At the age of 37, Jack Nicholson discovered that his “mother” was actually his grandmother, and that his “older sister” was really his mother.
This revelation did not come directly from his family, but by a Time magazine reporter who was working on a cover story of the actor.
Nicholson told Rolling Stone in 1984, “I was making The Fortune, and someone called me on the phone – I think it was turned over by the investigative reporting for the Time cover story they did on me. Ultimately, I got official verification from (aunt) Lorraine. I was stunned.”
Nicholson never found out who his father was.
Source
The Great Seducer: Jack Nicholson (Nancy Collins, Rolling Stone: March 29, 1984)