Veteran British actor SIR Derek Jacobi is convinced William Shakespeare's plays were written by more than one person - and is demanding further investigation into the playwright's life. The Gladiator star has signed a 287-strong petition on a "declaration of reasonable doubt" about the authorship of his plays, including MACbeth, Romeo + Juliet and Othello. The Shakespeare Authorship Coalition hopes this will instigate further research into the issue, which questions whether a 16th Century commoner raised in an illiterate household could have written plays about the upper classes and the law without referring to his own background. Jacobi says, "I subscribe to the group theory. I don't think anybody could do it on their own."