In 2007 Justin Bieber’s mom, Pattie Mallette, posted a video of her son, then 12, covering R&B star Ne-Yo’s “So Sick” at a kids’ singing competition in a little town near Toronto. “I told him, ‘Okay, you’ve never sung in front of anybody before, and you’ve never had singing lessons,’” Mallette told me. ‘Let’s adjust our expectations.’ ” Bieber finished third. “One of the judges actually left because she was upset that he didn’t come first,” Mallette said. Encouraged by his strong performance, Mallette started posting more videos on YouTube—mostly so Bieber’s grandmother could see him—and soon he had a...