Show me a person’s targeted adverts, goes no proverb (yet), and I’ll show you what they put in their online shopping basket but decided against buying at the last minute. Most internet users will be very familiar with the feeling that your computer is spying on you, with adverts trying to get your attention and reminding you what you’re missing out on.

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“The goal is to personalise those ads,” says Sean Donnelly, an analyst at Econsultancy, the digital research company. “The rationale is that the exposure to that ad will remind you that you looked at [something on that site], and at some point you will click on that ad and purchase it. The reason they do that is it works.”

Obviously companies don’t want those ads to appear on certain websites (extremist or porn sites, for instance) but it can be hard to control. “There are lots of different vendors out there, and if you think of the supply chain – sites that allow ads, the companies that sell space on those websites, the media companies that buy spaces – it gets quite fragmented and it can be difficult to see where your ad is going to appear.”



On a site such as YouTube, where millions of videos are uploaded, including extremist ones, brands can find themselves advertising on videos they would be horrified to be associated with – and unwittingly funding the uploader. The Times found that companies and organisations including this newspaper, Transport for London and Sainsbury’s had all had adverts on hate preacher Steven Anderson’s videos.



This afternoon, I spent 10 minutes looking at a couple of bike websites. A few minutes later, getting back to work and reading a news website, there was a big advert for a cycling chain, complete with a picture of the bike I’d just looked at, as well as offers on a helmet and lock. It’s the same on a US news site, though it doesn’t work everywhere: a TV site seems to think I should buy a car instead, while YouTube shows me an advert raising awareness about the danger of lungworm in dogs even though I’m pretty sure I’ve never stayed up late into the night surfing canine parasite pages. The other adverts, on Facebook and Instagram, are a trace of my browsing history over the last few weeks. Anyone looking at the adverts companies think I may be interested in will conclude I lead a pretty dull life.

Others should be more careful – in 2013, Conservative MP Gavin Barwell triumphantly criticised Labour on Twitter after clicking on a link in a press release, and seeing an ad for a dating site, reading “date Arab girls”. He’d learned nothing from John Prescott who, the year before, had criticised MP Grant Shapps for running ads for “Thai brides” on his website. Neither men realised the only thing they were exposing were their own browsing histories – or at least what Google had concluded would interest them.