It became a landmark experiment in sociology and cinematography, but when the first documentary in the Up series was made back in 1964, it was just going to be a one-off.

Seven Up took a group of seven-year-old Britons from a variety of backgrounds and interviewed them about their aspirations, hopes, fears and plans.

It was informed by the old adage, "give me a child until he is seven, and I shall give you the man": the idea that by the time someone is seven years old, the defining traits of their character are set in stone.

For Michael Apted, who worked as a researcher on the first film, it's also a snapshot of a time "when women were [seen as] nothing and most people who didn't have a good education had no chance of a good job; that kind of class imprisonment that some people went through".

Apted has been working in Hollywood since the late 1970s and returns to the UK to produce the Up series. ( Getty Images: Dave J Hogan )

Since then — alongside his work on everything from James Bond to Narnia movies — Apted has directed all the subsequent iterations, returning to the same group of people every seven years to track the arcs of their lives.

The series of films that resulted (7 Plus Seven, 21 Up, 28 Up, 35 Up, and so on) follow participants through marriages both failed and successful, career ups and downs, the birth of children and even one man's extended period of homelessness.

But while Apted promised his boss half a century ago he would keep working on the project as long as it had subjects, he fears his latest entry, 63 Up, could be his last.

How the Up series happened

Apted credits Dennis Foreman, then the head of Granada Television (now ITV Granada) in Manchester, with the idea of extending the project beyond one film.

"[Dennis] said, 'Have you ever thought about going back and visiting these children?' And I said, 'Ah, no, I haven't, but what a great idea,'" Apted tells RN's The Screen Show.

Apted says the course of the next 50 or so years of his life was set almost instantaneously.

"As soon as I went back to see them, I did one day's filming and I rang [Dennis] up and said: 'It's not my ambition to spend my working life in London or Granada or all that, but we should keep doing this, and I guarantee that if you could commit to doing this, for as many seven years as you've got people, then I guarantee I will come back and do it.'"

Tony was one of the original 14 Up participants. Two others have dropped out of the project and one has passed away. ( Supplied: SBS On Demand )

In the intervening years, Apted and his teams have covered a huge amount of ground, but they have had to set themselves limitations.

"It's a very big job to edit the film[s] … if you don't give it parameters, then I think you're dead in the water," he says.

The main parameter Apted is referring to is never attempting to capture events in his participants' lives outside each seven-year window.

"If I started making, you know, the lives of each of them as they happened, then I'd have a 64-hour program and no-one would ever go and see it," he says.

"So, you've got to draw the line somewhere."

Tracing the rise of women

In 49 Up (2005) one of Apted's participants took him to task.

Jackie, a woman who grew up in a working-class neighbourhood in east London, questioned whether Apted's line of enquiry was informed by gender bias.

In 63 Up, Jackie returns to the topic. "When we were younger I kept thinking: 'Why is he asking me questions about marriage and men … why is he not asking me questions about how the country is?'" she says.

"I felt that you treated us as women totally different. I didn't like it."

Apted denies that the earlier films were intentionally biased, but he is candid about the fact that the social mores of the time — that "men were [considered] more important than women" — informed the way he and his team shaped the documentaries.

Paul, one of the series' participants, left the UK soon after Seven Up was shot and resettled with his family in Australia. ( Supplied: SBS On Demand )

"For God's sake, we didn't choose enough women," he exclaims.

"We only chose four women, and we had 10 men. So, that was an imbalance from the very beginning."

But as the women's movement gained momentum in the UK and perspectives began to change, the Up series developed and made more room for its female participants.

"Women became more important in society as the series grew up, and so, I wanted to spend more time [with them]," he says.

"The change of women in society is one of the most important social changes, probably in any of the cultures I've been dealing with."

Finding profound truth

Up has spawned a number of imitators, but Apted says only three other series have worked: "The British one, which was the first one, the Russian one and the South African one."

"I reckon the reason ... no-one else has ever pulled it off is because, underneath the show, is a profound truth about the country," he says.

"In Great Britain, the class system has been with us … since the medieval times. It's still there and it's still the kind of touchstone of it all."

But through producing the series, Apted thinks he's seen a hopeful improvement in British society.

"I think class is what was, in fact, hurting Great Britain and is now easing off somewhat, and the country is all the better for it."

When he reflects on the possibility of continuing his 55-year record, Apted is a little less optimistic.

"I'm 78 now … I can't imagine I'd be in health to do this at 85, so I don't know," he says.

But his original commitment to keep making the Up series for as long as he can stays strong.

Reflecting on the possibility of a 70 Up, he says: "It would be nice, wouldn't it? It has a better sense of completion."

63 Up is available to stream on SBS On Demand.