 In the DVD 2004 release, George Lucas explained the reason behind  why Yoda told Luke that Darth Vader was his father. Lucas had  consulted with a child psychologist during the making of the film.  The psychologist said that unless it was unequivocally stated that  Vader was Lukes father, moviegoers age 12 and under would dismiss  Vaders claim to be Lukes father as a lie.