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Forensic Access’ Dr Angela Gallop was asked to participate in this event, as part of The Wellcome Collection’s new exhibition “Forensics – -the anatomy of crime“. This article contains the content that Angela presented at this excellent event.


Angela Gallop was asked by the Wellcome Collection to present a short lecture on ‘What counts as ‘evidence’ in a courtroom? Do forensic scientists, lawyers and the public all have the same understanding of truth and reasonable doubt? Does our notion of ‘evidence’ change over time? Join us for a conversation on how objects, witnesses and science fit into the legal process.

‘What Is Evidence?”

Forensic science evidence is all about the physical traces that can be used to link people with other people, places and vehicles and so on reflecting the principle advanced in 1910 by the French criminologist, Edmund Locard, now commonly expressed as ‘every contact leaves a trace’. What Locard was saying is that anyone entering a crime scene will leave something of themselves behind there, and will take away something of the crime scene with them. And the more cases I do, the more true I know this to be. Often it’s just a question of finding it and having appropriate techniques to be able to analyse and compare it.

These traces come in a wide variety of different types. The main biological ones include blood and other body fluids, and body tissues such as skin flakes, hair and nail fragments. The patterns body fluids make – particularly blood, can provide important information about what has gone on at a crime scene and the sequence of events there, and which samples might be best to take for DNA profiling to establish who the source of the body fluid could be.

Chemical traces include paint, glass and other building materials, textile fibres from clothing and furnishings, and then drugs of abuse and poisons in cases of suspicious death, fire accelerants where arson is suspected, and firearms discharge residues, bullets and cartridge cases when shots have been fired.

Marks are also very important, notably fingerprints but also shoe marks, and tool marks left behind when someone has forced entry into a premises, manufacturing marks and tyre marks. In recent times, digital devices such as mobile phones and computers have begun to provide a very rich new source of evidence showing who has been communicating with whom, about what, and where they were at the time.

One of the most exciting things about forensic science is the rich connections one can find between different types of trace which can confirm their relevance and improve the value of the evidence. For example, in the Damilola Taylor case, we found some of Damilola’s blood on the shoe belonging to one of the suspects and embedded in this blood was a textile fibre that could have come from Damilola’s jumper.

Also the way that searches for one type of trace can lead you to another. For example, in the Stephen Lawrence case – with which you will probably also be familiar, the search for evidence started with paint, moved on to textile fibres and that led in turn to the all-important DNA evidence. This enriches the evidence and can help provide real focus for an investigation, without which each case could take a lifetime if one were to examine everything for everything.

But for a scientific finding to be accepted as evidence, like for example what appears to be the victim’s DNA on the suspect’s clothing, it is extremely important to show that it could not alternatively have got there through some sort of legitimate contact before the event, or contamination afterwards. This means that it is very important to understand the overall context of the case including, for example, any relationship the victim and suspect might have had with each other prior to the event, what they had both been doing on the day in question, and precisely when and how the key items on which the evidence depends were seized and stored and then treated during their time in the laboratory – up until the moment when the proposed evidence was found.

As scientists are able to find and analyse increasingly small traces, it becomes ever more important to ensure that there has been no opportunity for any contamination to have occurred in the laboratory. This means they spend a lot of time cleaning their laboratories and then checking that the cleaning has been successful – all in addition to the work itself.

Of course, the situation can get quite complicated in complex historic cases where there may have been several investigations of the same items over an extended period of time and when laboratory practices were not so tightly controlled as they are today because it simply wasn’t necessary with the tests that were applied then.

In recent years there has been growing recognition of the potential dangers of what is commonly known as ‘cognitive bias’ in forensic evidence. Put rather too simply, this is usually when aspects of prior understanding about a case influence the scientist’s judgements and conclusions. This has been demonstrated in a series of experiments where fingerprint examiners were presented with fingerprints from a crime scene and asked if they matched the prints of a suspect. It was consistently found that judgements of qualified and experienced examiners were influenced by whether or not the suspect had confessed to the crime. Even the same examiner could reach different opinions about the same mark if provided with different contextual information.

And we have seen this in practice. For example, in the Madrid bombings the FBI positively identified Brandon Mayfield as having left a fingerprint found on a bag containing detonating devices. Soon after, however, the Spanish Authorities matched the print to the real bomber – an Algerian national.

‘Confirmation bias’ – a type of cognitive bias, was listed as a contributing factor to the original mis-identification.

Possible solutions include, for example, ensuring that people susceptible to cognitive bias are not given jobs in forensic science in the first place, training scientists how to recognise and avoid it, ensuring items from crime scenes are examined and analysed in advance of any attempt at comparison with samples from suspects, and adjusting requirements for peer review – the system whereby all significant positive or negative findings in a case are independently checked by another, similarly qualified scientist as part of our rigorous QM Systems .

Another emerging risk for the reliability of forensic evidence which I feel I should mention is the trend for police forces to undertake much of the initial searching for evidence themselves and then subcontract out just the analytical side of things to a variety of different specialist forensic firms. Aside from potential problems associated with police both investigating crime and being central to the generation and presentation of ‘impartial’ scientific evidence, this means that the work in individual cases is becoming increasingly fragmented with no one scientist with the full picture of what the results mean – either in relation to each other or the overall context of the case.

The way things are going, there is a real fear in the forensic science community that our ability to holistically investigate and interpret scientific findings, particularly in complex cases, is being compromised and that our Criminal Justice System will end up the poorer as will public confidence. But, of course, no-one will realise this until we have suffered another round of miscarriages of justice, and by which time we shall probably have lost some of the necessary skills.

In other words, forensic science evidence is increasingly powerful, but the outcomes are more likely to be increasingly unreliable unless we can use the changes inspired by austerity cuts to policing in a much more imaginative and principle based way so far as forensic science is concerned. But I remain the eternal optimist.

Anyway, I hope this has provided a flavour of what forensic science covers, and at least some of the issues involved in presenting scientific findings as evidence.