Late in the movie Chinatown, private investigator Jake Gittes orders his client, Evelyn Mulray, to identify a young woman she has protected. Between blows to the face from a disbelieving Gittes, Mulray utters the grim secret that reveals her tycoon father’s venality and captures the film’s unnerving ambiguity: “She’s my daughter. She’s my sister. . . . She’s my sister and my daughter.”

Petroleum company executives who plan mergers may feel that assessing antitrust risks entails Chinatown doses of ambiguity and uncertainty. Like Jake Gittes, business managers pummel antitrust lawyers for reliable predictions about what government enforcement agencies will do. Buffeted by shifts in the process and substance of antitrust oversight, the lawyers respond in terms that echo Evelyn Mulray: “It’s getting better. It’s getting worse. It’s getting better and worse.”