The effect of celebrity involvement on international activism?

As the world’s supply of elder statesmen consolidate their power, it occurs to me to wonder if there has ever been a quantitative measure of the effects of these international relations "superheroes" (hat tip to Opinio Juris for that comparison) on the success of their projects. So much persuasion on the world stage is accomplished by going public (in the political science sense of the phrase), but how much, exactly, does a personage like Jimmy Carter or Desmond Tutu add to the fight? Does their attachment primarily activate the public’s interest, or influence decision-makers directly? Does it serve as a catalyst for change itself, or is it simply a "tipping point" function of the activism that has already been done on the issue?