Abstract
We examine the role of private information on the impact of vertical mergers. A vertical merger can improve the information that is available to an upstream monopolist because, after the merger, the monopolist can observe the cost of its downstream merger partner. In the premerger world, because the costs of the downstream firms are private information, the monopolist has incomplete information and cannot implement the monopoly outcome: The expected premerger equilibrium price of the downstream product is lower than the monopoly price. After a vertical merger, the equilibrium input price that is charged to the downstream rival can either increase or decrease—depending on whether the downstream merger partner’s cost is low or high, respectively. However, in all cases the equilibrium price of the downstream product increases to the monopoly price. Therefore, the merger leads to consumer harm even when it leads to a reduction in the input price. The merged firm, however, cannot extract all of the monopoly profit: The merger causes production inefficiency (when the downstream rival has a relatively small cost advantage) and the downstream rival still earns an information rent (when it has a relatively large cost advantage). These results also have implications for vertical merger policy.
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Notes
Our main results do not change if instead the buyer uses a sealedbid firstprice auction.
We discuss those assumptions in Sect. 5.
Intuitively, dropping out before the price reaches one’s cost yields zero profit, while continuing to participate yields a positive profit if the other bidder is about to drop out; and continuing to participate when the price falls below one’s cost yields a negative profit if one wins.
Intuitively, the buyer asks each seller to submit, in a sealed envelope, the price at which the seller wants to drop out. The buyer then opens the two envelopes, selects the seller with the lowest bid as the winner, and pays to the winner a price equal to the bid of the other seller. This “replicates” the descendingprice auction, and thus bidding at cost is the dominant strategy for each seller.
The other solution of the quadratic equation does not satisfy \(W\in \left(VB,VA\right)\).
Given our assumption that D1 and D2 draw their costs \({C}_{1}\) and \({C}_{2}\) from the same distribution, D1 and D2 use the same bid function \(b(\bullet )\), i.e., their bids are \(b(W+{C}_{1})\) and \(b(W+{C}_{2}\)), respectively. While the bid function in the firstprice auction is different from that in the descendingprice auction, both are strictly monotonic and satisfy \(b\left(V\right)=V\). It follows that the probability that a downstream firm participates in the auction is the same for both types of auction, and hence U’s profit function and the optimal input price are also the same.
In the auction literature, this result is known as the Revenue Equivalence Theorem. For any given realization of the (independent) cost draws, the firstprice auction and the descendingprice auction lead to different prices; however, across all possible realizations, the expected or average price is the same for both types of auction.
The optimality of bidding \(W+{C}_{1}\) can be checked by considering UD1’s decision of whether to drop out of the auction or not, as the price descends gradually. If UD1 drops out when the price is \(P\), it will earn a profit of \(Wc\) by supplying the input to D2. If instead UD1 does not drop out, UD1 can win the auction and earn a profit that is equal to \(Pc{C}_{1}\) if D2 drops out in this bidding round; if D2 does not drop out, then UD1 has the option of dropping out in the next round when the buyer will reduce the price by another (small) increment. It follows that UD1 will drop out as soon as the price reaches the point where UD1’s profit from winning, \(Pc{C}_{1}\), is equal to its profit from losing: \(Wc\). That happens when \(P=W+{C}_{1}\).
UD1 is still following the downstream dominant strategy of bidding the minimum of \(V\) and its own costs (including the opportunity cost). But the function \(\widehat{W}\left({C}_{1}\right)\) is such that \(\widehat{W}\left({C}_{1}\right)+{C}_{1}>V\), so the optimal downstream strategy is to bid \(V\).
If the buyer’s budget is relatively large, with \(V\ge c+B\), then downstream production by UD1 is always viable, and thus only the second part of Proposition 2 applies. Figure 2 assumes that \(V<c+B\), so that downstream production by UD1 can be either viable or not viable.
The proof is provided in the “Appendix”.
When costs are uniformly distributed, partial foreclosure occurs when \({C}_{1}<2\left(V{W}^{*}\right)A\).
Intuitively, whereas premerger an increase in \(W\) above \({W}^{*}\) resulted in the loss of marginal sales regardless of which downstream firm was more efficient, postmerger the merged firm no longer loses those sales when D1 is more efficient, since with the elimination of double marginalization it can still profitably sell at price \(V\). At the optimal \(\widehat{W}\): the range of cost realizations where the buyer receives no surplus expands; the region where the price is below \(V\) shrinks; and the downstream price that results from the auction is higher at every point in that region. Thus, consumer surplus falls. D2 still receives some sales and profits when it is more efficient. Details are available upon request.
If the buyer uses a reserve price, she will start the auction at a price below her budget. In any event, the bidding strategies of D1 and D2 are not affected.
\(V<{C}_{1}+W\) implies \(V{C}_{1}c<Wc.\)
For \({C}_{1}=A\), we have \(F\left({\mathrm{C}}_{1}\right)=0\) and \(\left({C}_{1}A\right)f\left(A\right)=0\), but the argument goes through: UD1 increases profits by picking \(W\) in the interior of Region 2.
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Moresi, Reitman, and Sarafidis are Vice Presidents, Charles River Associates (CRA), Washington, DC. Salop is Professor of Economics and Law, Georgetown University Law Center, and Senior Consultant, CRA. The views expressed herein are those of the authors and do not represent or reflect the views of CRA or any of the organizations with which the authors are affiliated. The authors received financial support from Raytheon Technologies on an earlier version of this article.
Appendix
Appendix
Proof of Proposition 2
We begin with the case when downstream production by UD1 is not viable: \(c+{C}_{1}>V\). UD1 participates in the market only as a supplier to D2 and sets \(W\) to maximize upstream expected profits. Those profits are given by \(\left(Wc\right)F\left(VW\right)\), are strictly increasing in \(W\) at \(W=c\), and are strictly decreasing in \(W\) at \(W=VA\). Thus, the optimal input price \(\widehat{W}\in (c,VA)\). Since \(F\) is logconcave, profits are strictly quasiconcave in \(W\) for all \(W\in (c,VA)\). From the firstorder condition, \(\widehat{W}\) satisfies
When costs are uniformly distributed, \(\widehat{W}=\frac{1}{2}\left(VA+c\right)\).
To summarize, when \({C}_{1}>Vc\), downstream production by UD1 is not viable, UD1 sets the input price \(\widehat{W}\) given by (A1), UD1 does not participate in the auction, and either D2 participates and wins (if \({C}_{2}\le V\widehat{W}\)), in which case the buyer pays \(V\) to D2, or D2 does not participate (if \({C}_{2}>V\widehat{W}\)), in which case there is no sale.
We now turn to the case when downstream production by UD1 is viable: \(c+{C}_{1}\le V\). It is helpful to partition the range of possible values for \(W\) into three regions:
Region 1: \({\varvec{W}}>{\varvec{V}}{\varvec{A}}\)
If \(W\) is in this region, even the most efficient type of D2 would not bid, since \(W+A>V\). This is in effect an exclusionary strategy by UD1, where it is the only bidder in the auction. Without competition, UD1 always bids \(V\). Therefore, expected profit from an exclusionary input price strategy is \(V{C}_{1}c\).
Region 2: \({\varvec{V}}{{\varvec{C}}}_{1}\le {\varvec{W}}\le {\varvec{V}}{\varvec{A}}\)
Since \(V<{C}_{1}+W\) for \(W\) in (the interior of) this region, if D2 participates in the auction, then UD1 is better off letting D2 win, since the gains from losing and only supplying the input, which are \(Wc\), are larger than the gains from winning at price \(V\), which are \(V{C}_{1}c\).^{Footnote 15} If instead D2 does not participate, then UD1 maximizes its profit by bidding \(V\) and earning \(V{C}_{1}c\). Thus, in Region 2 the optimal downstream bid of UD1 is \(V\).
Given this auction strategy, it remains to determine what wholesale price within Region 2 is optimal and what is UD1’s (expected) profit at that price. Profits for UD1 in Region 2 are
The firstorder condition is
Note that when \(W=V{C}_{1}\), \(\Pi =V{C}_{1}c\) and \(\frac{d\Pi }{dW}=F\left({\mathrm{C}}_{1}\right)>0\); while at the other end of the region when \(W=VA\), \(\Pi =V{C}_{1}c\) and \(\frac{d\Pi }{dW}=\left({C}_{1}A\right)f\left(A\right)<0\).^{Footnote 16} Thus, UD1 increases profits by picking \(W\) in the interior of Region 2, and in so doing earns higher expected profits than \(V{C}_{1}c\) and, hence, higher expected profits than it receives from the exclusionary strategy in Region 1.
From the firstorder condition, \(\widehat{W}\) satisfies
When costs are uniformly distributed, the solution to the firstorder condition is halfway between the two endpoints of Region 2: \(\widehat{W}=V\frac{1}{2}\left(A+{C}_{1}\right)\). For later use, note that (A2) implies that \(\widehat{W}\) is a strictly decreasing function of \({C}_{1}\) (for \({C}_{1}\le Vc\)).
Region 3: \({\varvec{W}}<{\varvec{V}}{{\varvec{C}}}_{1}\)
In this region, UD1’s profit if D2 wins is equal to \(Wc\) (which is smaller than \(V{C}_{1}c\)) and UD1’s profit if it wins is at most equal to \(V{C}_{1}c\). Recall that \(c+{C}_{1}\le V\), and so a sale will be made regardless of the chosen value of \(W.\) Thus, if UD1 sets \(W\) in this region, it earns lower expected profits than if it sets \(W\) in Region 2.
To summarize, when \({C}_{1}\le Vc\), downstream production by UD1 is viable, UD1 sets the input price \(\widehat{W}({C}_{1})\) given by (A3), UD1 bids \(V\), and either D2 participates and wins (if \({C}_{2}<V\widehat{W}({C}_{1})\)), in which case the buyer pays \(V\) to D2, or D2 does not participate and UD1 wins (if \({C}_{2}\ge V\widehat{W}({C}_{1})\)), in which case the buyer pays \(V\) to UD1.
Proof that the main results in Proposition 2 also apply if the buyer uses a firstprice auction
We want to show that the following strategies and beliefs constitute a perfect Bayesian equilibrium:

If downstream production by UD1 is not viable (if \({C}_{1}>Vc\)), UD1 sets the input price equal to \(\widehat{W}\) (as given by (A1)), and does not bid. If instead downstream production by UD1 is viable (if \({C}_{1}\le Vc\)), UD1 sets the input price equal to \(\widehat{W}({C}_{1})\) (as given by (A3)), and bids \(V\).

If D2 observes \(\widehat{W}\), D2 believes that UD1 does not bid, and D2 either bids \(V\) or does not bid (depending on whether D2 is viable or not). If D2 observes \(\widehat{W}\left({C}_{1}\right)>\widehat{W}\), D2 believes that UD1 bids \(V\), and D2 either bids slightly below \(V\) or does not bid. If D2 observes an input price either strictly lower than \(\widehat{W}\) or strictly higher than \(\widehat{W}(A)\), D2 believes that UD1 does not bid, and D2 either bids \(V\) or does not bid.
One can check that (1) given the input price strategy of UD1, D2’s beliefs satisfy Bayes rule; (2) given the bid strategy of UD1 and the beliefs of D2, D2’s bid strategy is a best response; (3) given UD1’s input price strategy and D2’s bid strategy, UD1’s bid strategy is a best response. Finally, one can check that UD1 has no incentive to deviate and use a different input price strategy.
To see this last point, suppose first that downstream production by UD1 is not viable and, instead of setting the input price equal to \(\widehat{W}\), UD1 were to set \(W\in (\widehat{W},\widehat{W}\left(A\right)]\). Then, D2 would (wrongly) believe that downstream production by UD1 is viable and that UD1 will bid \(V\) (instead of not bidding). Thus, D2 would either bid slightly below \(V\) (instead of bidding \(V\)) or not bid if it is not viable to bid. Such deviation would reduce UD1’s expected profit because its effect on the probability of a sale is virtually the same as when the buyer uses a descendingprice auction (and we know from Proposition 2 that the deviation is not profitable). If instead UD1 were to set \(W\notin [\widehat{W},\widehat{W}\left(A\right)]\), D2 would still believe that downstream production by UD1 is not viable, and such deviation also would reduce UD1’s expected profit for the same reason.
Suppose now \({C}_{1}<Vc\) so that downstream production by UD1 is viable, but UD1 sets \(W>\widehat{W}\left(A\right)\) or \(W\le \widehat{W}\), instead of setting \(W=\widehat{W}\left({C}_{1}\right)\). Then, D2 would (wrongly) believe that downstream production by UD1 is not viable and that UD1 will not bid (instead of bidding \(V\)). Thus, D2 would either bid \(V\) (instead of bidding slightly below \(V\)) or not bid if it is not viable. Such deviation would reduce the expected profit of UD1, because (i) its effect on the probability of a sale would be the same as when the buyer uses a descendingprice auction, and (ii) when D2 is viable and \(W>V{C}_{1}\), UD1 and D2 would tie but UD1 would prefer D2 to win. If instead UD1 were to deviate and set a different price \(W\in (\widehat{W},\widehat{W}\left(A\right)]\), D2 would still believe that downstream production by UD1 is viable, and thus such deviation also would reduce UD1’s expected profit, as is the case when the buyer uses a descendingprice auction.
Proof of Proposition 3

(i)
If downstream production by UD1 is not viable, then \(\widehat{{\varvec{W}}}<{{\varvec{W}}}^{\boldsymbol{*}}\) .
From the main text, the firstorder condition for \({W}^{*}\) is
Rearranging terms, \({W}^{*}\) satisfies
The postmerger firstorder condition for \(\widehat{W}\) in the case when downstream production by UD1 is not viable (\(c+{C}_{1}>V\)) is
Using (A4) to evaluate this firstorder condition at \(\widehat{W}={W}^{*}\),
Therefore, in this case, the profitmaximizing value of \(\widehat{W}\) satisfies \(\widehat{W}<{W}^{*}\).

(ii)
If downstream production by UD1 is viable, then \(\widehat{{\varvec{W}}}\left({{\varvec{C}}}_{1}\right)>{{\varvec{W}}}^{\boldsymbol{*}}\) if \({{\varvec{C}}}_{1}\) is sufficiently low.
Rearranging the firstorder condition for \({W}^{*}\) once again, \({W}^{*}\) satisfies
The firstorder condition for \(\widehat{W}\left({C}_{1}\right)\) when downstream production by UD1 is viable is given by (A2). When we evaluate (A2) at \({W}^{*}\) with the use of (A5),
As shown in Proposition 1, \({W}^{*}<VA\), and so \({W}^{*}+{C}_{1}V<0\) if \({C}_{1}\) is sufficiently low (sufficiently close to \(A\)). Thus, for sufficiently low \({C}_{1}\), \({\left.\frac{d\Pi }{d\widehat{W}}\right}_{\widehat{W}={W}^{*}}>0\). Therefore, for sufficiently low \({C}_{1}\), \(\widehat{W}\left({C}_{1}\right)>{W}^{*}\).
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Moresi, S., Reitman, D., Salop, S.C. et al. Vertical Mergers in a Model of Upstream Monopoly and Incomplete Information. Rev Ind Organ 59, 363–380 (2021). https://doi.org/10.1007/s1115102109833y
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DOI: https://doi.org/10.1007/s1115102109833y
Keywords
 Vertical mergers
 Monopoly
 Foreclosure
 Incomplete information
 Antitrust
JEL Classification
 L1
 L12
 L4
 L41
 L42