Victim isn’t a word I’d use to describe the kind of girls I’ve seen, surviving and thriving in an atmosphere that has become very hostile to them much of the time. How can this be, when girls are graduating from college in higher numbers than ever before, when they’re becoming leaders in their chosen fields in greater numbers? From what we hear, American girls are among the most ­privileged and successful girls in the world. But tell that to a 13-year-old who gets called a slut and feels she can’t walk into a school classroom because everybody will be staring at her, texting about her on their phones.

So why do some girls post sexualized pictures? Why are they complicit in this potentially very self-­undermining aspect of social­media culture? “I think it’s just to get attention,” explains Lily, a 14-year-old in Garden City, N.Y., where I studied a group of girls for the book. “It’s to get the likes. Everything’s about the likes.”

If building a social-media presence is similar to building a brand, then it makes a twisted kind of sense that girls—­exposed from the earliest age to sexualized images, and encouraged by their parents’ own obsession with self-promotion—are promoting their online selves with sex. In so doing, they’re also following the example of the most successful social-­media celebrities.