Talking Points



DNA evidence is not foolproof

DNA is not foolproof so procedures need to be in place to ensure that matches between individuals’ DNA profiles and stored DNA profiles do not result in miscarriages of justice. The more DNA profiles that are compared the more likely errors are to occur, and problems can also result due to poor laboratory procedures, failure to require corroborating evidence, or if DNA evidence is planted at a crime scene.


Firstly, matches between one DNA profile and another can occur statistically by chance (known as ‘adventitious matches’): the likelihood of these depends on the DNA profiling system used and whether the profile is complete (crime scene DNA is often degraded and only a partial profile can be obtained). The likelihood of errors increases if there is a mixture of more than one person’s DNA; if the DNA is from a very small sample; or if the suspect is related to the perpetrator of the crime (because relatives share parts of the same DNA sequence).


Secondly, errors can occur in the process of collecting and analysing the DNA and reporting the results: one major source of error is laboratory contamination.


Thirdly, the presence of a person’s DNA at a crime scene can be misinterpreted: the person may have been there earlier in the day, or their DNA may have been planted at the scene (e.g. on a tea cup or cigarette butt) or transferred there accidentally.


Minimising the risk of miscarriages of justice requires:

1.    A requirement for the courts to require corroborating evidence, so that individuals cannot be convicted on the basis of DNA evidence alone;

2.   Measures to prevent contamination at crime scenes and laboratories, so that the DNA of innocent individuals does not become mixed up with crime scene DNA;

3.    A right for all defendants to be re-tested using a second DNA sample before trial;

4.    Restrictions on the numbers of individual and crime scene DNA profiles collected and stored on DNA databases, so that the DNA profiling system used has sufficient statistical power to minimise the likelihood of false matches occurring purely by chance.

5.     Careful retention of crime scene evidence and a right to re-testing of that evidence if a miscarriage of justice is suspected.


For more information, see:

William C. Thompson: The potential for error in forensic DNA testing:

Presentation (videos):





Bigger DNA databases are not better

Most countries with DNA databases measure matches between DNA profiles on the database. These can be matches between a newly loaded DNA profile from an individual and the database of stored crime scene DNA profiles, or between a newly loaded crime scene DNA profile and the stored DNA profiles of individuals. Matches can also occur between DNA profiles collected from several different crime scenes, suggesting the same person may have been involved in a string of crimes.


It is important to remember that matches are not convictions. As DNA databases get larger, many matches can occur between a crime scene DNA profile and people who were at the scene but did not commit the crime, including passers-by, family members and victims of the crime. At a murder scene in a house, for example, swabs might be taken from all over the house and many will match with the victim, visitors or members of the family who may not be suspects for the crime.


In addition, not all recorded matches require a DNA database of individuals’ DNA profiles. Some matches occur with DNA profiles taken from known suspects for a crime, who do not need to be on a database because their DNA profile can be compared directly with a crime scene DNA profile. Other matches occur with stored crime scene DNA profiles. Storing crime scene DNA profiles can help solve crimes without raising the same human rights concerns as storing individuals’ DNA profiles.


The number of matches is always likely to go up as a DNA database gets bigger, but this does not mean more crimes are being solved. The UK keeps figures on DNA detections, which are crimes which are prosecuted involving a DNA match. The number of detections is about half the number of matches and only about half the prosecutions lead to a conviction. This is because the DNA evidence may not be conclusive proof that the individual committed a crime, as there may be an innocent explanation of why their DNA was found there. Importantly, the UK figures show that the number of DNA detections did not increase when the UK expanded its database significantly in size, collecting DNA routinely on arrest and keeping the DNA profiles of innocent people indefinitely. However, DNA detections did increase when the number of crime scenes examined went up and more crime scene DNA profiles were loaded to the DNA database. Since the proportion of crime scenes analysed has levelled off, the number of DNA detections has stayed roughly constant as a proportion of recorded crimes and is about 0.35% of recorded crimes. This includes DNA matches with known suspects and matches between individuals’ DNA profiles and stored crime scene DNA profiles, which do not require individuals’ DNA profiles to be stored. The limiting factor in DNA detections is that useable DNA evidence is found at only about 1% of crime scenes.


Overall, the UK data shows that DNA databases can be useful to solve crimes but that increasing the number of crime scene DNA profiles collected and added to the database is much more cost-effective than expanding the number of DNA profiles collected and stored from individuals. This is because expanding the DNA database to include people who are not repeat offenders, or who do not commit the type of crimes for which DNA evidence is useful, does not help to solve more crimes.


Your DNA can track you and your relatives

DNA databases raise human rights concerns because:

• DNA can be used to track individuals or their relatives, so a DNA Database could be misused by Governments or anyone who can infiltrate the system;

• In order to be useful to track suspects, DNA records are linked to other computer records such as records of arrest, which can also be misused;

• DNA samples and profiles contain private information about health and genetic relationships (including paternity and non-paternity).


A person’s DNA can be left at a crime scene but it can also be left on a coffee cup at a political meeting or on everyday items such as a toothbrush. Anyone who can collect this DNA and gain access to a DNA database, which links people’s DNA profiles to identifying information such as their name or ID number, can identify them and probably find their whereabouts. This technique could be used by a totalitarian regime to track political opponents, or, if they can gain access to the database, by a criminal gang to track a victim who may be on a witness protection scheme, or an abuser to track a woman or child who has escaped to a place of refuge. DNA evidence could also be planted at a crime scene to implicate wrongly a person who is on the DNA database in the crime.


Because DNA profiles can be used to identify close relatives, a DNA database could also be used to find a person’s children or other family members. Non-paternity could be exposed, which could be used to blackmail or break up a family and could be particularly dangerous for women in some countries where sex outside marriage is illegal. If DNA samples can be obtained, they can be analysed further in a laboratory to reveal other personal information about the health of an individual and their family.


DNA databases normally contain additional information, or are linked to other databases using an ID number, so that a suspect can be tracked down if there is a match between their DNA profile and a crime scene DNA profile. This might include a record of the person’s arrest. In some countries this can be used to refuse them a job or a travel visa, even if they have never been convicted of a crime.


The negative effects of DNA databases can also impact disproportionately on ethnic minorities, who are often more likely to be arrested by the police. Whole groups of people could be targeted for surveillance using DNA if their ethnic group can be identified using their names or other stored information. There are also particular concerns about taking DNA from children or from vulnerable people, such as people suffering from mental illness, especially when the police have powers to take DNA by force from anyone who is arrested, or to keep innocent people’s DNA profiles on the database.


In 2008, in a case brought against the UK Government, the European Court of Human Rights stated ( ) that keeping innocent people on a DNA database for life was “a disproportionate interference with the applicants’ right to respect for private life and cannot be regarded as necessary in a democratic society”. The UK Government has since removed more than a million innocent people’s DNA profiles from its DNA database and destroyed more than 6 million DNA samples.

© 2014 Forensic Genetics Policy Initiative