A family is sitting down to supper. The mother announces that they are going to eat without the distraction of smartphones - so she collects everyone's phones and puts them in a glass jar with a timed lock. But shortly afterwards there is a crash: one of her daughters has got up and smashed the jar with a hammer.

This example of social media addiction was invented for the purposes of a documentary - but the rest of The Social Dilemma deals in serious facts. People who have held senior positions in companies such as Facebook outline why we cannot leave our phones alone, with the average person in the UK spending three hours a day staring at the small screen.

Tristan HarrisThe co-founder of the Center for Humane Technology in the US, he has been described as "the nearest thing Silicon Valley has to a conscience"., a former Google employee, sums it up: "If you're not paying for the product, you ARE the product." Social media companies make money not by charging us for their services, but by minutely analysing our online habits to work out what interests and excites us most.

They can then say to businesses, "Out of all the people in the world, we know which ones are most likely to buy your products. Advertise with us and you are sure to reach them.

"What I want people to know," says former Twitter executive Jeff Seibert, "is that everything they're doing online is being watched, is being tracked, is being measured - every single action you take is carefully monitored and recorded: exactly what image you stop and look at, and for how long. They know when people are sad, they know when people are depressed."

Facebook, Instagram, and Twitter, Harris says, are all competing to keep us looking at our screens. "There's the engagement goal, to keep up your usage, to keep you scrolling. There's the growth goal, to keep you coming back and inviting more friends ... Each of these goals is powered by algorithmsAny set of rules followed by a computer. In the context of social media, "the algorithm" refers to the intelligent AI that learns the interests of the user and presents them with posts that it thinks will interest them. whose job is to figure out what to show you."

In a forthcoming book, Reality and Other Stories, John LanchesterBritish journalist and novelist. His books include How to Speak Money: What the Money People Say - And What It Really Means. argues that we are being manipulated at a psychological level so deep that we do not realise it is happening.

Among weaknesses, these algorithms play on are "bad forecasting" - our inability to work out how much time we are going to spend on something; the fear of missing out; and the addictiveness of activities that require little or no effort, such as one-click ordering or automatically watching the next video suggested for us.

The effect of the algorithms is to reinforce our existing preferences - and prejudices. Two people googling "climate change" can get completely different results. If the computer thinks you are an environmentalist, it might supply the word "disaster"; if it identifies you as a sceptic, it might supply "hoaxA deceitful trick. It probably derives from the word "hocus", meaning a magician.".

The political result, says ex-Twitter engineer Justin Rosenstein, is that people are increasingly polarised. "You look over at the other side and think, 'How can these people be so stupid? Look at all this information that I'm seeing!' And the answer is that they're not seeing that information."

Some say, no. There are two sides to everything, and the documentary is taking a very negative view of social media, which is also a powerful force for good: it can reunite families and find organ donors. People join Facebook because they want to see what their friends are doing. Anyone with any intelligence can work out whether the sites Google sends them to are reliable sources of information.

Others, including Tristan Harris, argue that political polarisation because of social media, and the rise of fake news, has made civil war a real possibility. "Imagine a world where no one believes anything is true. Everyone believes the government's lying to them, everything is a conspiracy theory: 'I shouldn't trust anyone, I hate the other side.' That's where all of this is heading."