Original sentence: Many[who.] believe that Anno's four-year period of depression was the main source for many of the psychological elements of the series and its characters, as he wrote down on paper many of the trials and tribulations of his condition
Edited sentence: Many[citation needed] believe that Anno's four-year period of depression was the main source for many of the psychological elements of the series and its characters, as he wrote down on paper many of the trials and tribulations of his condition