Thanks to television detectives, the power of forensic DNA evidence is well known. Viewers are familiar with criminals leaving their DNA at the scene of a crime – allowing police to identify perpetrators with a high degree of accuracy, as well as any other crimes they may have committed.
Even if there is no arrest before the end credits, the crime scene DNA will be stored on a database, and later used to find a “matching” suspect.
In real life, this practice has been used successfully in widely reported court cases. Serial rapist Keith Samuels was caught after a 14-year hunt and a DNA test, while triple murderer Joseph Kappen was identified after his death when a DNA match was found with his exhumed remains.
The successful use of DNA in cases such as these, solving apparently unsolvable crimes, has led to significant public investment and political commitment to expanding forensic DNA databases. But the limitations of DNA evidence, and just how carefully it must be handled (by police officers, scientists, lawyers and juries) is rarely mentioned.
Recent research highlighted eight cases that were overturned in the last six years due to concerns over the DNA evidence.
Although the research indicates that there is a smaller risk of DNA being misleading than other evidence types, there needs to be clarification over issues such as DNA contamination, the interpretation of partial or mixed profiles, trace analysis and the transfer of DNA. When DNA science pushes boundaries, extreme care is required to avoid miscarriages of justice.
But there are perhaps greater concerns about DNA that do not revolve around the science (which in the majority of cases, is unproblematic). There have long been warnings over the potential for police to resort too quickly to DNA and the risk of abbreviating detective work. This is even more worrying when considering its actual effectiveness in resolving crime.
The National DNA Database “match rate” (the chance that a crime scene profile when loaded onto the database will match against the stored DNA of a person) is now at its highest level ever (63.3% in 2015-16). But the latest report from the Biometrics Commissioner, which oversees the use of DNA samples and profiles in the UK, said that DNA was “linked to outcome” (associated with a suspect being charged, or cautioned for example) in just 0.3% of all recorded crimes in England and Wales in 2015-16.
This rate remains unchanged since the national database was set up in 1995. Even in cases where it might be expected to be important, DNA is still, for the most part, insignificant as a crime solving tool.
For example in rapes, the rate is just 0.6%, and in domestic burglaries, 1.4%. It is most helpful in homicides, where it is linked to outcomes in 8.4% of cases.
Yet even when DNA is linked to outcome, the reality is that the police often already had a prime suspect. DNA is simply used to confirm their identity, and help construct a prosecution case against them (or persuade them to accept a caution).
The courts in England and Wales have now established that “where DNA is directly deposited in the course of the commission of a crime by the offender, a very high DNA match with the defendant is sufficient to raise a case for the defendant to answer”.
Although this indicates that “DNA only” cases could now go to court with no other evidence, this actually still depends on the overall circumstances of the case. It is difficult then, to exclusively link a conviction to DNA evidence.
This all highlights why the figures do not tell us a great deal about the independent value of DNA evidence. There is still very limited proof of how DNA evidence and DNA databases contribute to the fight against crime.
Where is the evidence?
There may of course be other advantages to holding the DNA of over 5m British citizens, such as crime deterrence and crime pattern analysis. But these supposed benefits are speculative, as there is no way of measuring them.
We can, though, measure its cost. Running the National DNA Database costs the government about £2.5m a year (it was £3.9m in 2014-15), while police forces must meet the costs of crime scene investigations and DNA analysis.
We are still a long way from gathering the evidence that could establish just how effective the National DNA Database is, and whether the taking of half a million DNA profiles a year and the storing of over 5m is actually worthwhile.
What we do know is that the limited knowledge of the effectiveness of DNA has been a prominent theme in the three annual reports of the Biometrics Commissioner. There needs to be a public debate over the continuing police use and retention of DNA. Far from being a catch-all solution to modern crime, a case still needs to be made for DNA evidence – apart, of course, from on TV detective shows.