THE secret to life, as every fan of the Peanuts comic strip knows, is to dread only one day at a time. That was the characteristically gloomy philosophy of Charlie Brown, the legendary “roundheaded kid” whose painful insecurities and pet beagle, Snoopy, can be numbered among the world’s greatest cartoon creations.

Yet the dread, melancholy and loneliness that became recurring themes in the antics of the Peanuts gang turn out to have been far from fictional.

A new biography of the late Charles Schulz, creator of the Peanuts strip, has upset his family and caused a stir in American literary circles by portraying him as a miserable womaniser who never recovered from childhood trauma.

Schulz and Peanuts: A Biography, by David Michaelis, will be published in America this week and has already won critical acclaim for its clever detective work in linking key upsets in Schulz’s life to events portrayed in his cartoons. “You’ll never read Peanuts the same way again,” wrote a critic in the New York Post.

The man who built a billion-dollar merchandising fortune on the backs of a cartoon boy and his dog once famously declared that “happiness is a warm puppy”. But Schulz emerges from Michaelis’s scrutiny as a moody, introverted and deeply unhappy artist who considered himself, like Charlie Brown, to be underappreciated.

The portrayal of Schulz as a comic curmudgeon has come as a shock to several of his children and his former wife, who had cooperated with Michaelis at least in part because Schulz had been reading one of the author’s previous books, a biography of N C Wyeth, the American illustrator the night before he died in 2000, aged 77.

Monte Schulz, one of the cartoonist’s five children, had been instrumental in persuading the family to grant Michaelis access to private papers. But after reading the manuscript, he told The New York Times last week that he found the biographer’s portrayal “preposterous”.

Amy Schulz Johnson, a daughter, declared that “the whole thing is completely wrong”. Jean, Schulz’s widow, agreed that her husband, known by his childhood nickname, Sparky, was sometimes melancholy but that was “not a full portrait. Sparky was so much more. Most of the time he loved to laugh”.

Yet most American reviewers have praised the book as a perceptive and compelling account of how Schulz’s life shaped the multilayered themes in his work.

“Children could enjoy the silly drawings and the delightful fantasy of Snoopy,” said Bill Watterson, creator of the Calvin and Hobbes comic strip, “while adults could see the bleak undercurrent of cruelty, loneliness and failure, or the perpetual theme of unrequited love.”

At the root of Schulz’s misery, Michaelis argues, was what the cartoonist acknowledged was his “greatest tragedy” the death of his mother in 1943 from cervical cancer. According to Michaelis, Schulz had grown up with parents who were emotionally remote and not especially interested in his first attempts at drawing.

He yearned to be closer to his mother but she fell seriously ill in 1938, when Schulz was 16. Her final words to him were “Goodbye, Sparky. We’ll probably never see each other again.”

Over the years Schulz would repeatedly despair that his mother had “never had the opportunity to see me get anything published”. The Peanuts strip first appeared under the title L’il Folks in 1950, yet as it was syndicated ever more widely, Schulz found little comfort in its success. He was particularly upset when the United Features Syndicate decided to rename it Peanuts, a name he detested.

When he married his first wife, Joyce, he warned her at the start of their honeymoon: “I don’t think I can ever be happy.” As the marriage slowly unravelled he began an affair. When he finally separated from Joyce in 1970 he marked the occasion in his comic strip by having Charlie Brown kick Lucy, whose bossy character was based on Joyce, off their children’s baseball team.

At the same time Snoopy was portrayed as yearning after another beagle he had seen. This was a code for Jean, who later became Schulz’s second wife.

Michaelis also argues that aspects of Schulz’s life and character pop up in his portrayal of Schroeder, the comic pianist obsessed with Beethoven; and also Snoopy himself, who regularly complains about being talented but unappreciated.

Michaelis said last week that he had done seven years of research on Schulz’s life and was convinced that he had got the story right. “The way his life had always been told was as a humble, very simple American story,” he said. “But I always had to believe there was more complexity.”

Michaelis noted that in one television interview Schulz had spoken of his “awful feeling of impending doom” and of waking up every day to “a funeral-like atmosphere”.

At the end of Schulz’s life the Peanuts strip was appearing in 2,600 newspapers around the world and he was earning up to $40m a year. Yet Michaelis writes that “all of his life he felt alone, spending most of his adult half-century yearning to be taken care of, to be understood. He would struggle to love and be loved”.

Michaelis notes that among all the 17,987 Peanuts cartoons that Schulz completed, Lucy always snatches the football away as Charlie Brown is trying to kick it, and Charlie Brown never gets a card on Valentine’s Day.

Schulz was often asked why his strip had no happy endings. He would reply: “Unhappiness is very funny.”

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