In 1974, Time magazine researchers learned, and informed Nicholson, that his "sister", June, was actually his mother, and his other "sister", Lorraine, was really his aunt. By this time, both his mother and grandmother had died (in 1963 and 1970, respectively). On finding out, Nicholson said it was "a pretty dramatic event, but it wasn't what I'd call traumatizing ... I was pretty well psychologically formed".
In 1974, Nicholson starred in Roman Polanski's noir thriller Chinatown, and was again nominated for Academy Award for Best Actor for his role as Jake Gittes, a private detective. The film co-starred Faye Dunaway and John Huston, and included a cameo role with Polanski. Roger Ebert described Nicholson's portrayal as sharp-edged, menacing, and aggressive, a character who knew "how to go over the top", as he did in One Flew Over the Cuckoo's Nest. It is that edge that kept Chinatown from becoming a typical genre crime film. Ebert also notes the importance of the role for Nicholson's career, seeing it as a major transition from the exploitation films of the previous decade. "As Jake Gittes, he stepped into Bogart's shoes", says Ebert. "As a man attractive to audiences because he suggests both comfort and danger ... From Gittes forward, Nicholson created the persona of a man who had seen it all and was still capable of being wickedly amused."