Lucy Letby and behavioural profiling

A leopard goes to the doctor.“Doc,” he says, “You gotta help me. Whenever I look at my wife, I see spots.”The doctor looks at him for a minute and says, “Well, what do you expect? You’re a leopard.”“I know that, Doc. But I’m married to a zebra.”

Healthcare serial murder enquiries are unusual in that they generally start without specific evidence of wrongdoing as such. Suspicion generally arises from an unusually large cluster of unexpected deaths. From there, prosecutors work backwards towards a perpetrator.

Their job is made harder by two inconvenient facts: firstly, hospitals generally, and intensive care units specifically, are places where “unexpected deaths” are a regrettable part of a normal operating environment. There is a “going rate” of unexpected collapse, and a “suspect cluster” is simply one that exceeds that going rate, usually not by much: a standard deviation or two above the hospital’s average. The elevated rate might be what you’d expect once in ten years, say, or fifty — but not one in a million. According to Professor Sir David Spiegelhalter’s evidence before the the Thirlwall Inquiry, the increase in deaths at the Countess of Chester Hospital’s neonatal unit in 2015 would be within the expected range for hospitals across the UK in any year:

“If we assume an underlying rate of 3 neonatal deaths per year, then the probability of getting 8 or more deaths in 2015 is 0.02. This would generally be considered as constituting an “alert” signal. Again, to put this in perspective, we would expect around 3 such signals each year in the UK, just by chance alone.”

Secondly, “malicious nurses furtively murdering patients” is a one-in-a-million sort of thing. You would not expect to see it “every so often” in a given ICU.

All other things being equal, you would take the “one in fifty” option over the “one in a million” option. From a starting point of there being “an unusual cluster of deaths”, there is a quite likely explanation — it’s just one of those things — and a highly unlikely one — there’s a serial murderer in the ward — and no evidence, as yet, to tell us which way to lean. But as the saying has it: when you hear hoofbeats, think horses, not zebras. At least, until you see some stripes.

A prosecution intent on proving murder will need “posterior evidence” to support its hunch and rebut the far more likely explanation that the cluster is just “one of those things”. Investigators must therefore scratch around looking for secondary indicators of foul play that were not picked up at the time of the deaths and, if they find any, a plausible suspect having both the opportunity and disposition to murder.

The first question — “was it murder?” — can be addressed by technical medical evidence. That is not the topic of this post.

This post is about the second question: “given that it was murder, whodunnit?” Here, generally, you need solid “identification evidence” incriminating a specific person. Eyewitnesses. Fingerprints. Confessions. That kind of thing.

The problem with healthcare serial murder cases is that they start and often end without compelling identification evidence. Prosecutors are often obliged to resort to unusual things the suspect did, or said, that do not specifically indicate that she committed a crime, but are more broadly “consistent with” it. This is a bit of a recipe for confirmation bias.

In the 1970s, the FBI’s legendary Behavioural Science Unit learned how subtle behavioural patterns could lead them to notorious serial killers. Would their techniques apply, or be any use, in a classic healthcare serial murder case?

When the lambs stopped screaming

Lecter: Oh, Agent Starling. You think you can dissect me with this blunt little tool?Starling: No! I thought with your knowledge —Lecter: You’re so ambitious, aren’t you.—Silence of the Lambs (1991)

In 1972, Howard Teten and Patrick Mullany established the FBI’s Behavioural Science Unit in Quantico, Virginia. You may remember it form the opening scenes of Silence of the Lambs. The FBI had long recognised that certain types of violent crime — “serial” ones, particularly — pose unique challenges to traditional policing methods. Serial killers murder in the shadows. They tend not to leave living witnesses, nor much in the way of physical evidence behind them.

By contrast, most normal violent crime happens between people who know each other and are embedded in stable, stationary, long-established social networks. When violence breaks out there tend to be many witnesses, lots of