Serial offenders sometimes catch the public interest, but only when
their crimes are particularly gruesome. Many criminals are serial offenders,
and this includes many burglars. Criminals establish a routine, a modus
operandi, and stick to it. It may be a lack of imagination, or simple
pragmatism. If a way of committing a crime gets them what they want without
detection, they persist in it. Far from being anything special, serial
offenders are simply criminals with a habit of offending. It’s a bit like the Olduvai Tool Set, a
collection of stone implements our ancestors made, with minimal variation, for
600,000 years. In that era we were very
conservative or not very bright, most probably both.
Anyway, back to South London. Burglary has a significant psychological
impact, particularly on the elderly, who are helpless. The implied sanctity
of the home is violated, with a consequent pervasive fear and loss of security
on the part of the victims. However,
Delroy Easton Grant was more than a burglar. He specialised in raping elderly women
and very occasionally elderly men. He did so with such violence that one woman’s
life was threatened by a perforated bladder. He probably committed over 200
offences (other estimates are much higher) starting in 1990, and was not caught
till May 2009.
Why did it take so long? Contrary to popular TV detective series,
it is hard to catch an accomplished criminal who strikes at random, with long
dormant periods, and who leaves very few forensic clues. This is no criticism
of the Police. One cannot protect every old lady in South London for years on
end simply because a particular violent rapist may strike again.
One possible reason for their failure is that Police encountered
difficulties because of the assailant’s race. Victims described him as a black male aged
between 25 and 40, about 5'9" to 5'11" tall, of slim athletic build. Police
at one stage issued a confusing photo-fit picture, in which the balaclava clad
assailant seemed to have a white face. That distraction aside, race was an
issue because an advanced DNA analysis of his sperm showed that he was very
probably from the Windward Islands ( St
Lucia, Barbados, St
Vincent, the Grenadines, Tobago or Trinidad). Police
identified around 21,000 possible suspects that fitted such a profile.
So, the solution seemed simple: DNA as many of those suspects until a match was
found.
Despite
a promise that DNA profiles which did not match the assailant would be
destroyed a number of putative suspects declined to give a DNA sample.
To some it seemed that the procedure was stigmatising an entire community, and
125 flatly refused to provide samples. This misunderstanding of the process, or
distrust of the Police, perpetuated the horror. Police were left with 1000
potential suspects.
However, consider the only other forensic
clue the rapist left: a size ten footprint of a particular brand of trainers.
Would it have been better to trace that? It might have been. A DNA profile is
unique, while anyone can buy a publicly available brand of trainers, though the
rarer brands such as the one found in this instance might be traceable. It was
a positive indicator, but hard to track down.
So, looking at the task from a Police
perspective, they were trying to find a black rapist without the full support
of the relevant target population of suspects, and had a DNA signature which
they simply had to match against a name.
Is
it easier to catch a burglar than an episodic gerontophile rapist? Frankly, the
Police have more experience of burglars. Every criminal has several signatures.
Almost as good as DNA, footprints, fingerprints and the like is the MO, the
modus operandi. What Police knew was that the assailant seemed to have an
unerring skill in tracking down the homes of the elderly. He never broke into a house
occupied by anyone other than a lone elderly occupant. He once targeted three
houses in a single street. He picked detached or semi-detached houses and bungalows but never flats. Therefore he must have spent much time reconnoitring.
To do this almost randomly over a wide geographic area he needed a car or
motorbike, or both.
The Police had hit a blank wall with their DNA
enquiries. They could only wait till those who had refused to proffer their DNA
got arrested for other crimes. Unknown to them, they were up against an even
bigger problem. Delroy Easton Grant had been incorrectly listed as having been
eliminated from suspicion. There are 63,000 Grants in the United Kingdom.
Delroy as a first name is disproportionately found among black men, so in this
case the nametag was a distractor, not an identifier. A young policeman made a
clerical error and accessed the wrong Delroy Grant, who was already on the
Police database but whose DNA did not match that of the rapist. One Delroy
covered for another. As a consequence the local police did not follow up the
car licence plate number that would have brought them to the real burglar
Delroy Easton Grant, whose DNA would have shown him to be the Night Stalker rapist.
While teams were trying to do the fancy stuff, and push the DNA analysis to its
outer limit, getting geneticists to come up with a possible estimate of the
assailant’s appearance, an elementary error was made on the tag which
constitutes a name. So, through carelessness, the unique identifier of the
genetic code was mistakenly thought not to match the supposedly unique code of
a given name.
Into this
impasse came a new team. They decided to try to catch a burglar who spent a lot of
time reconnoitring, and about whom they had some geographic leads. This was a
very staff intensive procedure (the total cost of catching Grant was £10
million) and involved staking out likely suspects and likely places. Car
license plates provided the other set of unique identifiers. In a way, they
looked where the light was brightest in terms of their procedural capacities.
This is often a sensible procedure.
At this point it might be interesting to
take a Bayesian approach, in which the evidence about the true state of the
world is expressed in terms of degrees of belief.
In The Theory That Would Not Die: How Bayes' Rule Cracked the Enigma Code, Hunted Down Russian Submarines,
and Emerged Triumphant (2012) Sharon Bertsch Mcgrayne described, amongst many other
things, the use of Bayesian statistics to catch submarines. The initial
indicator would be a radio location based on a transmission from the submarine.
Sending a plane straight to the location might have seemed sensible. However,
between the transmission and the plane arriving on site, many hours would have
elapsed. It would be better to guess where the submarine was going to. One could make some educated guesses and
express those as prior probabilities.
How best way to look? Operational research showed that if a spotter on
a plane saw anything, it was only in the first 15 minutes of their watch.
Looking down at the sea is deadly boring. So, they made everyone change windows
every 15 minutes. Research showed that if a spotter spotted anything it was on
the horizon, or just below the horizon, and the sunlight had to be in the right
place. So, they made the spotters look at that zone and nowhere else. In this
way, coupled with a well thought out search routine (using Bayesian estimates
of the most likely areas of sea given the time since the original intercepted
transmission) they increase their operational effectiveness.
Operational
research
can be applied to policing. Given that names are so important as tags on human
beings, making triple sure that the names are right should be a priority. Criminals
often change their names, or give false names, or use different variants on
their names in different circumstances. They also give false home addresses,
and conduct their businesses from yet other addresses, and use cars registered
to other people at yet other addresses.
Delroy Easton Grant, a Jehovah's Witness and father of eight, was a carer for his disabled
wife but also living with another woman. He turned
out to have been born in Jamaica, and was
dark-skinned. He had a very long criminal career, beginning with petty theft
and going on to armed robbery and vicious attacks on his partners. Neighbours
found him warm, charming and always friendly. He followed cricket,
liked to fish and enjoyed community barbeques where he would share jokes and
reminisce about his childhood in Jamaica.
Some of the errors:
1
Mixing up the names. The nametag must be firmly
attached.
2
Being more specific about ancestral DNA than was warranted.
3
Issuing a misleading photo-fit suggesting he was white or
light skinned
4
Searching for a specific rapist rather than an accomplished
burglar with a characteristic MO.
5
Under-financing the operation in the early stages.
One moral of this story is that the psychological pull
of DNA as a search tool may be
entrancing the police, to the detriment of systematic data collection and normal,
standard police work. It may be far better as a confirmatory technique in any setting in which a substantial
minority will not cooperate with testing.
Another moral is that the search for ancestral origins
identified the wrong Caribbean island, and added dangerous noise to the already
well known fact that he was a black man.
Another possible moral is that psychological profiling can be a distraction, since it offers
apparent specificity without sufficient reliability. It was another step too
far into sophistication.
It may be
best to look where the light is brightest, in this case for any person out late at night apparently
checking out houses but never flats. Grant was least at risk when he was raping
his victims, because he always disconnected electricity and phone lines. It was
the elaborate reconnoitring which put him most at risk. So, the task was not to catch him, but to
catch burglars like him, in the areas where his rapes had been reported, in the
hope that one of the burglars would be him.
Caution: This note was written
with the benefit of hindsight, and based on publicly available accounts of the
investigation, not on any internal documents. Almost every post hoc review of
persistent criminal operators shows that they might have been caught earlier. The
positive predictive value of criminal indicators is low, which is why so much
police work is boring and routine. Even a good indicator may not be specific
enough. Being a part-time cab driver, as Grant was, is a good cover for
burglary, but most part-time cab drivers are not burglars.
Catching a
wily criminal is hard. Searching extensively where the task is easiest (because a
high frequency behaviour leads in a few cases to a very low frequency
behaviour) may seem paradoxical, yet it has its advantages.
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