InStyle has the answer:
Robert Pattinson looked to Marc Jacobs for his all-black ensemble, but attributed his signature disheveled 'do to a "nervous tic" rather than a styling technique. "It doesn't take any time at all!" the Brit said, claiming that he gets the look simply by running his hands through his (seldom washed) hair. "I've never had so many people say, 'It must be mousse!'"