In 1973, Mexican anthropologist Santiago Genovés set out to test a hypothesis. He had been struck by the connection between violence and sexuality in monkeys. “Most conflicts,” he noted, “are about sexual access to ovulating females.”

Genovés’s advert for volunteers. Photograph: The Times

But would this apply to humans, too? To find out, Genovés asked a British boat builder to make a 12x7 metre raft called the Acali on which he planned to sail with 10 sexually attractive young people across the Atlantic from the Canary Islands to Mexico. It was like a prototype for the glut of reality TV shows since, a Floating Love Island or Big Brother at Sea, but with a twist – the participants were so isolated from the rest of the world that it would have been futile to cry: “Get me out of here!” The only ways out were drowning or getting eaten by sharks.

Genovés was a veteran of extreme rafting. A few years earlier he had been one of the seven-strong multinational crew on Thor Heyerdahl’s two Ra expeditions to sail reed rafts, like those used in ancient Egypt, across the Atlantic. The Norwegian adventurer wanted to show how people of different races could cooperate effectively.

Genovés had even grander motives in planning his voyage: he sought to diagnose and cure world violence. To that end, he placed ads in international newspapers and made his selection from respondents, choosing a crew of strangers from different races and religions so that he could create a microcosm of the world. Among the five women and five men were a Japanese photographer, an Angolan priest, a French scuba diver, a Swedish ship’s captain, an Israeli doctor and an Alaskan waitress who was fleeing an abusive husband. Genovés called his boat the Peace Project, but it rapidly became known in the world’s press as the Sex Raft.

The mixed-gender, multi-ethnicity, 11-strong crew, captained by Maria Björnstam (centre)

To spur conflict onboard, Genovés minimised opportunities for privacy. His human guinea pigs were allowed no reading material. When they wanted to use the loo, they had to sit on a hole perched above the waves in full view of the other 10 and hope that the sea would wash their bottoms. Sex was logistically tricky. Either you would have to do it in full view of the others, or wait until the opportunity offered by night-time. Even then, two people were on duty, one keeping lookout and the other steering. If you were quick about it, crew members related, you could have it off – so long as one of you kept a hand on the helm throughout.

The boat would have no engines and would sail towards the Caribbean, just in time for hurricane season. Genovés knew that the Acali was sailing into danger but thought science justified the risk. “I believe that in a dangerous situation people will act on their instincts and I will be able to study them.”

He put women in charge, in part to reflect what he thought was growing gender equality. The raft was captained by Maria Björnstam and Edna Reves was ship doctor; men were given menial tasks. “I wonder if having women in power will lead to less violence or more,” mused Genovés. “Maybe men will become more frustrated when women are in charge, and try to take over power.”

The Acali raft on its 101-day voyage. Photograph: Fasad

Armed with questionnaires and spreadsheets that matched up rises in aggression and sexual activity with phases of the moon and wave height, he yearned to discover what humanity must do to live in peace. It didn’t quite work out that way.

What happened in the next 101 days is now chronicled in Marcus Lindeen’s documentary The Raft. Using 16mm film from the journey spliced with new footage in which surviving crew members recall their experiences 43 years after the Acali’s voyage, Lindeen recreates one of the weirdest social experiments of all time. “I suspect that if Santiago were alive today, he would be working in reality TV,” says the Swedish artist, theatre director, playwright and documentary maker.

Marcus Lindeen, director of the documentary The Raft. Photograph: Emelie Asplund

Over Skype from Stockholm, Lindeen tells me he was looking for material to make a film akin to his debut documentary The Regretters. That 2010 film was about two Swedish men, both of whom had gone through sexual reassignment surgery to become women, regretted it, and transitioned back. “I was looking for another project that would involve a group coming back together and reflecting what had happened to them. I thought about a queer commune.” Then he read a book called Mad Science: 100 Amazing Experiments from the History of Science, which contained an account of Santiago Genovés’s Peace Project.

“I felt, ‘Oh my God. This is it!’” It was Homer’s Odyssey, an adult Lord of the Flies, with a hint of Fitzcarraldo and, fingers crossed, a rerun of 120 Days of Sodom. So he started to track down the crew, only to find many had died in the interim, including Genovés. “Maria, the captain, was Swedish so I got in touch with her quite quickly. But she was shy, and ashamed about her role on what had become called the Sex Raft, and initially didn’t want to take part.”

Then, having seen Lindeen’s earlier work, she changed her mind.

“She produced a box from her attic in Gothenburg that she had never opened before and we started looking thorough it.” Inside were photographs and blueprints for the construction of the raft, but most importantly a contacts book that set Lindeen off on the trail of the other crew members.

Survivors of the experiment (left to right: Mary Gidley, Edna Reves, Fé Seymour, Eisuke Yamaki, Maria Björnstam and Servane Zanotti) in Marcus Lindeen’s documentary The Raft. Photograph: Måns Månsson

Once he had tracked down five women and one man and secured their agreement to take part, he commissioned a full-size replica raft to put on the soundstage where he would film them reminiscing. They had not met since the Peace Project docked in Mexico 43 years earlier, so the reunion was poignant.

In one of The Raft’s most powerful moments, Fé Seymour, an African American engineer, tells her white compatriot Mary Gidley of the strange dreamy sense she had of making the same journey across the Atlantic as her African ancestors on slave ships. “I would sit on the starboard side and look into the water. I would start to hear voices coming from down there … I would hear my ancestors call me. They could feel my flying over their bodies and their tragedies. It was one of the best things that happened to me.”

Mary had a secret, too. She was fleeing a husband who had tried to murder her because he’d overheard her planning a divorce while talking to a friend on a boat. “I believed he was going to strangle me and so I ran and jumped off the boat.” On this new boat, Genovés’ raft, she felt safe and protected.

Not that Genovés’ raft was an antidote to the patriarchy. With a Caribbean hurricane brewing, Maria, the experienced ship’s captain, recommended they pull into a port to sit out the storm. Genovés, fearing the ruin of his experiment if they did so, mutinied and took control of the raft. “He wants to be very progressive and radical giving power to the women,” says Lindeen, “but when it comes to the crisis of the captaincy he’s very macho.”

But Genovés was symbolically castrated later, on the Atlantic crossing. A huge container ship bore down on the little raft and he panicked. Only Maria kept a cool head and organised flares to ward off the looming ship. After that, the guinea pigs turned on the scientist: Maria became captain again. Others on the raft even contemplated killing Genovés. Fé recalled imagining that everybody would put their hands on a knife and “plunge it into him so everybody was guilty. We would wrap him in a sheet, carry him over the railing and drop him.”

Overthrown, Genovés retreated below deck and collapsed into depression, made worse by news on the radio that his university wanted to be dissociated from the scandalous Sex Raft headlines. While lying there he started to cry for the first time since childhood and had an existential epiphany, writing: “Only one has shown any kind of aggression and that is me, a man trying to control everyone else, including himself.” The detached scientist had gone on a Conradian journey, ultimately realising that the heart of darkness was inside him.

‘We would plunge a knife into him, wrap him in a sheet, carry him over the railing and drop him’ … Genovés with Fé Seymour. Photograph: Fasad

After the crew rebelled, Linden argues, “he finds some kind of humbleness. I admire him.” Why? “For making this crazy experiment happen!”

Was the Peace Project a failure? Fé argues it was a great success, even though the anthropologist couldn’t see it: “He was so focused on the violence and conflict, but he had it right in his hands. We started out as them and us and we became us.”

For Lindeen, it’s poignant that Fé praises the experiment.

“If only [Genovés] had listened to why people were on the raft – Mary escaping an abusive husband, the racism Fé had suffered – he would have learned about the consequences of violence and how sometimes we can overcome it by overcoming our differences.”

And, just as Genovés learned a lot about himself through his supposedly detached scientific experiment, so Lindeen had an epiphany in making the film. “I was raising the money, making a full-size model of the raft, getting the crew back together, all the filming, a year editing the film – a really crazy project. It was painful to realise this, but I see something of myself in him. He was a master of manipulation, a control freak and a dictator. I am like him more than I want to admit.”

The Raft released in UK cinemas on 18 January

• This article was amended on 23 January 2019. Santiago Genovés was not directly involved in researching the connection between violence and sexuality in monkeys, as earlier version suggested. This has been corrected.