He was never just funny ha-ha. Look behind the silly walks and pratfalls and custard pies and you can see it for yourself: poverty, starvation, sickness, depression and mental illness were all portrayed in his films. Partly it was because they were issues Chaplin cared about (enough to earn him a big, fat FBI file, in fact), but also because he had been poor and sick and weak himself once. He was making films about the things he knew. He was making films about himself.

Stephen Weissman, in his biography Chaplin: A Life, has a wonderful term for this process: “clandestinely autobiographical”. Weissman believes most of the famous moments and scenes in Chaplin’s films were directly inspired by incidents and scenes in the film-maker’s childhood, even though the audience didn’t always know it.

In some cases, the links between the films and Chaplin’s own life are undeniable. For his 1921 masterpiece The Kid, he took the time and expense to have his studio built as a replica of one of his childhood homes, a flat in Pownall Terrace in London that was the final destination in the Chaplins’ descent into poverty. Another of his most famous films, Modern Times, in which he is sucked into grinding cogs and spinning wheels was inspired, says Weissman, by the time Chaplin spent working on a giant printing machine before he was famous. Even Chaplin’s famous wibbly, wobbly walk was copied from an old drunk he remembered from his childhood called, marvellously, Rummy Binks.

In uncovering these memories, what Weissman constructs is not a complete birth-to-death biography but a psycho-biography, a book that attempts to explain the actions of a man by looking for psychological clues in his childhood. And he wheels out an impressive star witness to support his case: Sigmund Freud himself, who said of Chaplin: “He always plays only himself as he was in his dismal youth.” The writer Thomas Burke, who spent time in the same orphanage as the writer, actor and director, is also quoted: “I never see a new Chaplin film without recognising the origin of at least two incidents.”

It is when Weissman attempts to extend this idea and suggest that not only was Chaplin inspired by his childhood, but that he also used his films as some kind of therapy, to interpret and understand what he had been through, that the elastic of the argument starts to stretch uncomfortably. Weissman suggests the tramp’s constant rescuing of women, particularly in City Lights in which he helps a flower seller find a cure for blindness, is Chaplin attempting to come to terms with the fact he could never rescue his mother from mental illness. Perhaps the only real prop the book provides for this argument is the moment when Chaplin the movie star returns to his former orphanage. The visit was, he said, like the dead returning to earth: it was cathartic, comforting, a way of coming to terms with his childhood. Perhaps – just perhaps – it made his films feel the same.

In digging down to the deepest layers of Chaplin’s life, Weissman suggests something else interesting, but only in passing: understanding the links between the childhood and the pictures can help us explain why, for some, the magic of the great comedian has faded to grey. The fact that Chaplin’s films were inspired by Victorian poverty can make them feel distant from our comfortable modern lives. Chaplin’s original audience at the beginning of the 20th century would not have been so far removed from the source material; many of them, watching the films projected onto sheets hung from a wall in filthy rooms, would have lived in aching poverty. They would laugh, as Chaplin once said, in order not to weep.

But just as this explains why, nearly 100 years later, some can no longer relate to Chaplin’s stories, it might also explain why he can still be powerfully relevant. His films aren’t just about being poor; they are about conquering poverty, the joy of the small overcoming the big, the little tramp kicking the giant bully. As Weissman explains, it’s a comedy of subversion that, even before Chaplin’s anti-fascist The Great Dictator, attracted the contempt of the Nazis: “The Nazis would sneer at the ridiculous comic logic of a puny underdog like Charlie triumphing over ubermensch heavies twice his size.” And the best thing about it, this mocking of the Aryan lie, this victory over aggressive bullies or thoughtless authority or bad luck, is that it doesn’t need historical context to work. It was funny and memorable and liberating then, and it still can be.

Chaplin: A Life by Stephen Weissman is published by JR Books, priced £18.99.