Trivia / The Cat o' Nine Tails

• Creator Backlash: Nine Tails is Argento's least favourite film made by him. It's also his only time working with writer Dardano Sacchetti, whom he had a very-public falling out with over a pay dispute.
• Fake Nationality: James Franciscus (American), Karl Malden (American), and Catherine Spaak (Franco-Belgian) play Italians.
• Science Marches On: At the time the film was made there was some speculation that individuals with XYY syndrome might be more susceptible to criminal behavior, writer Dardano Sacchetti got the idea from a magazine article on the very topic. Today, it's well-established that XYY syndrome is more likely to give people acne than homicidal urges, and the exact role of genetics in criminality is very much still up for debate.
• Shout-Out: During the nighttime burglary at the Terzi institute, a doctor can be clearly heard whistling Ennio Morricone's theme from Argento's previous film, The Bird with the Crystal Plumage.