The expansion of our digital universe — Second Life, Facebook, MySpace, Twitter — has shifted not only how we spend our time but also how we construct identity. For her coming book, “Alone Together,” Sherry Turkle, a professor at M.I.T., interviewed more than 400 children and parents about their use of social media and cellphones. Among young people especially she found that the self was increasingly becoming externally manufactured rather than internally developed: a series of profiles to be sculptured and refined in response to public opinion. “On Twitter or Facebook you’re trying to express something real about who you are,” she explained. “But because you’re also creating something for others’ consumption, you find yourself imagining and playing to your audience more and more. So those moments in which you’re supposed to be showing your true self become a performance. Your psychology becomes a performance.” Referring to “The Lonely Crowd,” the landmark description of the transformation of the American character from inner- to outer-directed, Turkle added, “Twitter is outer-directedness cubed.”