& Co., 388 U.S. 365, 379-80 (1967), overruled by Cont'l T.V., Inc. V. GTE Sylvania Inc., 433 U.S. 36 (1977). Networks in Telecommunications levels of concentration as to be tantamount to the same thing. 20 In Griffith, the Court went SO far as to indicate that liability could lie even in the absence of any intent or ability to acquire a second monopoly in another market (334 U.S. at 106-08). The Second Circuit took a similar position in Berkey Photo, indicating that "the use of monopoly power attained in one market to gain a competitive advantage in another is a violation of § 2, even if there has not been an attempt to monopolize the second market" (603 F.2d at 276). These decisions spawned a circuit split over whether Section 2 requires at least proof that the vertically related market was structured in a way that made the threat of monopolization credible.²1 Since that time, antitrust law has largely internalized the scholarly critiques reviewed in Chapter 4, which have emphasized that certain structural preconditions must generally be met before the producer of an input can plausibly have any ability to harm competition in a vertically related market. First, the firm must possess monopoly power over the input market. If not, any attempt to charge supracompetitive prices in any market will simply lead the firm's customers to turn to other sources. In addition and more importantly for our purposes, the vertically related market must be concentrated and protected by entry barriers for a threat to the competitiveness of that market to be credible. If not, any attempt to raise price in the vertically related market will cause existing competitors to increase their production or will attract new competitors until the price is forced back down to competitive levels.22 The post-Chicago models identifying circumstances under which input producers have the incentive to engage in vertical exclusion are based on dominant-firm and oligopoly market structures that presuppose that the relevant markets are highly concentrate