How big are forensic DNA databases, and what is a “cold hit”?

Very big. As of 2020, the UK's National DNA Database held 6.6 million profiles, from 5.6 million individuals once duplicates are excluded. As of September 2020, the national level of the US system held more than 14 million offender profiles, more than four million arrestee profiles and more than one million profiles from crime scenes.

A cold hit is what happens when a crime-scene profile is searched against such a database and unexpectedly matches a person whose profile is stored there. Because so many profiles are stored, cold hits can introduce a new suspect and help solve old cases. A cold hit is of particular value in linking a specific person to a crime scene, but it is of less evidential value than a DNA match made without a database search.

Why less? Statisticians disagree about how much a big database weakens the evidence. Stockmarr argued that searching a database increases the chance of coincidental matches, while Balding maintained that the evidence stays just as strong. Separately, DNA database searches showed false matches much more often than expected.

Rules differ by country. In England and Wales anyone arrested on suspicion of a recordable offence must give a DNA sample, and those not charged or not found guilty have their data deleted within a set period. In 2008 the European Court of Human Rights ruled that two British men should not have had their DNA and fingerprints kept by police.