I read DVD Savant's DVD review of Taxi Driver at http://www.dvdtalk.com/reviews/read.php?ID=47768 and...
any talk about what may have influenced "Taxi Driver" should start and end with the name Arthur Bremer. Arthur Bremer was an angry loner that kept a journal (later published) who was in love with an underage girl. His obsession with pornography frightened her away. Arthur Bremer seemed to lose his mind then shaved his head and later tried to assassinate presidential candidate George Wallace. Paul Schrader has claimed he knew nothing about Bremer, but that when he found out it proved he was on the right track with his screenplay. The big difference was Arthur Bremer (who has now been released!) was not declared a national hero (like Travis Bickle) and that the girl he frightened away did not return and flirt with him. That might have been what Bremer was hoping for, but it's the kind of thing that only happens in the movies.