How the method evolved
- 1980sThe method is first used in investigations: it both identifies the guilty person and clears the innocent.
- 1990sNational profile databases are created, making it possible to compare profiles across the country.
- 2000sSensitivity increases: a profile can be obtained from a touch, not just a stain.
- 2018A case is solved through open genealogy databases: a match is found not with the perpetrator, but with a distant relative.
That last development changed how cold cases were approached. Before, the absence of a profile in the database meant a dead end. Now, it is enough to find a third cousin in an open database and build the family tree from there, narrowing the field to a few people.
This same possibility sparked a debate about privacy. People uploaded their data to search for ancestors, not to help with criminal investigations, and usually had not consented to that use. Access rules have since become stricter.
The downside
The second tile is the key takeaway. Not fake evidence or bad intentions, but ordinary eyewitness identification turned out to be the most common source of wrongful convictions. The witnesses were usually highly confident.
The method itself has weak points, too. The higher the sensitivity, the easier it is to obtain a profile from someone who merely handled an object earlier or whose trace was transferred at the scene. There are known cases where this sent investigations far in the wrong direction.
This highlights an important distinction: the analysis answers whose trace it is, not how it got there. The second question is answered by the investigation itself, and the two must not be confused.
The approach to storing physical evidence has changed, too. Since an old case may be solved 20 years later, evidence is now stored longer and more carefully, and many countries have introduced mandatory retention rules for serious offenses.
Another complication has emerged: old evidence was often stored without today's precautions. As a result, some archived materials became contaminated and unusable for modern analysis.
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Sources
- Reviews of convictions using genetic analysis showed that most wrongful convictions relied on mistaken eyewitness identification
- increased sensitivity of the methods raised the risk of trace transfer and contamination