A merger investigation is the one realistic opportunity the government has to stop a potentially anticompetitive acquisition. Once the merger is completed it is far less likely that the government will launch enforcement action to stop anticompetitive practices such as price increases. It's much easier to prevent anticompetitive practices in the first place by helping antitrust agencies stop monopolistic mergers.

The type of information the government requests varies in each case. Typically, regulators are satisfied with interviews of key employees and, occasionally, some documents. If a case is headed for trial, an affidavit may be requested.

What complaints are most persuasive? In answering this question it's important to remember that the purpose of the antitrust laws is to protect consumers. Thus, the concerns that will be most persuasive to antitrust agencies are those that demonstrate that a merger is likely to result in higher prices, less innovation and, ultimately, harm to consumers.

It is extremely unlikely that the merging parties will ever discover the identity of complainants. All interviews are kept strictly confidential and government attorneys go to great lengths to guarantee they stay that way. Yes, if a case goes to trial a complainant could be selected as a witness. But the reality is that only a few cases a year go to trial.

Besides helping the government ensure a more competitive market, a company that cooperates could also put itself in a stronger competitive position. In some cases, concerns over a potentially problematic merger may be resolved by requiring the divestiture of various assets of the merged companies. A cooperating company might be able to put itself in a position to acquire those divested assets.

When not to answer the phone? The answer depends on your plans. If you plan to conduct acquisitions in the same market, it is not in your interest to cooperate and explain to authorities why an acquisition is problematic. If the deal is blocked, you may find the same arguments presented against your proposed acquisition.

Ultimately, a "merger is forever." Unraveling an anticompetitive merger is extraordinarily difficult. Assisting the government in preventing those mergers from occurring in the first place generally is the best course.

David Balto is a partner with the law firm White & Case LLP and former policy director of the FTC's Bureau of Competition.