Celebrities are increasingly front and centre in public debates on everything from solving world poverty to halting genocide, confronting obesity, and finding spiritual contentment. Bono, Bill Gates, Al Gore, Bob Geldof, Oprah, Madonna, and Angelina Jolie are just some of the entertainers, politicians, pundits, elite business people, and policy-makers whose highly visible political activism has become an integral part of their public personas. These...