Nobody knows how many people were killed by Jack the Ripper. Dr. Percy Clark, who assisted at some of the post-mortems on the victims, thought that one person was responsible for just three murders. He attributed the others to a weak-minded individual trying to emulate crimes published in the newspapers. Given that dozens falsely confessed to being the Ripper, including some in the twentieth century, it is possible that one or more were tempted to copy his crimes.

In my childhood there was a debate about the influence of television violence. Campaigner Mary Whitehouse cited a case where a boy attempted to drown his brother after watching a scene in Doctor Who. Today there is easy access to violent games, movies and pornography and anyone can read a book, or watch a documentary, containing explicit details about Jack the Ripper and other serial killers.

With these influences one might expect a rise in offences. In fact, the number of serial killers is declining. This could be due to improvements in detection and technology, suggesting that the potential to offend remains. Most known serial killers were active in the late twentieth century. Around the time that Mary Whitehouse was campaigning.