THE potent combination of three-martini lunches and creative genius in “Mad Men”, a television show about 1960s Madison Avenue, is a fair representation of the advertising industry’s past. For its future, though, look to a 2002 film, “Minority Report”, starring Tom Cruise and set in 2054. Mr Cruise, as usual, spends a lot of time on the run. When he dashes past a digital billboard it takes note of his exertions, remarking, “You could use a Guinness right about now.”

Modern advertising is not far from the world of “Minority Report”. Internet ads now account for around a quarter of the $500 billion global advertising business, and the confluence of mobile devices and social networks allows advertisers to track and target people to a degree once reserved for fiction. As people spend ever more time online, thousands of firms are invisibly gathering intelligence about them, as our special report explains.

By monitoring the websites people visit, these companies can infer their location, income, family size, education, age, employment and much more. One data firm has compiled a billion profiles of potential customers, each with an average of 50 attributes. Consumers are lumped into “segments” such as “men in trouble”—presumed to have relationship problems because they are shopping for chocolates and flowers—or “burdened by debt: small-town singles”. When people visit websites, advertisers bid to show them precisely targeted ads. The auctions take milliseconds and the ad is displayed when the website loads.

Targeted advertising has advantages for consumers. It pays for many popular websites which people can enjoy free of charge. Relevant ads are probably more useful to consumers than irrelevant ones. But any business based on covert surveillance is vulnerable to a backlash.

You’ll never click alone

Most consumers have no idea how closely they are being followed online. They do not know that Facebook’s “Like” and Twitter’s “Tweet” buttons on other websites carry a code that allows those companies to track users, even if they don’t click on them. They have no notion that on the most popular websites up to 1,300 companies are watching what they do.

Although companies that buy and sell data insist that they use numbers, not names, to identify individuals, it is getting easier and easier to pinpoint people with supposedly anonymised online tracking information. New real-world tracking tools, like beacons, which let retailers use wireless technologies to see when particular customers walk into a store, will make individual identification even easier.

Such information could be used against consumers. Someone who is categorised by a data broker as a “motorcycle enthusiast” might find his rates for medical or accident insurance rise. “Men in trouble” might find it harder to get a job. Until objections were raised, OkCupid, a dating website, used to sell data about people’s drug and alcohol consumption. It is not going to be to anybody’s advantage to have such information about them widely available.

People should be able to find out if they are being tracked and what information companies are holding about them, and they should be able to stop companies tracking them if they want. America has relied on the industry to regulate itself, but it has done too little to protect people’s privacy. The European Union’s approach is more transparent. European websites are required to make it clear to consumers when they are being tracked by third-party cookies, and people opt into data collection rather than opting out. And just as consumers in America can get a credit report on their financial affairs, so they should be able to get access to the digital dossiers companies hold on them.

Since revelations of surveillance can damage a firm’s reputation—as the outrage over a letter sent by OfficeMax, a department-store chain, to “Mike Seay, daughter killed in car crash”, showed—advertisers would do well to go further voluntarily. In the same way that some consumers turned against clothing companies that used sweatshop labour, consumers could rebel against firms that invade their privacy. Advertisers should take a good look at their data supply-chains and strengthen consumers’ right to control their own digital information. Surveillance works in the movies, but it may not be a sustainable business model.