The internet is one big experiment, and you’re part of it. Every day, millions of trials are manipulating what you see when you browse online, to find out how to keep your attention, make you click more links – and spend more money. And these experiments are often secret. You’ll probably never know you were part of them.

This is all thanks to something now well-known in the tech industry, called A/B testing. It means that the web pages served to you are not necessarily the same as those shown to the next person – they might have slightly different colours, an alternate headline or, on social networks, you could be shown different personal information about your friends and family.

What started as a way to tweak website design is becoming increasingly controversial – in the most divisive cases, A/B testing can help companies sway people’s mood or even their love life. This summer, it emerged that Facebook used the technique to experiment on users, without their knowledge, in an effort to influence their emotions. And more recently came the revelation that dating network OkCupid lied to some of its users about their suitability to be “matched” romantically with another member of the site. The company was hooking people up with unsuitable potential partners, and then tracking their interactions.

So, at what point does all this experimentation become outright manipulation?