One day in 2004, Paula Bernstein, a thirty-five-year-old New York writer, woke up to learn that she had an identical twin whom she'd never met. That same day, Elyse Schein, a thirty-five-year-old filmmaker living in Paris, made the same discovery. Schein and Bernstein suddenly found their most basic assumptions about identity challenged. Their attempts to piece together what happened to them as infants, and for each to become acquainted with her doppelganger in midlife, led to their co-authored book Identical Strangers, an unusual combination of memoir and detective story.

Despite all the Tina Fey/Sarah Palin jokes, there appear to be only a few hundred twins who have ever really been separated at birth. What makes these twins' journey even stranger is that they were divided from each other intentionally, without their adoptive parents' informed consent or knowledge, for a medical research study that was never published.

Schein and Bernstein each tell overlapping versions of what happened, from perspectives that can be surprisingly different. We follow as they attempt to unearth their past, understand the researchers' motivations and, ultimately, see if they can find their shared birth mother. Early on, they learn that their genetic mother had been hospitalized for schizophrenia. Their evolving story raises some interesting and challenging questions about "nature vs. nurture," and how little we still know about mental health.

The book has a depth that goes far beyond the "twins reunited as adults" headline. Identical Strangers has an unusual emotional honesty, particularly given that it has two co-authors with such a complicated relationship. Bernstein, with a stable marriage and a new daughter, openly acknowledges her initial ambivalence at being "found," and Schein is clearly shocked by that ambivalence. Both freely confess their insecurities about their physical attributes, bouts of depression, how they might be seen by the other, and so on. It is intimate rather than tawdry, and it makes it much more possible to imagine how we might react if we found ourselves in their newly shareable shoes.

I recently caught up with Bernstein in a Brooklyn café. Now a vivacious auburn-haired forty-three-year-old with two daughters, she sipped a mint iced tea as we talked about the writing process and life after publication. I'd wondered about how she and her sister navigated co-authoring a book while simultaneously coping with such -- well, family drama. She laughed, and agreed that it might have been easier to just write their sections and let an editor sort it all out. Writing collaboratively can be a challenge under the best of circumstances and, as her sister wrote, "[They] are strangers who inhabit the same body."

As we get an insider's view of Schein and Bernstein's complicated journey, we are reminded of questions posed by Wim Wenders' "Wings of Desire," a film they both love: "Why am I me, and why not you? Why am I here, and why not there?" They -- and we -- are still searching for those answers.