Lacey Schwartz, a 37-year-old Harvard Law School graduate turned filmmaker, moves with ease in circles in which her identity as both black and Jewish seems unremarkable. What makes her biography striking is that Ms. Schwartz, a woman with light brown skin and a cascade of dark curls, grew up believing she was white.

How and why that happened is the subject of her film, “Little White Lie,” which has its offical world premiere on Sunday at the San Francisco Jewish Film Festival, (after screenings at the Woods Hole Film Festival in Cape Cod and the BlackStar Film Festival in Philadelphia) and continues on the festival circuit before being broadcast on PBS next year. With Ms. Schwartz narrating, the camera travels to a funeral, girlfriend gab sessions and even her therapy appointments. At each stop, in raw conversations with family and friends, Ms. Schwartz asks over and over, how and why did she pass as white?

“I come from a long line of New York Jews,” she says early in the film, as photographs of her white relatives flash across the screen. “My family knew who they were, and they defined who I was.”

Ms. Schwartz was an only child who grew up in the mostly white town of Woodstock, N.Y. Her parents, Peggy and Robert Schwartz, told her that she favored her father’s swarthy Sicilian grandfather. It was not until she went off to college that she learned the truth.