The Daily Mail–I can hear some of you groaning already–claims that an amateur sleuth and a scientist have finally unmasked Jack the Ripper. They claim that their DNA analysis of a scarf ostensibly found on Catherine Eddows (3rd victim) is that of Whitechapel hairdresser Aaron Kosminski.

For The Mail this is a big wow, but for anyone who’s watched suspects come and go for any part of the last century and change, there are reasons for skepticism.

Susannah Bodman at OregonLive spells some of them out:

1. The Daily Mail’s reporting on science and scientific evidence is — let’s say — not known to be robust. For example, Media Matters took them to task last year for their reporting on climate change.

2. The chain of evidence or provenance on the shawl is less than stellar. In the piece on the Daily Mail’s website, the aforementioned amateur sleuth writes that the shawl is “said to have been found next to the body of one of the Ripper’s victims, Catherine Eddowes, and soaked in her blood. There was no evidence for its provenance, although after the auction I obtained a letter fromits previous owner who claimed his ancestor had been a police officer present at the murder scene and had taken it from there.” (Emphasis mine) I am of the camp that believes extraordinary claims require extraordinarily clean and robust evidence. A shawl with no provenance record and an association based on a family claim is not what I call extraordinarily robust.

3. While Russell Edwards, the sleuth, and Jari Louhelainen, the scientist, detail some of the methods of sample recovery and analysis in the Daily Mail’s piece, this is not the same as reporting and publishing your methods in a peer-reviewed journal (and Edwards’ “Naming Jack the Ripper” cited at the end is not what I’d call a PRJ). I’d want to see such a journal article and any subsequent critiques by other scientists before I decide whether to accept this claim that the Ripper’s been identified. I did a quick search of Google Scholar and found nothing on this particular work from Louhelainen, but perhaps something will be forthcoming.

The only part of this I’d quibble with is her invocation of Sagan’s “extraordinary claims require extraordinary evidence dictum.” What Sagan was talking about here were UFO sightings and the advisability of searching for more mundane explanations for them before assuming the arrival of alien visitors. In the Ripper case, an extraordinary claim would be that Elvis did it, or aliens, or Redjac from Star Trek, or Eugene Victor Tooms, or Amanda Knox. (Don’t whisper that last one to the Italians. The woman has enough problems.) Aaron Kosminski, by contrast, crosses at least a minimum threshold of credibility as a Ripper suspect, insofar as Kosminski was in Whitechapel at the time of the killings, and police did mention someone named Kosminski as a suspect. Whether this Kosminski was Aaron Kosminski is another matter. Since the detective who named Kosminski said his man had been committed in 1889, and Aaron Kosminski was still at large at the time, there’s room for doubt. That said, the claim that Kosminski is the killer may be hard to substantiate given the amount of time that’s passed and the state of the evidence, but it’s not extraordinary.

The part of the Mail story that troubles me most is the issue of the chain of evidence, since the scarf would be the only physical evidence that links Kosminski to a Ripper crime scene. Reliable eyewitness testimony doesn’t place him at any of the five murder scenes. Kosminski was young and slender in 1888. Witnesses who thought they caught sight of the Ripper generally described him as older and stockier (this includes the witness who swears he saw Kosminski).

Another issue that strikes me as even more troubling than the lack of eyewitness evidence (eyewitness testimony is usually unreliable anyway) is that the Ripper killings in Whitechapel ceased after the frenzied slaying of Mary Kelly (The 5th victim). Mary Kelly’s death was exceptionally gruesome, leaving a mutilated corpse. Few criminologists think that a serial killer would just stop after such a murder. The thinking is that after Mary Kelly, the Ripper either died, moved, or went to prison on an unrelated charge. But Kosminski remained at large and in London until 1891, when he was committed to an insane asylum for paranoid schizophrenia. During his stays in asylums over the next three decades, he was never violent or troublesome, but he did increasingly suffer from the hallucinations and delusions common among the severely mentally ill.

The idea that Aaron Kosminski could go on a kill-crazy rampage, remain undetected, and afterwards have the self control to suppress all violent impulses for the remaining 30 years of his life despite his deepening mental illness may not be as extraordinary as Alien Jack The Rippers Harvesting Organs, but it’s more than a little hard to believe.

One of the things to remember when dealing with any claim is that a single piece of evidence is rarely powerful enough to confirm it.  We have to assess the reliability and quality of each particular piece of evidence then fit it together with the other most reliable pieces we have to see if, and where, the evidence converges. Jack the Ripper was hard to catch not because he was a homicidal genius, but because in 1888 reliable scientific evidence was practically non-existent (fingerprints wouldn’t be used as crime scene evidence for another 18 years), because psychology, both clinical and criminal, was in its infancy, and because serial killers, who usually lack obvious social connections to their victims, were (and are) hard to catch. For 126 years the evidence in the Ripper case has never converged around a credible suspect. To my mind, The Daily Mail notwithstanding, it still hasn’t.

Update: Smithsonian magazine has more on the issue of the scarf’s provenance as evidence and possible issues with the authors’ DNA testing procedure.

Update #2: NBC News also has an excellent write-up on the problems with the Kosminski evidence.

Related: Bigfoot DNA tests. (Spoiler: There’s still no evidence of Bigfoot. There is evidence, however, of an attention seeking crank.)