A child at loose ends needs help, and someone steps in--a Big Brother, a Big Sister, a mentor from the growing ranks of volunteers offering their time and guidance to more than two million American adolescents. Does it help? How effective are mentoring programs, and how do they work? Are there pitfalls, and if so, what are they? Such questions, ever more pressing as youth mentoring initiatives expand their reach at a breakneck pace, have occupied...