GCOE Discussion Paper Series
Global COE Program

Human Behavior and Socioeconomic Dynamics
Discussion Paper No.191

What factors determine the number of trading partners?

Noriaki Matsushima and Ryusuke Shinohara

April 2011

GCOE Secretariat
Graduate School of Economics

OSAKA UNIVERSITY

1-7 Machikaneyama, Toyonaka, Osaka, 560-0043, Japan

What factors determine the number of trading partners?∗
Noriaki Matsushima†
Institute of Social and Economic Research, Osaka University
Ryusuke Shinohara‡
Faculty of Economics, Shinshu University
April 22, 2011

Abstract
The purpose of the paper is to provide a simple model explaining buyer-supplier
relationships and show what factors determine the number of trading partners. We
show that when the supplier is able to determine the number of trading partners, theoptimal number is small if the supplier’s bargaining power with them is weak, theeconomy of scope in the supplier’s variable costs is signiﬁcant, and that in its sunkinvestment is weak. Investment may be greater when the number of trading partners
is small. The results may be consistent with the formation of Japanese buyer-supplier
relations.

Keywords: buyer, supplier, investment, economies of scope
JEL Classiﬁcation codes: L14, M11∗

We would like to thank Shingo Ishiguro, Hideshi Itoh, Shinsuke Kambe, Toshihiro Matsumura, Takashi
Shimizu and the seminar participants at Contract Theory Workshop (Kwansei Gakuin University) and Law
and Economics Workshop (Hitotsubashi University) for their helpful comments. We would like to thankRisa Gotoh and Shizuru Yamagiwa for their research assistance. Any errors are the responsibility of theauthors.
†
Noriaki Matsushima, Institute of Social and Economic Research, Osaka University, Mihogaoka
6-1, Ibaraki, Osaka 567-0047, Japan. Phone: (81)-6-6879-8571. Fax: (81)-6-6879-8583. E-mail:nmatsush@iser.osaka-u.ac.jp‡
Ryusuke Shinohara, Faculty of Economics, Shinshu University, 3-1-1 Asahi, Matsumoto, Nagano, 3908621, Japan. Phone: (81)-263-37-2951. Fax: (81)-263-37-2344. E-mail: ryusukes@shinshu-u.ac.jp1

I

Introduction

Manufacturers have sought the optimal number of buyers, which aﬀects revenues from
customers and manufacturers’ ﬁxed/variable costs including labor, material, and investment. For instance, in the context of patent licensing, licensors determine the numberof licensees (Kamien and Tauman (1986) and Kamien et al. (1992)).1 In the context ofbuyer-suppler relations, suppliers consider whether strategies with broader customer scope
lead to superior performance (Nobeoka et al. (2002)). In fact, in the Toyota keiretsu group(recognized as among the tightest), 41.7 percent of its aﬃliated ﬁrms (deﬁned here as thosethat are more than 20 percent owned by Toyota) sold 40–80 percent of their products tooutsiders (Sato (1988, p. 121) and Nishiguchi (1994, p. 115)). This fact implies that some
suppliers choose a broader customer scope, while others choose a narrower scope. In otherwords, taking into account their technological environments, including the values of theirproducts, production eﬃciencies, and investment capabilities, suppliers choose their optimal number of customers. We therefore investigate what factors determine the number of
trading partners (the customer scope).
We provide a simple model to explain the strategies of suppliers. The setting is asfollows. Consider a situation in which there is one supplier and two buyers. The suppliercan provide a good that is used by the two buyers. However, the buyers cannot produce
the good on their own. In this situation, the supplier ﬁrst decides with whom to negotiate.
Second, the supplier invests to improve the value of the good for the buyer(s) and at thisstage decides the amount of investment. The investment cost is sunk before the next stage.
The (additional) sunk investment cost for the second buyer (the second unit of the good)is smaller than that for the ﬁrst buyer. Third, there are negotiations between the supplierand the buyer(s) who were designated by the supplier in the ﬁrst stage. In this stage, weapply a simple Nash bargaining approach used by Chipty and Snyder (1999) and Raskovich
1

Kamien and Tauman (1986) and Kamien et al. (1992) and subsequent researchers investigate how a(monopolistic) patent holder determines the number of licensees that compete in ﬁnal product markets.
Downstream competition is an essential factor in the adoption of exclusive licensing oﬀers for the licensees
in those papers.

2(2003).2 The characteristic of this approach is that if the supplier chooses to bargain withtwo buyers in the ﬁrst stage, the supplier and each buyer conduct simultaneously andseparately the Nash bargaining. The supplier only provides and sells the good to thebuyer if its variable production cost is covered as a result of the bargaining. When the
supplier produces the good for the buyer(s), it incurs a variable production cost. The(additional) variable production cost for the second buyer is smaller than that for the ﬁrst
buyer.
We show several results that may be consistent with the formation of Japanese buyer–
supplier relations.3 When the supplier is able to determine the number of trading partners—
one or two—the optimal number is one for the supplier if the supplier’s bargaining powerwith its trading partners is weak, and the supplier’s additional sunk investment cost is
relatively large. The supplier prefers to trade with one buyer if the variable production
cost is neither large nor small relative to the value of the good. In other words, when
the supplier’s variable production cost is small, it should have broader relationships withpartners. As the product eﬃciency of the supplier improves, the performance (payoﬀ) of
the supplier becomes higher because of the higher investment level caused by the broader
customer scope. This seems consistent with the ﬁnding in Nobeoka et al. (2002): a
broader customer scope strategy should result in superior performance, primarily becauseof superior learning opportunities. We also show that the equilibrium investment level
when the supplier trades with one buyer can be larger than with two buyers if the sunkinvestment cost is large relative to the value of the good, the supplier’s bargaining powerwith its trading partners is weak, and the variable production cost is large.4 This may
2

For theoretical discussions of bargaining solutions, see, for instance, Binmore et al. (1986), Chae andYang (1994), Krishna and Serrano (1996), and Okada (1996, 2010).
3

Konishi et al. (1996) formulate a bargaining model of buyer–seller relationships from the buyer’s

viewpoint. Their purpose is to provide an alternative solution for the hold-up problems, in contrast to
the vertical integration approach advocated by Grossman and Hart (1986) and Hart and Moore (1990).
Although their model provides a plausible explanation why typical auto assemblers in Japan do not vertically integrate with a single part supplier but transact with two potentially competitive suppliers, they
do not explain suppliers’ decisions concerning the number of buyers.
4

The outcome in this paper (a narrower customer scope) may evoke the term “exclusive dealing,”3

also be consistent with the following ﬁnding. Japanese automakers and their suppliers
are more specialized than their US counterparts, and there is a high correlation between
supplier specialization and automaker proﬁtability (Dyer (1996)). Although this statement is based on the viewpoint of buyers (automakers), the correlation may be because
the higher investment levels caused by the narrower customer scope of suppliers leads tohigher proﬁtability of the automakers.
This paper is closely related to those of Chipty and Snyder (1999) and Raskovich(2003).5 However, to analyze a buyer–supplier relation with cooperative investments, we
add two new elements to these papers. One is the supplier’s decision regarding the numberof buyers, and the other is the supplier’s investment to improve the quality of the buyers’
products. Inderst and Wey’s (2003) study is also closely related to ours. They discuss
how the equilibrium market structure is determined in a bilateral oligopoly with choice oftechnology. Although they comprehensively discuss bargaining, mergers, and technology
choice in simple bilateral oligopolistic markets, they do not discuss the relation between
bargaining power and equilibrium market structure (the number of trading partners) as
our paper does.
This paper is also related to literature on the source of buyer power.6 The model
structure in this paper is related to those in Battigalli et al. (2007) and Inderst and Wey(2007, 2010). Those papers discuss situations in which a monopoly upstream supplier sells
an input to downstream ﬁrms (buyers) and engages in quality-enhancing/cost-reducing
investments. The number of buyers is exogenously ﬁxed in those papers.
This paper is relevant to the literature on the hold-up problem because in our model
the supplier’s investment is not fully compensated by the buyers (a classic form of “holdalthough the main concern is quite diﬀerent from those in the literature on exclusive dealing. Several
papers discuss whether exclusive contracts foster relationship-speciﬁc investment by an incumbent supplier(e.g., Segal and Whinston (2000) and de Meza and Selvaggi (2007)). They discuss whether the exclusionof potential suppliers enhances the incentive of the incumbent supplier to invest.
5

Stole and Zwiebel’s (1996a, b) study is also relevant to the discussion in Chipty and Snyder (1999).

6

Inderst and Mazzarotto (2008) and Inderst and Shaﬀer (2007) provide surveys in discussions of buyer

power. Rey and Tirole (2007) provide a comprehensive survey of vertical relations.

4up”). The literature mainly discusses ways to overcome the hold-up problem (e.g., Klein
et al. (1978) and Williamson (1979)) and examines a pair of buyers and sellers in isolation,
whereas this paper considers the hold-up problem in a situation with multiple buyers.7
This study is also relevant to studies of buyer and seller networks. While there aremany papers discussing buyer–seller networks (e.g., Kranton and Minehart (2000, 2001))
that compare vertically integrated ﬁrms and networks of manufacturers and suppliers, thepurpose of this paper is diﬀerent.8 Although in most of these papers product quality is
exogenously given, we discuss quality investment by the upstream supplier who anticipates
subsequent negotiations between the buyers.
The remainder of this paper is organized as follows. Section 2 presents the basic model.
Section 3 presents the result of the basic model. Section 4 incorporates the investment
decision of the supplier into the basic model. Section 5 discusses the buyers’ incentive fora horizontal merger. Section 6 investigates whether an exclusive incumbent buyer allows
a monopoly supplier to expand the customer scope of the supplier when the incumbent
buyer contracts exclusively with the monopoly supplier. Section 7 concludes.

II

A simple model

We consider a situation in which there is one supplier and two potential buyers (buyers
1 and 2). For example, in the case of the automobile industry, the supplier corresponds
to an auto parts manufacturer and the buyers correspond to automotive manufacturers.
The supplier can produce a good and sell it to the buyers. Although the buyers need one
7

The other methods are changing ownership structure (e.g., Grossman and Hart (1986), Hart andMoore (1990), and Aghion and Bolton (1992)) and contractual solutions (e.g., Chung (1991) and Aghion
et al. (1994)). Che and Hausch (1999), however, showed that when investments have a cooperative
nature (e.g., the seller’s investment improves the buyer’s valuation of the good), contracting has no value
if the contract must remain subject to renegotiation (see also Hart and Moore (1999) and Segal (1999)).
This paper also concerns cooperation, because the supplier’s investment improves the quality of buyers’
products.
8

Jackson and Wolinsky (1996) and Bala and Goyal (2000) provided formal models of network formation.

Belleﬂamme and Bloch (2004), Billand and Bravard (2004), Goyal and Joshi (2006), and Furusawa andKonishi (2007) applied the theory to the models of oligopoly.

5

unit of the good, they cannot produce it on their own, and therefore must buy it from the
supplier.
We model the negotiation between the supplier and the buyers as the following twostage game. In the ﬁrst stage, the supplier decides with whom to negotiate. In the second
stage, the supplier and the buyers designated in the ﬁrst stage negotiate. If the negotiation
is successful, the supplier produces the good and sells it to the buyer. Otherwise, the good
is not provided and the buyer obtains nothing.
We now explain the details of the two stages. In the ﬁrst stage, if there is one negotiation partner, the cost of the investments for production is F , and if there are two, this cost
is dF where d ∈ [1, 2]. This investment cost is assumed to be sunk; that is, the suppliercannot recover the cost in the second stage. This assumption means that the per-buyer(sunk) investment cost is smaller when the supplier trades with two partners than with
only one, and that the investment cost is related to several kinds of sunk set-up costs; forinstance, designing a large plant to manufacture the inputs of partners, conducting basic
surveys to build it and building ﬁrm-speciﬁc facilities to make the inputs.
In the second stage, we consider a form of bilateral and simultaneous negotiation. If
the supplier negotiates with two buyers, then it enters into simultaneous bargaining witheach of the two buyers separately.9 Bargaining determines whether one unit of the good isprovided and the amount of money the buyer transfers to the supplier. Each bargaining
session, if successful, provides one unit of the good. The cost of providing the good is c
if one unit of the good is produced, and (a + 1)c if two are produced, where a ∈ [0, 1]
and this cost is not sunk. These assumptions mean that the per-buyer production cost issmaller when the supplier trades with two partners than with only one, and is related to9

We have to mention two remarks about the assumption. First, this assumption implies that each pairof the supplier and a buyer bilaterally negotiate the amount of payment after the product characteristic is
determined. In the Japanese buyer-supplier relationships, Asanuma (1985) observes that parts prices arerevised at regular intervals, by bilateral negotiation, incorporating both risk and incentives for innovationand eﬀort. We think that the assumptions about the timing structure and the negotiation procedure
capture this stylized fact in Asanuma (1985). Second, some may assume that our results depend on thesimultaneous bargaining procedure. Fortunately, the results of sequential bargaining are similar to thoseof simultaneous bargaining and are available in Appendix 2.

6

several variable costs, such as material, labor, and natural resources.10
Payoﬀs to the supplier and the two buyers are determined as follows. Let v denote the(gross) value of each buyer for the good. If buyer i obtains the good and pays Ti , then it
receives the payoﬀ v − Ti . Otherwise, its payoﬀ is zero.11 When the supplier negotiateswith the two buyers, the supplier’s payoﬀ is T1 + T2 − (a + 1)c − dF if it successfully
negotiates with buyers 1 and 2, Ti − c − dF if it succeeds only in the negotiation withbuyer i (i = 1, 2), and −dF if none of the bargaining is successful. When the suppliernegotiates with one buyer, the supplier’s payoﬀ is Ti − c − F if bargaining is successfuland −F otherwise.
We assume that the outcome of the second stage is determined as follows.12
1. The outcome of each negotiation is given by the Nash bargaining solution in the
belief that the outcome of bargaining with the other party is determined in the sameway.
2. The joint surplus is divided between the buyer and the supplier in the proportion of
1 − β to β, in which β ∈ (0, 1) represents the bargaining power of the supplier, andthe supplier has the advantage over the buyer if and only if β > 1/2.
In this model, there is an externality in bargaining. Although each bargaining session
decides whether one unit of the good is produced so as to maximize joint surplus, the costof providing the good depends on the outcome of the bargaining with the other party. The
cost of the good is ac if the other negotiation is successful, and c otherwise. Thus, the10

If prices of some widgets needed to produce the buyer’s inputs depend on the following functional

form, the assumption that a ∈ [0, 1] is reasonable: F/Q + w, where F (> 0) and w(> 0) are exogenous
parameters and Q is the quantity demanded by the supplier. That price schedule is often called quantity
discount (Jeuland and Shugan (1983)). The price schedule is equivalent to the case in which the total
payment for widgets is equal to F + wQ (= Q × (F/Q + w)), the so-called “two-part tariﬀ.”
11

This payoﬀ form of buyers implicitly assumes that the buyers’ demand for the good is inelastic. This

assumption is to simplify the analysis.
12

We follow the bargaining procedures in Raskovich (2003). A diﬀerent approach, building on the

Shapley value, has been used, for instance, in Inderst and Wey (2003) and de Fontenay and Gans (2005,2007).

7

joint surplus of each bargaining session varies according to the belief about the bargaining
with the other ﬁrm. Note that according to this assumption, the two participants in eachbargaining session (the supplier and one buyer) believe that the other bargaining session
is conducted eﬃciently, and this belief is justiﬁed in equilibrium.13

Remarks

We have implicitly assumed that the supplier cannot supply more than two

buyers. This constraint reﬂects technological diﬃculties of the suppliers. For instance,
using a common platform, Nissan (a major Japanese automobile manufacturer) produces
two brands of cars (CUBE and MARCH). It is not easy, however, for Nissan to makea new type of car under the common platform because it restricts the car’s design, size,
drivability, and so on.14 Therefore, we consider that the implicit assumption is reasonable.
We have also implicitly assumed that only two buyers exist. This assumption implies thatthe supplier would have diﬃculty in ﬁnding a new buyer. Small suppliers often face sucha diﬃculty due to a shortage of information about potential buyers. Moreover, trade
frictions can cause thin trade networks among buyers and suppliers (Mahoney (2001) and
Chatain and Zemsky (2009)). We have also assumed that the supplier does not tradewith new buyers when it has decided to trade with only one. Once the supplier incurs
a sunk investment cost of production for one buyer, it has diﬃculty changing production
factors, such as size, design, and process. Moreover, the supplier faces a time constraintthat impedes the expansion of its production capacity. For instance, small and mediumsized technology-based enterprises can hardly compete in volume, because they may nothave suﬃcient resources to expand production capacity swiftly (Qian and Li (2003)). We
implicitly assume that the supplier forgoes the opportunity to sell the right to obtain
the good. This implies that a buyer cannot buy out the supplier. Small ﬁrms are often
13

The assumption that each participant in a bargaining session believes that bargaining with the otherparty is conducted eﬃciently is essential to our result. In another bargaining procedure, such as the
Shapley bargaining, we may not obtain the same results. This is available in Appendix 3.
14Some researchers also point out that this kind of component sharing has a trade-oﬀ: the beneﬁt is

a reduction in the cost of designing and purchasing additional components, but the cost is an increase inmismatch costs associated with using existing components with excess capability (Fisher et al. (1999),
Ramdas and Sawhney (2001), Ramdas and Randall (2008)).

8

organized by owners who have speciﬁc skills in production. In other words, techniciansmanage their own ﬁrms. Those self-managing technicians may have an intrinsic motivation
to operate their ﬁrms. They may also like the privilege of determining when they work
and what they produce. Those factors would discourage the suppliers/technicians fromselling their ﬁrms.

III

Analysis

We solve the two-stage model using backward induction. First, we calculate the Nashbargaining outcomes of the second stage. Second, we consider the ﬁrst stage and examine
the number of buyers with whom it is optimal for the supplier to negotiate.

III(i)The second stage

We need to consider the following two cases: the supplier negotiating with only one buyer,
and with two buyers.
III(i).1

Negotiation with one supplier and one buyer

If the supplier decided to negotiate with one buyer, then the surplus of this negotiation isv − c. Let T denote a payment from the buyer to the supplier. The buyer and the supplier
split the surplus in a way that satisﬁes v − T : T − c = 1 − β : β. Therefore, we obtainβ[v − T ] = (1 − β)(T − c) or To := βv + (1 − β)c.
Then, the proﬁt of the supplier is(1)π1∗ = To − c − F = β(v − c) − F.

As the value of β increases, the positive/negative eﬀect of an increase in v/c on π1∗ getsstronger. The signiﬁcance of each eﬀect is β (|∂π1∗ /∂v| = |∂π1∗ /∂c| = β).
The proﬁt of the buyer is(2)

π1B = v − To = (1 − β)(v − c).

9III(i).2

Negotiation with one supplier and two buyers

Let us consider a case in which the supplier negotiates with two buyers. In this case,
we need to consider two possibilities. One is that each buyer pays not less than c, andthe other is that each buyer pays less than c and the sum of the payments of the twobuyers is greater than (1 + a)c. In the ﬁrst case, one unit of the good can be provided andthe buyer in the successful negotiation can obtain the good even if the other negotiation
breaks down. In this situation, the buyer is nonpivotal to the production of two units ofthe good. In the other case, if one of the buyers withdraws from the negotiation, thennone of the goods are provided. In this situation, each buyer is pivotal to the provision of
the two units of the good (the term “pivotal” is used in Raskovich (2003)).
Nonpivotal buyers

First, we consider a situation in which the two buyers are nonpiv-

otal to production; production is successful even if one of the buyers withdraws from thenegotiation. In this situation, the buyer and the supplier split the surplus v − ac, wherethe ﬁrst (resp. the second) term is the additional beneﬁt (resp. cost) from the trade giventhat one trade will be executed. The buyer pays T to the supplier in a way that satisﬁesβ(v − T ) = (1 − β)(T − ac) or T2n := βv + (1 − β)ac.
The proﬁt of the supplier is(3)π2n = 2T2n − (1 + a)c − dF = 2βv − (1 − (1 − 2β)a)c − dF.

As the value of β increases, the positive/negative eﬀect of an increase in v/c on π2nbecomes stronger. The signiﬁcance of the positive eﬀect is 2β and that of the negativeeﬀect is 1 − (1 − 2β)a (|∂π2n /∂v| = 2β and |∂π2n /∂c| = 1 − (1 − 2β)a).
The proﬁt of each buyer is
(4)

B
π2n
= v − T2n = (1 − β)(v − ac).

Because the buyers are nonpivotal, we must have T2n − c > 0. This is given as
βv > (1 − (1 − β)a)c.
10

Pivotal buyers Second, we consider a case in which every buyer is pivotal to production.
In this case, the production fails if one of the two buyers withdraws from the negotiation.
If T1 and T2 designate payments from buyers 1 and 2, respectively, the surplus of thenegotiation with buyer 1 is (v − T1 ) + (T1 + T2 − (1 + a)c) = v + T2 − (1 + a)c and thatof the negotiation with buyer 2 is (v − T2 ) + (T1 + T2 − (1 + a)c) = v + T1 − (1 + a)c. In
each equation, the terms in the ﬁrst set of parentheses are the buyer’s beneﬁt from thetrade and those in the second set are the supplier’s beneﬁt from the trade, given that theother trade will be executed. The buyers and the supplier split the surplus in a way thatsatisﬁes the following conditions:β(v − T1 ) = (1 − β)(T1 + T2 − (1 + a)c), andβ(v − T2 ) = (1 − β)(T1 + T2 − (1 + a)c).
The simultaneous equations have only one solution: we obtain
T1 = T2 = T2p :=βv + (1 − β)(1 + a)c.
2−β

In the two negotiations, they take into account the total sum of the three players’ bargaining positions, β + 2(1 − β) = 2 − β. The share of the supplier’s bargaining position isβ/(2 − β). The coeﬃcient of v reﬂects the share of the supplier.
The proﬁt of the supplier is(5)π2p = 2T2p − (1 + a)c − dF =β(2v − (1 + a)c)
− dF.
2−β

As the value of β increases, the positive/negative eﬀect of an increase in v/c on π2p becomesstronger. The signiﬁcance of the positive eﬀect is 2β/(2 − β), and that of the negativeeﬀect is (1 + a)β/(2 − β) (|∂π2p /∂v| = 2β/(2 − β) and |∂π2p /∂c| = (1 + a)β/(2 − β)).
The proﬁt of each buyer is
(6)

B
π2p
= v − T2p =(1 − β)(2v − (1 + a)c)
.
2−β

11

Summary

The proﬁts of the supplier and each buyer are summarized as follows:

βv



 π2n = 2βv − (1 − (1 − 2β)a)c − dF if c ≤ 1 − (1 − β)a ,π2∗ =β(2v − (1 + a)c)
βv


− dF
if c ≥.
 π2p =2−β
1 − (1 − β)a

βv

B

,
if c ≤
 π2n = (1 − β)(v − ac)
1 − (1 − β)a
π2B =

βv
(1 − β)(2v − (1 + a)c)B
if c ≥
.
=
 π2p
2−β
1 − (1 − β)a

(7)

(8)

Note that when c = βv/(1 − (1 − β)a),π2n = π2p =III(ii)βv(1 − a)(1 − β)v(1 − a)B
B
− dF and π2n
= π2p
=
.
1 − (1 − β)a
1 − (1 − β)aThe ﬁrst stage

We now consider the supplier’s choice concerning the number of buyers. Suppose that the
supplier chooses the number to maximize its own proﬁt. Then, the supplier negotiateswith one buyer if and only if π1∗ > π2∗ . ¿From (1) and (7), the diﬀerence between π1∗ and
π2∗ is given as(9) ∆π ∗ ≡ π1∗ − π2∗ =


βv



 (d − 1)F − βv + (1 − β − (1 − 2β)a)c if c ≤ 1 − (1 − β)a ,
β 2 v + β(1 − β − a)c (d − 1)F −(2 − β)if c ≥βv.
1 − (1 − β)a

Before we ascertain the properties of ∆π ∗ , we must ﬁrst ascertain whether there exists
F such that π1∗ > 0 and ∆π ∗ > 0.15 A simple rearrangement of (1) and (9) leads to thefollowing condition by which the supplier trades with only one buyer:(10)βv − (1 − β − (1 − 2β)a)cβv< F < β(v − c) if c ≤,d−1
1 − (1 − β)aβ 2 v + β(1 − β − a)c< F < β(v − c)(2 − β)(d − 1)

if c ≥βv.
1 − (1 − β)a

In (10), when the right-hand-side value of the second inequality is larger than the lefthand-side value of the ﬁrst inequality, there exists F such that π1∗ > 0 and ∆π ∗ > 0. This15

If π1∗ ≤ 0, the optimal number of buyers is not 1 even though ∆π ∗ > 0.

12

condition is given as

(11)

d>d≡


2βv − (1 − (1 − 2β)a)cβ(v − c)

if c ≤βv,
1 − (1 − β)a
2v − (1 + a)c(2 − β)(v − c)

if c ≥βv.
1 − (1 − β)a

We summarize this in the following proposition:
Proposition 1 If F satisﬁes (10), then π1∗ > 0 and ∆π ∗ > 0. That is, the supplier prefers
trading with one buyer to trading with two buyers if F satisﬁes (10).
We ﬁrst exclude the case in which β > 1/2 and concentrate our discussion on the casein which β ≤ 1/2. This is because the inequality d > d in (11) is satisﬁed only if d > 2
when β > 1/2.16
We now check the property of ∆π ∗ . When β ≤ 1/2, the relations between ∆π ∗ and theexogenous parameters are given as (the mathematical procedure is available in Appendix
1)
(12)

∂∆π ∗
< 0,
∂a
∂∆π ∗
> 0,
∂a

∂∆π ∗
> 0,
∂c
∂∆π ∗
< 0 iﬀ 1 > a + β,
∂c

∂∆π ∗
βv
< 0, if c ≤,∂β
1 − (1 − β)a∂∆π ∗βv< 0, if c ≥.
∂β
1 − (1 − β)a

When the buyers are nonpivotal (c is small), an increase in a decreases ∆π ∗ . As the
additional cost generated by the second production increases, the supplier tends to tradewith two buyers. This is because the supplier and one buyer take into account only theadditional cost ac, even though the supplier incurs the variable production cost (1 + a)c.
This means that the “uncompensated” production cost of the supplier is (1 + a)c − 2ac =(1−a)c. Because the uncompensated cost decreases as the value of a increases, the suppliertends to trade with two buyers when the value of a is large. This tendency does not holdwhen the buyers are pivotal (c is large). The buyers take into account the full cost (1 + a)c
when they negotiate with the supplier. An increase in a only increases the additional
production cost when the supplier trades with the second buyer. Thus, the supplier tends
to trade with one buyer when the value of a is large.
16

When c = 0, d = 2. ∂d/∂c > 0 for all c when β > 1/2.

13

When the buyers are nonpivotal (c is small), an increase in c increases ∆π ∗ . The“uncompensated” production cost of the supplier (1 − a)c increases as the value of cincreases. This eﬀect induces the supplier to trade with only one buyer.17 When the buyers
are pivotal (c is large), the eﬀect of an increase in c on ∆π ∗ depends on the exogenous∗ /∂c| = (1 + a)β/(2 − β).
values. We have already shown that |∂π1∗ /∂c| = β and |∂π2p

An increase in a increases the total production cost where the supplier trades with two
buyers, (1 + a)c. Moreover, in general, a stronger bargaining position of the supplier leads
to greater gain and cost of the supplier from trade. In this context, the negative eﬀect
of an increase in c strengthens with the bargaining position of the supplier. Because thetotal production cost when the supplier trades with two buyers is larger than that when it
trades with one, the negative eﬀect in the two-buyer case is also stronger than that with
one buyer. Because of those two eﬀects, the sign of ∂∆π ∗ /∂c can be positive when β anda are suﬃciently large (a + β > 1).
An increase in β decreases ∆π ∗ . As the bargaining power of the supplier increases,
the supplier tends to trade with two buyers. The bargaining power is positively correlatedwith the gross gain of the supplier in the negotiation stage. A stronger bargaining power ofthe supplier weakens the relative importance of the additional investment costs, (d − 1)F .
We brieﬂy discuss the relation between d and c. This is summarized in Figure 1.
d

d

2.0

2.0
Β=14
Β=14

1.5

1.5
Β=110Β=110

1.0

1.0

0.5

0.5

0.0

0.2

0.4

0.6

0.8

1.0

cv

0.0

0.2

(a = 0)0.4

0.6(a = 3/4)

Figure 1: The optimal number of buyers can be one
Note: horizontal axis, c/v; vertical axis, d.

17

If a = 1, d in (11) is 2 for any c ≤ v. That is, this eﬀect disappears if a = 1.

14

0.8

1.0

cv

d is minimized when c = βv/(1 − (1 − β)a), which is the threshold value regardless ofwhether the buyers are pivotal. As mentioned above, the cost of the investments for
production is F (dF ) if the number of its negotiation partners is one (two), respectively,
where d ∈ [1, 2]. The additional investment cost of the second round of production is(d − 1)F (≤ F ). This means that trading with two buyers is more proﬁtable when d issmaller.
This ﬁgure shows an interesting property: nonmonotonicity of the relation between
the optimal number of buyers and c. This means that the supplier with a lower or higher
c prefers trading with two buyers, whereas one with an intermediate value of c prefers
trading with one buyer. The reason for this concerns the relation between ∆π ∗ and c,which has been discussed above.

IV

Quality choice

We incorporate quality investments into the basic setting. Between the ﬁrst and the second
stages, the supplier invests to improve the (gross) value of each buyer for the good. In thisstage, if the supplier invests q ≥ 0 and has one (two) negotiation partners, the gross valueof each buyer becomes q and the cost of the investments is f q 2 (df q 2 ), respectively, whered ∈ [1, 2] and f is a positive constant. That is, the supplier endogenously determines thevalue for the good. Note that in the basic model, the supplier incurs the investment costs
F (dF ) when it trades with one buyer (two buyers), respectively. This investment cost is
assumed to be sunk; that is, the supplier cannot recover the cost in the second stage. We
employ this assumption in this section. The rest of the model assumptions are the sameas in the basic model.18

18

To simplify the analysis, we assume that product quality is common to both buyers. We only haveto consider two cases concerning buyers’ positions: (i) they are pivotal; (ii) neither is pivotal. If we allow
heterogeneity of product quality, we have to consider the third case: (iii) one of the buyers is pivotal. The
additional case complicates the comparison of the three cases.

15

IV(i)The negotiation stage

We use the same general mathematical procedures with the basic model. We replace thegross value of each buyer by q and the investment cost by f q 2 (df q 2 ) if the number of its
negotiation partners is one (two), respectively.
If the supplier decided to negotiate with one buyer, the proﬁt of the supplier is (see(1))π1 (q) = β(q − c) − f q 2 .

(13)

If the supplier decided to negotiate with two buyers, two situations can occur: (i) thetwo buyers are nonpivotal to production; (ii) they are pivotal to production. If the buyers
are nonpivotal to production (βq > (1 − (1 − β)a)c), the proﬁt of the supplier is (see (3))π2n (q) = 2βq − (1 − (1 − 2β)a)c − df q 2 .

(14)

If both buyers are pivotal to production (βq ≤ (1 − (1 − β)a)c), the proﬁt of the supplieris (see (5))(15)

IV(ii)

π2p (q) =β(2q − (1 + a)c)
− df q 2 .
2−β

The investment stage

We derive the optimal investment level q in the three situations.
If the supplier decided to negotiate with one buyer, the optimal q is given as (see (13))
q1∗ = arg max π1 (q) =

(16)

q

The optimal q1∗ leads to(17)

π1 (q1∗ ) = β

(

β
.
2f

)
β
−c .
4f

The proﬁt of the buyer is (see (2))(18)π1B (q1∗ ) = (1 − β)(q1∗ − c) = (1 − β)

(

)
β
−c .
2f

If the supplier decides to negotiate with two buyers and they are nonpivotal to production (βq > (1 − (1 − β)a)c), the optimal q is given as (see (14))
(19)

∗
q2n
= arg max π2n (q) =
q

16

β
.
df

∗ leads to
The optimal q2n∗
π2n (q2n
)=

(20)

β2
− (1 − (1 − 2β)a)c.
df

The proﬁt of each buyer is (see (4))
B
∗
∗
π2n
(q2n
) = (1 − β)(q2n− ac) = (1 − β)

(21)

(

)
β
− ac .
df∗ is an interior solution if and only if f < β 2 /(d(1 − (1 − β)a)c).
q2n

If the supplier decides to negotiate with two buyers and both are pivotal to production(βq ≤ (1 − (1 − β)a)c), the optimal q is given as (see (15))∗
q2p= arg max π2p (q) =

(22)

q

∗ leads to
The optimal q2p∗
π2p (q2p
)=

(23)

β
2−β

(

β
.
df (2 − β))β
− (1 + a)c .
df (2 − β)The proﬁt of each buyer is (see (6))
(24)

B ∗
π2p
(q2p ) =∗ − (1 + a)c)
(1 − β)(2q2p
1−β
=
2−β
2−β

(

)
2β
− (1 + a)c .
df (2 − β)∗ is an interior solution if and only if f ≥ β 2 /(d(2 − β)(1 − (1 − β)a)c).
q2p

Nonpivotal or pivotal

For f ∈ [β 2 /(d(2 − β)(1 − (1 − β)a)c), β 2 /(d(1 − (1 − β)a)c)),the two cases in which the supplier trades with two buyers have interior solutions. In∗ and q ∗ are local optimal solutions for this range of f . q ∗ (q ∗ ) is theother words, q2n2p2p2n∗ ) ≥ π (q ∗ ) (π (q ∗ ) ≤ π (q ∗ )), respectively.
global optimum if and only if π2p (q2p
2n 2n
2p 2p
2n 2n
∗ ) ≥ π (q ∗ ) if and only if
π2p (q2p
2n 2n

f≥

[
)
β2
β2
β 2 (3 − β)∈,.
2cd(2 − β)(1 − a + aβ)d(2 − β)(1 − (1 − β)a)c d(1 − (1 − β)a)c¿From (20) and (23), if the supplier decides to negotiate with two buyers, the optimal
investment level q2∗ and the proﬁt of the supplier π2 (q2∗ ) areββ 2 (3 − β)

∗

q
=
if
f
<
,

 2n df
2cd(2 − β)(1 − a + aβ)
∗
q2 =
(25)


β 2 (3 − β)
β
∗

 q2p
if f ≥,=
df (2 − β)2cd(2 − β)(1 − a + aβ)
17

 2
β
β 2 (3 − β)


−
(1
−
(1
−
2β)a)c
if
f
<
,

 df
2cd(2 − β)(1 − a + aβ)∗(26) π2 (q2 ) =
(
)


β
β
β 2 (3 − β)− (1 + a)c
if f ≥.
2 − β df (2 − β)2cd(2 − β)(1 − a + aβ)¿From (16) and (25), we ﬁnd how the number of buyers aﬀects the equilibrium investment level.
Proposition 2 The investment level in the case of one buyer, q1∗ , is larger than that in
the case of two buyers, q2∗ , if and only ifd > 2/(2 − β) and cdf ≥β 2 (3 − β).
2(2 − β)(1 − a + aβ)

Now suppose that the supplier trades with two buyers that are pivotal. When theinvestment level is q, following their bargaining positions, they split the total gain fromtrade, 2q. A supplier and buyer pair i consider the gain from another trade between the
supplier and buyer j, Tj (i = 1, 2, j ̸= i). As mentioned above, in the two negotiations
they consider the sum of the three players’ bargaining positions, β + 2(1 − β) = 2 − β.
The share of the supplier’s bargaining position is β/(2 − β). π2p (q) in (15) reﬂects theshare of the supplier’s bargaining position, and the total gross gain of the supplier fromthe trades is 2qβ/(2 − β) (see (15)). Note that this is smaller than that in which eachbuyer is nonpivotal, 2βq (see (14)). The smaller gross gain in the pivotal case diminishes
the incentive of the supplier to invest.
Proposition 2 implies that a narrow buyer-supplier relationship intensiﬁes the supplier’s incentive to invest if the supplier’s investment cost parameters c, d, and f arelarge. As mentioned in the introduction, Japanese automakers and their suppliers are
more specialized than their US counterparts and there is a high correlation between supplier specialization and automaker proﬁtability (Dyer (1996)). Although this statement
is based on the viewpoint of buyers (automakers), this correlation may occur because thehigher investment level caused by the narrower customer scope of suppliers leads to greater
proﬁtability of the automakers.

18

IV(iii)

The ﬁrst stage

We need to ﬁnd the highest value of f such that both π1 (q1∗ ) and π2 (q2∗ ) are positive. We
now impose the following assumption:
Assumption 1 We assume that a = 0 and f < β/4c.
We impose Assumption 1 to simplify the exposition and the analysis. The inequality inAssumption 1 ensures that π1 (q1∗ ) and π2 (q2∗ ) are positive for the exogenous parameters(β, c, d, and f ). ¿From (17) and (26), the diﬀerence between π1 (q1∗ ) and π2 (q2∗ ) is given
as19∆π(q ∗ ) ≡ π1 (q1∗ ) − π2 (q2∗ )
4cd(1 − β)f − β 2 (4 − d)




4df
=


β(β((2 − β)2 d − 4) − 4cd(2 − β)(1 − β)f )
4(2 − β)2 df

if f <β 2 (3 − β),2cd(2 − β)if f ≥β 2 (3 − β).
2cd(2 − β)If the following condition holds, ∆π(q ∗ ) > 0:
β 2 (4 − d)β((2 − β)2 d − 4)<f <.
4cd(1 − β)4cd(2 − β)(1 − β)(27)

Depending on the exogenous parameters, f can be empty. If the right-hand-side value inthe latter inequality is larger than the left-hand-side value in the former inequality, there
exists f such that ∆π(q ∗ ) > 0. The following inequality shows the condition in which the
optimal number of buyers may be one. The right-hand side is increasing in β.
(28)

d>

2(1 + 2β − β 2 ).
2−βWe summarize the above discussion in the following proposition:
Proposition 3 Under Assumption 1, if f satisﬁes (27), then π1 (q1∗ ) > 0 and ∆π(q ∗ ) > 0.
That is, the supplier prefers trading with one buyer to trading with two buyers if f satisﬁes
(27).
19

Note that β 2 (3 − β)/2cd(2 − β) is not always smaller than β/4c in Assumption 1. That is, in somecases, only the nonpivotal case appears.

19

The relations between ∆π(q ∗ ) and the exogenous parameters are given as (the mathematical procedure is available in Appendix 1)(29)∂∆π(q ∗ )> 0,∂c∂∆π(q ∗ )< 0,
∂β∂∆π(q ∗ )< 0,∂c∂∆π(q ∗ ) >β 2 (3 − β)> ˜,
< 0 if d < d if f ≥∂β
2cd(2 − β)if f ≤β 2 (3 − β),2cd(2 − β)where d˜ ≡ 8β/((2 − β)(β(2 − β)2 − 2(2 − 4β + β 2 )cf )).20 Essentially, ∆π(q ∗ ) has a similar
property to ∆π ∗ . The property of ∂∆π(q ∗ )/∂β is diﬀerent from that of ∂∆π ∗ /∂β when
the buyers are pivotal. In particular, when d > 16/((2−β)(6−4β +β 2 )) and the buyers arepivotal, ∂∆π(q ∗ )/∂β > 0 for any f under Assumption 1. As in Proposition 2, when d andf are large, the equilibrium investment level is larger when the supplier trades with one
buyer (q1∗ > q2∗ ). This property is quite diﬀerent from that in the case where the qualityof goods is exogenous. When d and f are large, the stronger bargaining position of the
supplier generates a greater gross gain from trade when it trades with one buyer rather
than two. Because a larger value of d enhances the diﬀerence between the equilibrium
investment levels q1∗ − q2∗ , a stronger bargaining position for the supplier can encourage it
to decrease the number of buyers when d is large.

V

Buyer merger

We brieﬂy discuss whether the buyers have an incentive to merge.
If the supplier decides to negotiate with a merged buyer, then the surplus of thisnegotiation is 2v − (1 + a)c. Let T denote a payment from the buyer to the supplier. The
buyer and the supplier split the surplus in a way that satisﬁes 2v − T : T − (1 + a)c =1 − β : β. Therefore, we obtainβ[2v − T ] = (1 − β)(T − (1 + a)c) or Tm := 2βv + (1 − β)(1 + a)c.
Then, the proﬁt of the supplier is
(30)
20

∗
πm
= Tm − (1 + a)c − dF = β(2v − (1 + a)c) − dF.

The denominator is positive under Assumption 1.

20

The proﬁt of the buyer isBπm= 2v − Tm = (1 − β)(2v − (1 + a)c).

(31)

B is larger than 2π B (see (8) and (31)), the buyers merge. We easily ﬁnd that the buyers
If πm2∗ and π ∗ is positive.21
merge if and only if they are pivotal. The diﬀerence between πm1

When the supplier invests to improve its product quality, q, the proﬁt of the supplierisπm (q) = β(2q − (1 + a)c) − df q 2 .

(32)¿From (32), the investment level is given as∗
qm= arg max πm (q) =

(33)

q

β
.
df

∗ leads to
The optimal investment level qm∗
πm (qm)=(34)

β2− β(1 + a)c.
df

The proﬁt of the merged buyer is
(35)

B ∗
∗
πm
(qm ) = (1 − β)(2qm− (1 + a)c) = (1 − β)()
2β
− (1 + a)c .
df

B (q ∗ ) is larger than 2π B (q ∗ ) (see (26) and (35)), the buyers merge. We easily ﬁnd
If πm
m
2
2
∗ ) andthat the buyers merge if and only if they are pivotal. The diﬀerence between πm (qm

π1 (q1∗ ) is positive.22
21

The diﬀerence is given by∗
π1∗ − πm
= (d − 1)F − β(v − ac) < (d − 1)β(v − c) − β(v − ac)= −β(2v − (1 + a)c − (v − c)d) ≤ −β(2v − (1 + a)c − 2(v − c))= −β(1 − a)c ≤ 0.

22

The diﬀerence is given by«„«„β(4 − d)β(4 − d)∗≤β c−π1 (q1∗ ) − πm (qm) = β ac −
4df
4df
„
«
2
β(4 − d)β (2d − 4)
β
< β
−
=
≤ 0.
4f
4df
4df

21

Based on the model in Raskovich (2003), Adilov and Alexander (2006) and Clark etal. (2008) also discuss buyer mergers. They clarify how being a pivotal merged buyer
aﬀects the payoﬀs of the players. Adilov and Alexander (2006) focus on heterogeneityof bargaining power among buyers, and Clark et al. (2008) consider several cases inwhich players sequentially bargain as in Stole and Zwiebel (1996a). Their basic results are
consistent with ours.

VI

Decision on the number of trading partners: Buyer’s
decision

So far, we have considered several cases in which the supplier determines the number ofbuyers. We now consider two cases in which a buyer determines the number of buyers. The
only diﬀerence between the analysis in this section and that in the previous section is the
structure of the ﬁrst stage. The number of trading partners is determined by the current
buyer in this section. The discussion is motivated by the following situation. A potential
buyer emerges when a buyer and a supplier trade exclusively with each other. It is not
obvious whether or not expanding the number of buyers beneﬁts the incumbent buyer. If
the expansion harms the incumbent buyer and if the contract between the pair allows thisbuyer to prevent the supplier from increasing the number of buyers, the incumbent buyer
does not allow the supplier to do so.23
The model setting in this section is related to Nobeoka (1996). Focusing on the sourcing
concentration and the sharing of common suppliers with competitors, he examines thecomponent sourcing strategy of the Japanese automobile manufacturers. In his paper, he
proposes two strategic dimensions in component sourcing. One of the dimensions is thedegree of supplier sharing with competing assemblers. Some assemblers may buy a certain
type of component from a supplier that exclusively sells it to the manufacturer, while others
23

In the introduction, we mention the case in which aﬃliated ﬁrms in the Toyota keiretsu group sold aportion of their products to outsiders. This case may be taken as an example of the buyer deciding thenumber of trading partners served by the supplier. If Toyota has an exclusive contract with its aﬃliatedﬁrms, then the aﬃliated ﬁrms cannot trade with a new partner without its permission. In that situation,
Toyota’s permission implies that the aﬃliated ﬁrms can expand their number of trading partners.

22may rely on a supplier that sells the same type of component to other manufacturers as
well.24
We now consider the buyer’s choice concerning the number of buyers. Suppose thatthe buyer chooses the number to maximize its own proﬁt. From (2) and (8), the number
is one if and only if

π1B = (1 − β)(v − c) ≥ π2B = (1 − β)(v − ac)if c ≤βv.
1 − (1 − β)a(1 − β)(2v − (1 + a)c)

2−β

if c ≥βv.
1 − (1 − β)a

We easily ﬁnd that the inequality does not hold for all exogenous parameters (note thatc ≤ v). That is, the current buyer always prefers to expand the customer scope of the
supplier. We next show that this result changes when we incorporate quality investments
into the basic model.
We now consider the case with quality investments. The only diﬀerence between theanalysis in this and the previous part is the structure of the ﬁrst stage. To simplify the
analysis, we impose Assumption 1 in this case.
B (q ∗ ) > π B (q ∗ ) (see (18) and (21)). This means that theFirst, we easily ﬁnd that π2n2n11

optimal number of buyers is two for the current buyer when f < β 2 (3 − β)/(2cd(2 − β)).
B (q ∗ ) is given as (see (18) and (24))
Second, the diﬀerence between π1B (q1∗ ) and π2p2p
B ∗∆π B (q ∗ ) ≡ π1B (q1∗ ) − π2p(q2p ) =(1 − β)(β((2 − β)2 d − 4) − 2cd(2 − β)(1 − β)f ).
2df (2 − β)2This is positive if and only if(36)β((2 − β)2 d − 4)
β 2 (3 − β)≤f <.
2cd(2 − β)2cd(2 − β)(1 − β)∗ inNote that the ﬁrst inequality is the condition under which the supplier chooses q2p

the investment stage. Depending on the exogenous parameters, f can be empty. If theright-hand-side value in the second inequality is larger than the left-hand-side value in the24

The other dimension is the sourcing concentration that determines the degree of reliance on a small

number of suppliers such as on a single supplier. This dimension is similar in concept to the number ofsuppliers from which a ﬁrm procure a certain type of components.

23B (q ∗ ). The following inequality showsﬁrst inequality, there exists f such that π1B (q1∗ ) > π2p2p

the condition under which the optimal number of buyers can be one. The right-hand side
is increasing in β.
(37)

d>

4 + 3β − 4β 2 + β 3.
(2 − β)2

We summarize the above discussion in the following proposition:
Proposition 4 Suppose that the supplier engages in quality investment. Under Assumption 1, when a current buyer determines the number of buyers, the optimal number ofbuyers is one for the current buyer if f satisﬁes (36).
This result is also related to Proposition 2. When the supplier trades with two buyers,
the equilibrium quality becomes lower, although per-unit production cost is reduced. A
broad customer scope strategy is not preferable for the supplier and the current buyer if
the investment technology of the supplier is not good (f and d are large).
Our result may have a potential to explain the diﬀerence among the sourcing strategies of major Japanese automobile assemblers. Nobeoka (1996) investigates the sourcing
strategy of six Japanese car assemblers (Toyota, Nissan, Honda, Mitsubishi, Mazda, andSuzuki) regarding 95 components. He classiﬁes two types of sourcing strategy into thequasi-market strategy and the quasi-hierarchy strategy. The former is related to an expansion of the supplier’s customer scope and the latter is related to an exclusive traderelation. He shows that Nissan and Honda employ the quasi-hierarchy strategy while Toyota, Mitsubishi, and Suzuki do the quasi-market strategy. He also shows that ﬁrms using
a broad manufacturer-supplier network tend to be more proﬁtable. Our result clariﬁes
the condition that ﬁrms using a broad manufacturer-supplier network are more proﬁtable.
Our result implies that a broad customer scope strategy is employed by the supplier andthe current buyer when the supplier has a good investment technology (f and d are small).
As a result, in our model, a broad manufacturer-supplier network leads to superior performance. Our result may be consistent with the ﬁnding in Nobeoka (1996).

24

VII

Concluding remarks

We provide a simple model and investigate what factors determine the number of trading
partners. We show several results that may be consistent with the formation of Japanesebuyer-supplier relations. When the supplier is able to determine the number of trading
partners, one or two, the optimal number is one for the supplier if the supplier’s bargaining
power with its trading partners is weak and the supplier’s additional sunk investment costis relatively large. The supplier prefers to trade with one buyer if the variable production
cost is neither large nor small for the value of the good. That is, there is nonmonotonicity
in the relation between the optimal number of buyers and the variable production cost.
This means that a supplier with a lower or higher cost prefers trading with two buyers,
whereas one with an intermediate level of variable production cost prefers trading with
one buyer. We also show that the equilibrium investment level when the supplier tradeswith one buyer can be larger than that with two buyers if the sunk investment cost islarge relative to the value of the good, the supplier’s bargaining power with its trading
partners is weak, and the variable production cost is large. This may be related to thefollowing ﬁnding. Japanese automakers and their suppliers are more specialized thantheir US counterparts and there is a high correlation between supplier specialization andautomaker proﬁtability (Dyer (1996)). Although this statement is based on the viewpointof buyers (automakers), this correlation may occur because the higher investment levelcaused by the narrower customer scope of suppliers leads to greater proﬁtability of theautomakers.
Our model may be applicable to the cable television industry. As mentioned in Chipty
and Snyder (1999), a program service provider (supplier) has bilateral relationships withcable operators (buyers). Those operators tend to be regional monopolists and do notcompete with each other. To make TV programs, such a program service company must
incur higher (sunk) costs. In some cases, the program service company may have to pay
per-buyer copyright fees for artists who participate in these TV programs (this is related
to variable costs of the supplier). In other cases, this company would not have to do so
25because its own staﬀ members make its programs. When we apply our results to thisindustry, we can say that a program service provider (supplier) that has to incur higher
ﬁxed costs and relatively higher variable costs should have a narrow relationship withcable operators.
In our model, buyers are independent in their ﬁnal product markets. As discussed
in Matsushima (2004, 2009), competition among buyers is an important research topic.
To simplify the analysis of our model, we consider the transactions of only one supplier.
Markets with multiple suppliers are also an important research topic. The wider investigation allows us to discuss competition among suppliers, although this complicates theanalysis. Moreover, the topic of repeated interactions between suppliers and buyers is important. This would arise in examining the reason why technological improvements induce
the gradual decrease of trading prices over time in Japanese buyer–supplier relationships.
These are signiﬁcant topics for future research.

Appendix 1: comparative statics
We now explain the results concerning the comparative statics of ∆π ∗ and ∆π(q ∗ ).
When β < 1/2 and c ≤ βv/(1 − (1 − β)a) (the buyers are non-pivotal), we have thefollowing result:∂∆π ∗= −(1 − 2β)c < 0,∂a∂∆π ∗= (1 − β − (1 − 2β)a) > 0,∂c∂∆π ∗= −v − (1 − 2a)c ≤ 0.
∂βNote that (1 − β − (1 − 2β)a) = 1 − β > 0 when a = 0 and (1 − β − (1 − 2β)a) = β > 0when a = 1. Therefore, for any a ∈ [0, 1], (1 − β − (1 − 2β)a) is positive. Note also that−v−(1−2a)c = −v < 0 when c = 0 and −v−(1−2a)c = −(1+β)(1−a)v/(1−(1−β)a) ≤ 0
when c = βv/(1 − (1 − β)a). Therefore, for any c ∈ [0, βv/(1 − (1 − β)a)], −v − (1 − 2a)c
is non positive.

26

When β < 1/2 and c ≥ βv/(1−(1−β)a) (the buyers are pivotal), we have the following
result:
∂∆π ∗
βc
=
> 0,
∂a
2−β
∂∆π ∗
1−β−a
= −
< 0 iﬀ 1 > a + β,
∂c
2−β
∂∆π ∗
2(1 − a)c + β(4 − β)(v − c)
< 0.
= −
∂β
(2 − β)2
When β < 1/2 and f ≤ β 2 (3 − β)/(2cd(2 − β)) (the buyers are non-pivotal), we havethe following result:∂∆π ∗4d(1 − β)f
=
> 0,
∂c
4df
2cdf + β(4 − d)
∂∆π ∗
= −
< 0.
∂β
2df
When β < 1/2 and f ≥ β 2 (3 − β)/(2cd(2 − β)) (the buyers are pivotal), we have thefollowing result:∂∆π ∗4d(2 − β)(1 − β)f
= −
< 0,
∂c
4(2 − β)2 df∂∆π ∗β(d(2 − β)3 − 8) − 2cd(2 − β)(2 − 4β + β 2 )f=.
∂β2(2 − β)2 df

Appendix 2: sequential bargaining
We present two sorts of sequential bargaining models. In the ﬁrst model, the suppliernegotiates with buyers bilaterally and sequentially. In the second model, the supplier andthe buyers participate in the bargaining modeled through the Shapley value.
A sequential bilateral bargaining
We ﬁrst consider the following sequential bargaining (see Stole and Zwiebel (1996a, b)):First, the ﬁrst buyer negotiates with the supplier. If the negotiation reaches an agreement,
the buyer’s payment T1 is determined; otherwise, no transfer occurs and the buyer exits
the game. Observing the outcome of the negotiation, the second buyer negotiates with the27

supplier. If the negotiation reaches an agreement, the buyer’s payment T2 is determined;
otherwise, no transfer occurs and the buyer exits the game.25
We ﬁrst analyze bargaining between the second buyer and the supplier. Given theoutcome of the bargaining, we then examine the bargaining among the ﬁrst buyer and the
supplier.
Given that the ﬁrst buyer’s payment T1 is determined, we consider the negotiation
between the second buyer and the supplier. We need to consider the following two cases:
one is the case of T1 > c (the second buyer is non-pivotal) and the other is the case ofT1 ≤ c (the second buyer is pivotal).
Case 1. T1 > c.

When T1 > c, the second buyer is non-pivotal. The additional surplusof the trade with the second buyer is v − ac. The second buyer pays T2 to the supplier ina way that satisﬁesβ(v − T2 ) = (1 − β)(T2 − ac) or T2 = βv + (1 − β)ac.
Assuming that the second negotiation reaches the agreement mentioned above, the ﬁrst
buyer negotiates with the supplier. It is worth noting that T2 > c holds if and only
if the ﬁrst buyer is also non-pivotal.26 Because T2 > c implies βv > (1 − (1 − β)a)c,βv > (1 − (1 − β)a)c must be satisﬁed for the ﬁrst buyer to be non-pivotal.
(1.1) If βv > (1 − (1 − β)a)c, the ﬁrst buyer is non-pivotal and then the additional surplusof the trade with the ﬁrst buyer is v − ac. The ﬁrst buyer pays T1 to the supplier ina way that satisﬁesβ(v − T1 ) = (1 − β)T1 − ac or T1 = T2 = βv + (1 − β)ac.
This satisﬁes the condition that T1 > c if and only if
βv > (1 − (1 − β)a)c.
25

The order of bargaining does not aﬀect our result.

26

If T2 ≤ c, then it depends on the value of T1 whether two units of input are supplied by the supplier.

28

(1.2) If βv ≤ (1 − (1 − β)a)c, then the ﬁrst buyer is pivotal and the additional surplus of
the trade with the ﬁrst buyer is (v − T1 ) + (T1 + T2 − (1 + a)c) = v + T2 − (1 + a)c.
The ﬁrst buyer pays T1 to the supplier in a way that satisﬁesβ(v − T1 ) = (1 − β)(T1 + T2 − (1 + a)c) or T1 = β 2 v + (1 − β)(1 + aβ)c.
This satisﬁes the condition that T1 > c if and only if
βv > (1 − (1 − β)a)c.
However, since we now consider the case in βv ≤ (1 − (1 − β)a)c, case (1.2) does notappear in equilibrium.
If βv > (1 − (1 − β)a)c, then T1 > c is supported as an equilibrium and then T1 = T2 =βv + (1 − β)ac.
Case 2. T1 ≤ c.

When T1 ≤ c, the second buyer is pivotal. The additional surplus of

the trade with the second buyer is (v − T2 ) + (T1 + T2 − (a + 1)c) = v + T1 − (a + 1)c. The
second buyer pays T2 to the supplier in a way that satisﬁes(38) β(v − T2 ) = (1 − β)(T1 + T2 − (a + 1)c) or T2 = βv + (1 − β)(a + 1)c − (1 − β)T1 .
Assuming that the second negotiation reaches the agreement mentioned above, the ﬁrst
buyer negotiates with the supplier.
(2.1) If T2 > c, then the ﬁrst buyer is non-pivotal and the additional surplus of the tradewith the ﬁrst buyer is v − ac. The ﬁrst buyer pays T1 to the supplier in a way thatsatisﬁesβ(v − T1 ) = (1 − β)(T1 − ac) or T1 = βv + (1 − β)ac.
Substituting it into T2 in (38), we obtain
T1 = βv + (1 − β)ac, T2 = β 2 v + (1 − β)(1 + βa)c.
29

This satisﬁes the condition that T1 ≤ c if and only if
βv ≤ (1 − (1 − β)a)c.

(39)

We obtain T2 > c if and only if
βv > (1 − (1 − β)a)c.

(40)

There is no exogenous values that satisfy the two inequalities. Therefore, case (2.1)
does not appear in equilibrium.
(2.2) If T2 ≤ c, then the ﬁrst buyer is pivotal and the additional surplus of the trade withthe ﬁrst buyer is (v − T1 ) + (T1 + T2 − (a + 1)c) = v + T2 − (a + 1)c. The ﬁrst
buyer pays T1 to the supplier in a way that satisﬁes (we substitute T2 in (38) into
the following equation)β(v − T1 ) = (1 − β)(T1 + T2 − (a + 1)c) = β(1 − β)(v + T1 − (a + 1)c).
The equation leads to
T1 = T2 =βv + (1 − β)(a + 1)c.
2−βThis satisﬁes the condition that T1 ≤ c if and only if βv ≤ (1 − (1 − β)a)c and T2 ≤ c
if and only if βv ≤ (1 − (1 − β)a)c.
If βv ≤ (1 − (1 − β)a)c, then T1 ≤ c is supported as an equilibrium and then T1 = T2 =(βv + (1 − β)(a + 1)c)/(2 − β).
We can summarize the results mentioned above as follows:
if βv > (1 − (1 − β)a)c,
 βv + (1 − β)ac
T1 = T2 =(41)
βv + (1 − β)(a + 1)c
if βv ≤ (1 − (1 − β)a)c.

2−βThese transfer payments by the buyers are equal to those derived in the main text.

Appendix 3: a sequential bargaining modeled through the Shapley value30

We consider the bargaining modeled through the Shapley value. Each player comesto negotiate in a given order and receives the marginal surplus from his/her arrival. The
marginal surplus of the arrival may depend on the order of arrival, which is stochasticallydetermined. We assume that the probability of each arrival order is the same. The Shapley
value of each player is their expected marginal surplus. By applying the Shapley value,
we calculate each player’s share of the bargaining surplus.
We ﬁrst examine the case in which the supplier negotiates with two buyers. Because
there are three players (the supplier, buyer 1, and buyer 2), there are 3! = 6 orders.
Each order occurs with probability 1/6. The bargaining surplus is calculated from acharacteristic function. A natural characteristic function of our model, denoted by V 2 :{S, B1, B2} → R+ , in which S, B1, and B2 represent the supplier, buyer 1, and buyer 2,
respectively, is as follows. We normalize V 2 (∅) = 0. No one can gain by him/herself; hence,
V 2 (i) = 0 for each i ∈ {S, B1, B2}. B1 and B2 can earn nothing; hence, V 2 (B1, B2) = 0.
Groups of the supplier and at least one buyer generate a surplus; V 2 (S, B1) = V 2 (S, B2) =v − c and V 2 (S, B1, B2) = 2v − (1 + a)c. The marginal contribution of player i to a set ofarrived players, S ′ , such that i ∈/ S ′ is given by V 2 (S ′ ∪ {i}) − V 2 (S ′ ).
The expected contribution of player i constitutes the Shapley value, SVi2 (i ∈ {S, B1, B2}).
2 , isBuyer 1’s expected contribution, SVB1
2
SVB1
=

+
+
=

V 2 (B1) − V 2 (∅) V 2 (B1) − V 2 (∅)+
66
V 2 (B1, B2) − V 2 (B2) V 2 (S, B1) − V 2 (S)+66V 2 (S, B1, B2) − V 2 (S, B2) V 2 (S, B1, B2) − V 2 (S, B2)+
6
6
3v − c − 2ac.
6

2 , and that of S, SV 2 , are
By a similar calculation, the expected contribution of B2, SVB2S
2 = (3v − c − 2ac)/6 and SV 2 = (3v − (1 + a)c − c)/3, respectively.27
SVB2
S

Second, we examine the case in which the supplier negotiates with one buyer. Consider
a situation in which the supplier negotiates with buyer Bi (i = 1, 2). We can similarly
27

22Note that SVS2 + SVB1
+ SVB2= V 2 (S, B1, B2).

31

introduce a characteristic function V 1 : {S, Bi} → R+ : V 1 (∅) = 0, V 1 (j) = 0 for eachj ∈ {S, Bi}, and V 1 (S, Bi) = v − c. The expected contribution of S, SVS1 , and that of Bi,
1 , are SV 1 = (v − c)/2 and SV 1 = (v − c)/2, respectively.
SVBi
S
Bi

Based on these analyses, we examine the optimal number of buyers with whom the
supplier negotiates. When the supplier negotiates with one buyer, his/her payoﬀ is
π1∗ ≡ SVS1 − F =

(42)

v−c
− F.
2

When he/she trades with two buyers, his/her payoﬀ is
π2∗ ≡ SVS2 − dF =(43)

3v − (1 + a)c − c− dF.
3

Subtracting (43) from (42) yields
π1∗ − π2∗ =(44)−3v − c + 2(1 + a)c+ (d − 1)F.
6

We examine whether there is an exogenous parameter in which the supplier chooses totrade with one buyer. The supplier chooses to trade with one buyer if and only if π1∗ −π2∗ ≥0 and π1∗ ≥ 0. We have
π1∗ − π2∗ ≥ 0 if and only if F ≥3v + c − 2(1 + a)6(d − 1)andπ1∗ ≥ 0 if and only if

v−c
≥ F.
2

Thus,
(45)

v−c
3v + c − 2(1 + a)c≥F ≥.
2
6(d − 1)

We show that there exists an exogenous parameter that satisﬁes (45).
Lemma 1 Condition (45) holds if and only if a = 1 and d = 2.
Proof. Suﬃciency is trivial. We show its necessity. We suppose that either a ̸= 1 ord ̸= 2. The left-hand side of (45) minus the right-hand side of (45) is equal to(46)

3(d − 2)v − (3d − 2)c + 2(1 + a)c.
6(d − 1)32

Because v > c, we can describe v = γc for some γ > 1. Substituting v = γc into (46), we
obtain(3(d − 2)γ − (3d − 2) + 2(1 + a))c.
6(d − 1)(47)
Because γ > 1 and d ≤ 2,

(3(d − 2)γ − (3d − 2) + 2(1 + a))c(3(d − 2) − (3d − 2) + 2(1 + a))c≤.
6(d − 1)6(d − 1)(48)

If d = 2 but not a = 1, the right-hand side of (48) is (a − 1)c/3, which is negative. If
a = 1 but not d = 2, then (48) holds with strict inequality and the right-hand side of (48)is zero. Thus, there is no exogenous parameter that satisﬁes (45) in either case. ¥By Lemma 1, if there exists an exogenous parameter at which the supplier chooses totrade with one buyer, then a = 1 and d = 2. However, when a = 1 and d = 2, the supplieris indiﬀerent between trade with two buyers and trade with one buyer. No parameter
supports the supplier (strictly) preferring trade with one buyer to trade with two buyers.
Therefore, in bargaining based on the Shapley value, the supplier rarely trades with one
buyer.
We examine the merger incentive of buyers. Suppose that B1 and B2 merge. The
merged buyer is denoted by B12.

In this situation, a characteristic function V m :{S, B12} → R+ is as follows: V m (∅) = V m (S) = V m (B12) = 0 and V m (S, B12) =2v − (1 + a)c. In the Shapley value, S receives SVSm = (2v − (1 + a)c)/2 and B12 receivesm = (2v − (1 + a)c)/2. The buyers merge if and only if SV m ≥ SV 2 + SV 2 . We
SVB12
B12
B1
B2
m − (SV 2 + SV 2 ) = c(a − 1)/6 ≤ 0. Thus, in bargaining modeled by theobtain SVB12
B1
B2

Shapley value, the merger does not beneﬁt the buyers.
In the Shapley value, the probability of each order is the same. This reﬂects the factthat the supplier and the buyers are treated symmetrically and the players have the samebargaining power.28 One of the ways to investigate the eﬀect of asymmetric bargaining
power, introduced in simultaneous bargaining, is to assign diﬀerent probabilities to the28

There are similarities between the Shapley value analysis and the simultaneous bargaining analysis

with β = 1/2. In the case of the simultaneous bargaining analysis with β = 1/2, d ≥ 2. This means thata trade with one buyer rarely occurs in equilibrium.

33

order of the three players. In our game, as you can see in the characteristics functions,the marginal contribution of a player increases as he/she comes to the negotiation later.
If we interpret this to mean that a higher marginal contribution entails greater bargaining
power, then we can represent bargaining power by order of players. If the supplier has
relatively strong bargaining power, the probability that the supplier arrives in the third
place is relatively high and vice versa. If the supplier arrives in third place, he/she canextract the entire surplus. Thus, if the supplier negotiates with two buyers and if theprobability that the supplier is the third arriver is suﬃciently low, the supplier’s payoﬀ
in the case of one buyer may be greater than his/her payoﬀ in the case of two buyers.
Asymmetric treatment of the order may create the possibility that the supplier beneﬁts
from trading with two buyers and that the buyers beneﬁt from merging.
Stole and Zwiebel (1996a, b) introduce a sequential and bilateral bargaining model.
They examine intraﬁrm wage bargaining between an employer and employees. They do
not examine a buyer–supplier network. Unlike our sequential bargaining model, Stole andZwiebel (1996a, b) assume that contracts between players are nonbinding and players canrenegotiate contract details. They show that the equilibrium outcome of the sequential
bargaining with renegotiation coincides with the Shapley value. Clarke et al. (2008) andJeon (2006) apply the Shapley value to the buyer–supplier network and analyze the merger
incentive of players. They do not examine the optimal number of buyers. Shapley (1953)
and Kalai and Samet (1987) introduce a generalization of the Shapley value. In the generalized Shapley value, the order of players’ arrival is treated asymmetrically. Applicationof the generalized Shapley value may change our result above.

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