Terrorism, crime and illegal immigration are fuelling state surveillance, and are being used to justify it to the public, who too often seem asleep to the risks of abuse. This is particularly true of national DNA databases, where in several countries there is an insidious creep to log not only serious offenders but also other classes of the population, such as immigrants and minor offenders.

So it was refreshing to see resistance articulated this month in France and the United Kingdom. Prominent French scientists led public protests against a government bill to use DNA tests on immigrants to see whether they are related to family members already resident in the country. Such protests might seem an overreaction. Many countries already practise DNA testing of immigrants, with varying rules for use. In 1985, the first use of DNA fingerprinting for legal purposes led to a Ghanian boy being allowed to join his family in the United Kingdom after he proved kinship (A. J. Jeffreys et al. Nature 317, 818–819; 1985).

But the objectors are correct to argue that the French proposal, far from promoting greater fairness, is aimed at erecting another obstacle to immigration. The scientific opposition is also linked to a strong bioethical and legal tradition in France of the concept of the family as a social unit, not reduced to mere biological ties, reflecting the reality that (as in all countries) many children are not the biological offspring of their legal father. Given this culture, there is no reason why only immigrants with a biological link should qualify for integration with their families in France. Furthermore, DNA testing of immigrants elsewhere has destroyed families by uncovering true biological relationships.

The scientists' case has enjoyed public and political support, and has embarrassed the government, which sought to defuse the controversy last week by postponing a final decision to 2009. The outcry has also thrown an overdue spotlight on issues surrounding such population databases — issues being tackled in Britain, which has the world's largest DNA fingerprint database. The National DNA Database contains samples of 4 million people or 6% of the population, and one in ten males. The Nuffield Council on Bioethics, in a landmark report this month, does a service by drawing attention to the dangers of proposals to expand the database (see http://tinyurl.com/2upt8x).

There is a widespread misperception, encouraged by governments and media success stories, that DNA evidence is infallible in clinching convictions or acquittals. The technology is sound, but errors or deliberate falsifications in sample taking and handling are not uncommon, and a match with a sample at the scene of a crime may amount to proof only that the person was present at some point.

Since 2003, DNA samples and fingerprints have been compulsorily taken from Britons arrested for criminal offences. But the government now proposes extending the database to include fingerprints and DNA from anyone arrested, even for minor offences such as dropping litter. And voices within the UK government and the judiciary have suggested that the entire population should be sampled. The US government, meanwhile, is proposing to extend its database to include DNA from anyone arrested by federal agents.

The Nuffield report is right to denounce the infringements on liberty and privacy represented by such extensions as being disproportionate to any possible benefits. Suspicion of involvement in a minor offence cannot justify taking a biological sample without consent. In the United States, the largest group likely to be affected is illegal immigrants — and there is no reason to suspect this group of being more likely to engage in serious crime.

DNA fingerprints themselves contain relatively little personal information, but the biological samples are open to misuse. Although supposedly limited to direct matching of individuals for crime cases, DNA data are already used for the much less scientifically robust practices of searching for family relatives of a crime's perpetrator, and to try to reduce possible suspects to ethnic groups.

History teaches us that it is a fallacy that only those without a clear conscience need fear a knock on the door at midnight. Governments' enthusiasm for DNA databases needs to be matched by commensurate statutory protection, transparency and oversight — and vigilance by citizens.