Sickert sent letters addressed from the Ripper

The argument

During the period of the Ripper's crimes, multiple letters were sent to Scotland Yard and various newspapers by individuals purporting to be Jack the Ripper. Many of these "Ripper letters" were preserved, enabling DNA analysis of the stamps and envelopes used once that technology was developed. Their preservation is what gives them their weight in the case, because they constitute some of the last remaining physical evidence: they are objects that were handled, sealed and stamped by whoever sent them, and so they retained trace material that could still be examined long after the events themselves. Author Patricia Cornwell had some of the Ripper letters analyzed by forensic scientists, who determined that mitochondrial DNA which could have belonged to Walter Sickert was present on the letters. The significance claimed for this finding rests on the nature of the sample. Stamps and envelopes are moistened and handled by the person posting them, so biological material recovered from those surfaces is argued to come from the sender rather than from a later reader or archivist, which is what makes the result relevant to the identity of the writer at all. A comparison between the Ripper letters and Sickert's own letters also revealed that both contained the watermark of the Aberdeen paper manufacturer Alexander Pirie and Sons. A watermark identifies a particular manufacturer's stock, so on this reasoning the match narrows the field of possible senders to people with access to that paper, and Sickert is demonstrably among them because his own correspondence carries the same mark. Two findings from some of the last remaining physical evidence in the Ripper case thus point to the same individual: Walter Sickert. The strength claimed for the identification lies in that convergence. The documentary trail and the biological trail were examined independently and by entirely different methods, and on this account they arrive at one name rather than at two unrelated suspects.

Premises

Counter-arguments

Ripperology specialists have challenged this identification on several grounds. Most of the hundreds of "Ripper letters" are widely regarded, including by police at the time, as hoaxes written by journalists and cranks rather than the killer, so DNA or watermarks linking a letter to Sickert would show he wrote a letter, not that he committed the murders. The mitochondrial DNA reported is not unique to an individual and could match many thousands of people, and the Pirie and Sons paper was a mass-produced commercial stock, not a personal marker. Critics also note that Sickert is documented as having been in France during at least some of the killings. On this view the letter evidence does not establish that Sickert was the Ripper.

Rejecting the premises

[Rejecting P1] Mitochondrial DNA is shared among large numbers of maternally related people and is not individually identifying, so a "consistent" match does not single out Sickert. [Rejecting P2] Pirie and Sons paper was mass-produced commercial stock; a shared watermark shows common paper, not common authorship. [Rejecting P3] Since most Ripper letters are considered hoaxes, linking a letter to Sickert would at most show he wrote a hoax letter, not that he was the murderer — and evidence places him abroad during some of the crimes.