Basic principles of forensic science
By Dave Barclay
Prof. Edmund Locard wrote in various papers and books published around the time of the first world war that every contact leaves a trace. This is also called Locard’s exchange principle, and refers to the transfer of trace evidence like fibres, soil, dust and hair, from one person or location to anyone or thing that contacts it. When two garments are in contact, fibres shed from one will be picked up by the other; if we find transfers both ways, the significance is greatly increased.
If a hammer strikes a window frame, the hammer will pick up paint smear and fragments from the window, and an impression of the hammer will be impressed onto the wood of the frame. Further, as we look more closely, the hammer will have picked up dust, soot, etc. which originally had been deposited on the surface of the paint, and the paint surface will now display microscopic abrasions which include the fine detail of any scratches, chips or defect on the surface of the hammer. As scientific instrumentation has improved, we are now able to look at materials which Locard would simply have characterised as ‘dust’ and completely resolve them using the Scanning Electron Microscope to visualise particles of ‘dust’.
The image below shows a Scanning EM image of glass in the eye of a needle
Gas chromatography and mass spectrometry are used to separate and then identify picogram quantities of chemicals such as drugs, fire accelerants or environmental contaminants. When we use picograms we are talking about a millionth of a millionth of a gram. Locard would have worked at the milligram level – a thousandth of a gram. Nowadays, we routinely express results of drugs as nanograms per millilitre – a million times smaller than milligram, and on a much smaller sample size as well. When blood was transferred in Locard’s day, the best that could be done was say that it was human and classify it under the ABO system, which was described by two scientists around 1900; around 40% of the population were blood type O. Now we can discriminate a single hair root using nuclear DNA to the order of 1 in trillions, although it is normally expressed as ‘more than 1 in a billion’.
Locard still absolutely applies and is particularly useful in transfer of physical material such as paint, glass, fibres, makeup, soil. But nowadays the trick is to FIND that trace and assess its significance in each particular circumstance. Sure, we know that a suspect has been in a room, shedding flakes of skin, so his DNA is there somewhere – but how do we find and analyse the right flake? And what is the significance, because we all have flakes of other people’s skin and their hairs on us, and maybe that secondary transfer is what deposited the analysed skin flake.
So it’s very important to realise that the essence of forensic science is not at all the clever test results, but our interpretation of the significance of those results; and significance is entirely dependent on the case circumstances.
- Forensic science is the interpretation of results in context, not the results per se
- Forensic science is absolutely context-dependant
So having got the two basic principles, how do we set about actually doing all this science, and assessing and interpreting? By using an even more basic method the normal Scientific Method we were all taught at school:
- recognition and formulation of a problem
- collection of data through observation and experiment
- formulation and testing of a hypothesis
And then repeat until you are satisfied that your hypothesis is correct. And in our case, that any propositions about actions that must be part of the crime have been tested.
Investigators in the UK and many countries are taught a variant of the same system as the basic investigative process-they assemble a list of all the persons (nominals) who could have committed the crime, assemble the evidence for and against them, and then eliminate nominals until they are left with the likely perpetrator, or prime suspect. It is only at that stage that ‘evidence’ is assembled against an individual, and in fact the totality of the evidence can be processed and collated without ever knowing the identity of the suspect.
This is an open process, with no colours being nailed to masts, and therefore no ownership of a particular suspect until late on. This contrasts with the ‘Life on Mars’ approach common until the late 1980s, in which the senior investigating officer nominated a suspect and evidence was accumulated against them in an effort to prove their guilt. In retrospect it is easy to see that this was the primary cause of many of the miscarriages of justice that occurred up to the 1990s. And of course, when you think you know who is responsible, or what happened, there is a natural tendency to make everything fit, and disregard or ignore competing information.
And allied to that is the fourth principle, ably described by Mr Sherlock Holmes in my favourite quote from Sir Arthur Conan Doyle:
Watson: ‘Is there any point to which you would wish to draw my attention?’
Holmes: ‘To the curious incident of the dog in the night-time.’
Watson: ‘The dog did nothing in the night-time.’
Holmes: ‘That was the curious incident.’
(Extract from Silver Blaze)
So, often in forensic science, what you do NOT see can be as important as what you do. Just in case there are any readers not familiar with the story [SPOILER ALERT!], an attempt had been made to nobble (lame) a racehorse, the eponymous Silver Blaze. The dog that didn’t bark was the stable dog, and Holmes deduced that the explanation for this must be that the owner of the dog – the trainer – had been responsible. So what we do not see – such as traces we do not find when we should – are just as significant as what we do see.
A pre-school child was abducted from her parent’s bedroom in a ground floor maisonette in the North East of England. She had been put to bed early so the parents could go out to the pub, and they returned to find the window forced open and the child missing. There had been a number of burglaries through these windows in other flats in the block. The concrete window sills on that side of the block were covered in lichen and moss, and the window opening was restricted to 18 inches by a safety bar; it also opened outwards at the top, again as safety measure. Having noticed the restricted opening the CSI paid particular attention to the outside of the window sill, which both the offender and child must have scraped over.
However, there was no sign of any disturbance on the sill, and the parents claimed that the house was locked when they returned. The circumstances and the apparent jemmy marks on the window (right) were evidence of staging, which always indicates an inside job. The absence of marks on the sills meant that the parents’ story could not be true, and eventually they admitted that the child had accidentally got hold of the father’s methadone and subsequently died. They had disposed of the body from a bridge over a major river. It was never recovered.
Finally, in this wee principles section we should talk about contact points which brings us back to Locard. This is not really a principle, but a way of working, and contact points are identified by ‘thinking like the criminal’ to understand and predict where physical contact or interaction is likely to have taken place between offender and victim, or the scene.
Contact points are crucial to identify where possible evidence can be recovered, particularly in cold or stuck cases where most of the obvious stuff will already have been done.
As an example, a man attacks a woman to force her to perform oral sex on him. She resists and eventually he kills her. If she has a pony tail it is almost certain that he will have grasped that in an attempt to control her head, and he will have left his DNA on the scrunchie, or even on the hair itself. The pony tail/scrunchie is a contact point. In this example, he may not have had any intimate contact with the victim, and the best chance of getting DNA profile will be the scrunchie contact point; indeed if it is a cold case from the 90s or earlier it is very likely that the scrunchie has been kept nice and dry and bagged up, and no-one has ever done anything with it. The DNA will be as good now as it was 20 years ago.
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