L-G (1999) Strategy-proof allocation of indivisible goods. Soc Choice Welfare 16: pp557–67.
[10] Takamiya K (2001) Coalition strategy-proof and monotonicity in Shapley-Scarf housing markets. Math Soc Sciences 41: pp201–13.
[11] Takamiya K (2003) On strategy-proofness and essentially single-valued cores: A converse result. Soc Choice Welfare 20: pp77–83.

8