The “little white lie” in Lacey Schwartz’s documentary is hardly trivial.

The first-person narrative follows the 30-something filmmaker’s effort to learn the identity of her biological father and, more importantly, force her parents to acknowledge and confront their painful secret.

Lacey Schwartz with her mother, Peggy

A fascinating modern mystery that paradoxically chooses not to explore the most interesting aspects of identity and race, “Little White Lie” has its world premiere at the San Francisco Jewish Film Festival. The doc screens four times during the festival, beginning Aug. 3 as the closing-night film at the Castro. A PBS broadcast will follow later this year or in 2015.

Schwartz grew up in Woodstock, N.Y., with doting parents. Her mother, Peggy, was married at 21, sticking to a path her parents had instilled.

“We didn’t think outside of the box,” Peggy recalls. “And sometimes it was easier that way.”

Going with the flow seems to have been Peggy and husband Robert’s credo. After Lacey was born in 1977, and relatives or friends would observe that she was darker-skinned than her parents, Robert would point to a photograph of his swarthy, Sicilian Jewish grandfather by way of explanation.

Schwartz offers numerous childhood pictures of herself, and it’s obvious that one of her parents is black. Was she adopted? That would make sense, and certainly wouldn’t be a shanda, but no, there’s a photo of a very pregnant Peggy.

Did Peggy have an affair? If so, neither she nor Robert ever said a word about it while they raised Lacey like any other Jewish girl.

“I wasn’t passing,” Schwartz tells us. “I actually grew up believing I was white.”

One of the odder aspects of this bizarre saga is that Peggy and Robert seemingly never anticipated that one day Lacey would have questions and demand answers.

It wasn’t until Lacey started high school — in a neighboring town with African-American students — that she began to experience serious cognitive dissonance. The black kids assumed she was black, though she thought she was white.

Schwartz gives the impression that in the ensuing years, through college and into adulthood, she had to work out her identity issues on her own with little to no help from her parents.

The perfectly titled “Little White Lie” eventually clears up the paternity mystery, but along the way the emphasis shifts to Schwartz’s ongoing confusion, frustration and insecurity. In its weaker moments, the film becomes a therapeutic record of, and a vehicle for, her rocky process of acceptance.

Peggy and Robert’s inability to take responsibility for the messy secret at the family core deprives Lacey of the catharsis she seeks, and “Little White Lie” of a poignant climax.

More regrettable, though, is Schwartz’s disinterest in pursuing a deeper discussion of identity, and the comparative influences of genetics and upbringing. The film operates on a relentlessly personal level that perhaps precludes a broader perspective, but it’s therefore baffling that Schwartz never specifies which Jewish and African-American practices and traits she maintains and cherishes.

Schwartz’s wedding partially addresses this oversight. The filmmaker joins in the hora circle and is lifted with her husband on chairs; a bit later she dances to an African-American rhythm.

The greater disappointment is that “Little White Lie” squanders a unique opportunity to bring Jewish values to a wide audience, and African-American culture and principles to a Jewish audience.

“Little White Lie,” 7 p.m. Aug. 3 at the Castro Theatre in S.F., 6:40 p.m. Aug. 4 at the California in Berkeley, 7 p.m. Aug. 7 at the Parkway in Oakland, 3 p.m. Aug. 9 at the Smith Rafael Film Center in San Rafael. The filmmaker will attend the S.F. and Berkeley screenings. (65 minutes)