People can find out a lot on the internet. More and more, the internet is finding out a lot about them. Google said this week it will follow Yahoo and AOL and tailor web adverts to its users' preferences, divined by monitoring their individual browsing patterns. This causes excitement among advertisers – and sends shivers down the backs of privacy activists. Both are justified.

Much advertising is utter waste. Any advert that does not help people find things they want to buy amounts to money spent in vain by the advertiser and time lost (and sometimes mood soured) for the consumer. “Behaviourally targeted” advertising can therefore in theory be a great improvement: the advertiser gets more value for money, consumers avoid ads for things they have no interest in, and businesses that depend on advertising revenue, such as Google but also the media, gain a more solid economic footing.

Google attempts to do this by having websites leave small files, called cookies, on the user's comp-uter. Adverts can be programmed to glean the user's browsing behaviour from the cookies and display matching advertising. But search histories, like browsing histories, provide detailed information about people who thought they were acting anonymously. The risk of abuse seems low today, but as long as the assembled information exists, the possibility of harm is always there. It is imperative to develop rules to govern such information: who may store it and how, for how long, and to which uses it may be put.