Paul-Julien Robert was born into a commune in 1979 and spent the first 12 years of his life there. "Everything in the outside world was described to us as evil. I knew what a nuclear family was, but it was something distant and seen as destructive."

His idealistic Swiss mother, Florence Desurmont, joined the Friedrichshof commune, near Vienna, when she was 26, before Paul-Julien was born. She hoped, after a difficult upbringing of her own, that living there would be a kind of psychotherapy. Like her fellow members, she renounced the traditional family model. "Her family would have been my family," says Paul-Julien, 34, "but she traded it for an alternative lifestyle.

The commune manifesto, written in 1973 by its founder, artist Otto Mühl, states: "The attempt to live together in communes is an important social experiment that will allow the transformation and evolution of the nuclear family society in the long term."

Dissolution of the nuclear family, free love and communal property and childcare were the basic principles at Friedrichshof: those who joined thought they could raise healthy, independent and untroubled children like this but many grew up to become emotionally scarred, scared and confused adults.

Mühl, an Austrian who died in May, said he set up the commune out of loneliness after his marriage collapsed. But he was cruel, controlling and authoritarian. He separated parents and children and created a hierarchical structure, which defined the members' status, and regularly "directed" them in disturbing performances, including dance, song or acting, based on "self-exposure", designed to break down the idea of the nuclear family – the central building block of society.

How did Paul-Julien feel about his mother in relation to the other women in the commune and to the collective upbringing principle? He pauses for a long time: "My mum was the only person with whom I felt safe. I had a feeling that she was something more and so she meant more to me," he says.

Paul-Julien in the early 80s, at Friedrichshof.

In 1983, Florence – along with the other Swiss members – was forced to go Zurich to earn money for the commune. She had to leave behind Paul-Julien, then four. "I was very lonely. Other women replaced her, but they were never close to me. The ideology was that all relationships were bad for the group, so it was never possible to truly bond with someone."

Everything changed for him when his mother left and the early period of his childhood, which he recalls as happy, came to an end. The children he now spent his time with, playing and participating in various creative activities and role plays (in one, Mühl asks them to recite lines such as "My mum drove off to Zurich. Since then, I feel better and better every day") became a kind of surrogate family.

Did he feel loved? "Never. I grew up believing love was something bad. The feeling of being loved, and of expressing love, was something I really had to learn and to accept later."

Paul-Julien's film My Fathers, My Mother and Me has just won the Grierson award for best documentary in the 57th BFI London Film Festival for directing. It is a powerful, moving, sometimes disturbing film about his experiences in Friedrichshof and combines archive footage with present-day conversations with his mother.

Paul-Julien with his mother, Florence, in a scene from the film My Fathers, My Mother and Me (in German, Meine Keine Familie).

Kate Ogborn, president of the BFI jury, said they wanted to recognise Paul-Julien's bravery for "taking us on such a personal journey with the documentary. It is a thought-provoking and disturbing film, intimate while also raising larger questions of power, parental responsibility and abuse. The incredible archive footage combined with the personal journey of a mother and son left us disturbed, angry and feeling that this is a film that deserves to be seen by a wider audience."

On film, when Paul-Julien asks Florence if she had ever worried about him while she was away in Zurich, she said she hadn't: she had absolute trust in the commune.

Like all the children born there, Paul-Julien was a member of a very large family, but had no real father. Although the children were to grow up without specific father figures, legal fathers' names werer required for the birth certificates. A year after Florence left, Paul-Julien's legal father, Christian, killed himself with a knife.

Paul-Julien with his mother, Florence, and 'legal' father, Christian, named on his birth certificate.

Did he feel a sense of loss? He says not. "When they told me, I remember I started crying. Not because I was sad but because everyone was looking at me, making me feel that something very bad had happened."

In 1991, Mühl was convicted of sexual abuse of minors and sentenced to seven years in prison; the commune fell apart. At that point, it was agreed to run paternity tests for all children born in the commune.

Thus, Paul-Julien was 12 when he found out that his biological father was a German, Egon Redent, who worked at the commune's winter home, El Cabrito, on the island of La Gomera, in the Canaries. Florence wanted the three of them to become a family, but Egon was already in a relationship with another woman whom he later married and had a son with; Paul-Julien's half-brother, Adrien, 19.

Until that point, Paul-Julien hadn't been interested in his father's identity. "We didn't have any closeness to men. For Otto, any intimacy between men was an expression of homosexuality – something sick. There was a general lack of affection from the adults – no one held me or was tender towards me as a child."

Has his relationship with Egon developed? "I had a lot of expectations, but he couldn't fulfil them. A few years ago, I realised that, as much as I like him as a person, I could not have him in my life as a father. Having a relationship with my half-brother was also difficult. I felt like my father felt, I guess. I wanted to play a role that I couldn't, also because we lived far from each other. I wish we had a closer relationship, but we don't."

A common trait among the communards seems to have been an unwillingness or inability to take responsibility and to adopt specific roles, which is what a family needs, at least in principle. That was also the case for Egon who, in the film, recalls how it felt to become a father to a 12-year-old boy overnight. It was a level of responsibility for which he wasn't ready and he'd felt unable to make an emotional connection with his son.

Paul-Julien also remarks on the paradox that most of the men in the commune found a father figure in Mühl, while the children born there had no fathers in the conventional sense. Was Mühl an alternative father figure to them too? "Yes, in that he was the only one who had authority. If we behaved badly, the adults could not punish us, but did threaten to tell Otto who would then decide what to do."

After the commune collapsed, Paul-Julien went to live in Zurich with Florence. He went to school and led a "normal" life, but this new world was foreign to him, and getting on with his schoolmates was difficult. "I didn't know how to communicate with them or what to talk about. The image of friendship I had until then was very different. I had also grown up in the countryside and was used to being close to nature, so adjusting to life in a big city took time."

Paul-Julien and Florence, left, with other women and children at Friedrichshof.

What about his relationship with his mother once they were reunited? Surprisingly, he says they never talked about the past until six years ago, when he decided to make the film. In fact, at some point, Florence told Egon that until Paul was 25, he didn't talk to her much at all. "I hadn't realised it, but I guess it was true."

Does he wonder how his life might have been if he had grown up more like other children? "Sometimes I actually think that my childhood, compared to other children's, was normal. The commune had some positive elements too. I grew up close to nature and within a group of children who were my friends. Being so far from the society outside, it felt like a place where we could have freedom."

Towards the end of the film, Florence confesses to her son that when she was young she had felt insecure as a mother and didn't know how to play that role. She also expresses the doubts she still feels about the nuclear family. She considered the exclusive mother-child relationship was bad and tried something different in the hope that it would be better.

Film director Paul-Julien Robert was presented with the Grierson award for best documentary at the BFI London Film Festival 2013. Photograph: Tim P Whitby/Getty Images for BFI

How does Paul-Julien feel about nuclear families? He pauses for a long time and smiles: "I think even to live with myself is a challenge. To live with another person, and for her to accept me as I am, is an additional challenge. For me, a nuclear family is like a small commune in a way. Each individual has their own opinion and to find a way to be together without losing one's self is very difficult."

Why did he make the film? There are moments during his conversations with Florence in the documentary when one gets the impression that anger simmers beneath his questions. He says this isn't true. "I was not angry. Maybe sometimes I was disappointed. It was not about finding answers for myself, more about putting up questions so that my parents could find some answers for themselves. I did not want them to justify themselves, but to put them in a position to go back emotionally and reflect on what happened."

Paul-Julien will soon have to confront his feelings on family – his girlfriend is pregnant and he will become a father in January. He admits that even if he has changed over the years, he occasionally finds relationships hard. "My girlfriend is different to me. She wants to have a relationship where you share a lot and that feels a bit unusual sometimes."

But fatherhood does not scare him. "I will learn from day to day and I will handle it. It's life."

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