Mr. Langley said he believes the lack of motivation and a definitive backstory for the Joker, whose schemes range from mutating smiley Joker fish to murdering Robin, makes him more interesting.
"Clearly, we are influenced by popular culture," Mr. Langley said. "The tricky part is quantifying it. There are millions upon millions of influences on us all the time. Our culture is one of those influences, but we don't know to what degree. Even if it turns out that Holmes is preoccupied with Batman and the Joker, there are so many other variables involved."