"Psycho" was a comedy.

Brilliant.

Okay, so the source is Wikipedia, which quotes Hitchcock: The director and producer Alfred Hitchcock popularized both the term "MacGuffin" and the technique. According to the Oxford English Dictionary, Hitchcock explained the term in a 1939 lecture at Columbia University: "[We] have a name in the studio, and we call it the 'MacGuffin.' It is the mechanical element that usually crops up in any story. In crook stories it is almost always the necklace and in spy stories it is most always the papers."
Okay, what's the MacGuffin in "Chinatown"? One or many?