Forensics: What Bugs, Burns, Prints, DNA and More Tell Us About Crime

by Val McDermid

the forensic scientists’ watchword, ‘every contact leaves a trace.’

Niamh Nic Daéid’s mother

why is this person only referred to in this way? lmao

wrote a surprisingly popular book, called Les faune des cadavres (Fauna of Corpses),

(which he detailed in an earlier book, Faune des Tombeaux (Fauna of the Tombs).

A Manual of Forensic Entomology,

‘It was just extraordinary to watch. Although there were other scavengers, like hyenas and vultures, the maggots were probably responsible for 40 to 50 per cent of the biomass loss.’

maggots can devour 60 per cent of a human body in under a week,

The cartoon is striking but it’s also misleading. Maggots cannot tell us when a murder took place. They can indicate when flies laid eggs on a corpse, and that reveals the point by which the person was definitely dead.

Maggots, Murder and Men (2000),

leaving her husband to babysit their three children.

Excuse me?

Lethal Witness (2007),

every case is the same: an autopsy is supposed to be ‘a non-judgemental, scientific, acquisition of facts’, no matter who the victim is.

It’s up to the police to make the tricky call about how much a pathologist should know of a case before the first crucial examination of a body. If the pathologist knows too much it might bias the autopsy. If he knows too little he might overlook something important.

It was once thought that bodies lose heat at a constant 1.8°F per hour until they reach ambient temperature. So, for example, if a person dies with the average body temperature of 98.6°F in a 68°F room, there will potentially be a 17-hour window when the time of death can be roughly estimated. But research has shown significant variables: a thin body cools faster than a fat one; the larger the surface area to weight ratio and the less subcutaneous fat, the faster the cooling process; whether the body is sprawled or curled up will have an impact; clothing will affect the cooling; shade or sunlight; in the shallows or beside a river. Even so, an early and careful reading can be a useful starting point and pathologists can factor in variables like ambient temperature and body weight using a graph with multiple axes called a nomogram.

The symptoms of rigor are useful to the pathologist for about two days after death because of a recognised cycle. At first a body relaxes completely, then after three or four hours the small muscles of the eyelids, face and neck begin to stiffen. Rigor progresses downwards, from head to toe towards the larger muscles. After twelve hours the body is completely rigid and will stay fixed in the position of death for around twenty-four hours. Then the muscles gradually relax and stiffness goes away in the order in which it appeared, starting with the smaller muscles and progressing to the larger ones. After a further twelve hours or so all the muscles reach a state of complete relaxation. But even a process as well documented as rigor mortis is a very imperfect indicator of time of death. The hotter the ambient temperature, the faster each step of the cycle occurs. Also, bending and stretching a corpse breaks up its muscle fibres and eliminates rigor, a piece of information that murderers have been known to employ to confound the investigation.

No two bodies will decompose in the same way, and at the same rate. You can have two bodies that are literally six feet apart and they will decompose in entirely different manners. It could be the amount of fat on the body. It could be the drugs they were taking, or the medication. It could be the type of clothing they’re wearing. It could be that one has a particular odour that is more attractive to flies than the other. Absolutely anything.’

Researchers have worked out a general rule of thumb: one week exposed above ground equals eight weeks below and two weeks in water.

Paracelsus, the founder of modern toxicology, expressed this idea neatly when he wrote in 1538, ‘The dose makes the poison.’

in 1851 Parliament passed the Arsenic Act, making it harder to buy arsenic over the counter. Sellers had to be registered and buyers had to sign and give a reason for their purchase. Unless it was for medical or agricultural use, all arsenic had to be coloured with soot or indigo, so it looked less like sugar or flour.

more than 90 per cent of convicted spouse murderers in nineteenth-century Britain were men. But men were far more likely to stab or strangle their wives; twice as many wives as husbands stood trial for the more indirect murder method of poisoning.

James C. Whorton, author of The Arsenic Century (2010), has written: ‘As with arsenical candles and papers and fabrics, items become established in commerce before their dangers are recognized, ensuring that any attempt to curtail their use will be resisted by manufacturers … and fought or ignored by politicians ideologically opposed to government interference

How much toxin and where it can be found in the body depends on how it was taken. If it was inhaled, it will be predominantly in the lungs. If injected intramuscularly, it will be mainly in the muscles around the injection site; injected intravenously, it will all be in the blood and there will be very little or nothing in the stomach and liver. If it was swallowed, then it will be mainly in the stomach, intestines and liver.

the Poison Garden at Alnwick Castle

If ricin is swallowed, its symptoms are nasty but not fatal. But if it is injected or inhaled or absorbed through the mucus membranes, a dose the size of a few grains of salt will kill an adult man.

poison extracted from the beautifully flowering monkshood plant, also known as Indian aconite, thought to be the deadliest plant in the world.

Aconite stops the heart and other internal organs from working. After the severe vomiting, the victim feels like they have ants crawling over their body, then they lose sensation in their limbs, their breathing becomes slower and slower, and their heartbeat weakens, disturbing the rhythm of the heart. However, the mind remains clear throughout.

try to identify them systematically using anthropometry, the science of measuring humans. He chose eleven body measurements, including width of head and distances from elbow to end of middle finger. Bertillon put the odds of two people sharing all eleven of his measurements at one in 286 million. He recorded the individual measurements on file cards, and in the middle of the cards he stuck two photographs – full face and profile – and thus the mug shot was born.

For the rest of his career Macnaghten kept photographs on his desk of Jack the Ripper’s mutilated victims to remind him to try harder.

What the fuck???

When a print is clear, the chances of an officer making a wrong call are next to nothing. But when a print is smeared, overlaid with other marks, made in blood, one officer may see points of agreement that another does not.

‘I’ve been doing this from the mid-1990s,’ she says, ‘and I’ve come to it as a scientist. I tear my hair out at the assumption people have made right the way through that it is an absolute rock solid science. It’s not a science at all. It’s a comparison.’ The rhetoric used to support forensic fingerprinting has always been scientific in tone.

recommended that fingerprint evidence should from now on be regarded as ‘opinion evidence’ not fact, and thus should be treated by courts ‘on its merits’. But this message has not trickled down to all fingerprint officers, says Catherine Tweedy: ‘They are not being trained to think that an opinion is an opinion. Once you are trained to see things as facts it is extremely difficult to be pulled back to understand that there are shades of grey. You can’t be 100 per cent certain in a lot of cases because you only get a fraction of a fingerprint.’

their fingerprint experts had failed to separate the analysis and comparison stages of their examination. First of all the expert should analyse the mark in detail, describing as many minutiae as she can. Only afterwards should she examine possible matches and carry out a comparison. When analysis and comparison happen simultaneously, experts run the risk of finding matching minutiae because they are looking for them.

‘Examiners don’t think in any percentages except 100 and 0.’

Sally Mann’s series ‘What Remains’. Sally Mann, ‘Untitled’, 2000,

Death of a court lady,

‘Nutshell Studies of Unexplained Death’.

Watson is nearly sick at the scene, which seems frankly improbable given he’s a medical practitioner who has served in the Afghan Wars. But then, I am a writer whose work features blood and gore and yet I am squeamish about blood.

A mood tbh

As with a real fingerprint, a genetic fingerprint should not be enough to secure a conviction on its own. According to Gill, ‘DNA doesn’t lie. It’s an exceptionally good lead and exceptionally strong evidence but there is human interaction in the process [of profiling]. So the error rate is exceptionally low but it’s not zero … DNA shouldn’t be a lazy way to not do an investigation.’

A defence barrister once put the framing idea to Val Tomlinson in court, claiming that his client’s LCN DNA had been planted at the scene by an anonymous other. To prove it, he asked Val a hypothetical question: ‘If you were going to set somebody up for this how would you go about doing it?’ ‘I don’t think I could,’ said Val. In Val’s experience most set-ups fall down on basic points. ‘Children go over the top when they are trying to cover up for their mistakes. And you tend to find that people who frame others distribute too much blood in the wrong way, or a whole bucketful of glass instead of the two little pieces which is what you would expect to persist on a piece of clothing recovered a week after a crime.’ Like any powerful tool, DNA can be misused. But, as always, the analysis of evidence is not simply about the collection of data – whose DNA is or isn’t there – but also about the interpretative skills of the scientists who deal with it. This is what should – and mostly does – protect the innocent.

But, as well as human origins, he was interested in the origins of human evil, and studied the anthropometric characteristics of criminals and ‘normal’ Americans to discover whether the measurements of wrongdoers were different. By 1939 he was able to announce: ‘Crime is not physical, it is mental.’

The Psychopath Inside,

researching and writing about geographical profiling. Just as law-abiding citizens tend to go back to the same street to do their shopping time and again, most criminals like to commit their crimes in the same areas. They feel safer in places they know. David Canter came up with a circle hypothesis: if you draw a circle with a circumference going through the sites of the two crimes furthest apart from each other, the culprit’s home will likely be near the centre of that circle. Research has shown this to be true of the majority of criminals who strike more than five times. Canter has found that a serial killer can usually be found living within a triangle formed by the sites of his first three murders, as Duffy was.

The Jigsaw Man (1998) Paul Britton

You can work on something for several hours and then something says, “No,” so you throw it out. I think sometimes police officers and profilers make mistakes when they have a hunch and they stick with it. What we all have to do is to learn to let go. If the evidence doesn’t support a hypothesis, you throw it away, start on Plan B and work your way through to Plan Z.’

Defending the Guilty, criminal lawyer Alex McBride

Furthermore, by its nature, science is provisional: theories are open to rejection or modification in the light of new evidence. Fiona Raitt says, ‘Much expert testimony goes to the core of scientific development, which is in a constant state of discovery and refinement. What we know today is sometimes very different from what we knew yesterday.’

they can afford. The civil rights campaigner Clive Stafford-Smith’s book Injustice (2013)