M ost moviegoers view Alfred Hitchcock's Psycho as the spoiler of showers and the death knell of motels.

After seeing the film, and witnessing what happens to Janet Leigh, who could ever again completely relax beneath the cascading water? Who could check into a motel without wondering if the crazy-eyed rube behind the counter is another Norman Bates?

Psycho has haunted the public mind more than any other horror film in the nearly 50 years since its 1960 release. Yet many critics consider it one of Hitchcock's lesser works, not on an artistic par with North by Northwest, Rear Window or Vertigo.

Critic and historian David Thomson is among them – Psycho wouldn't be his desert-isle Hitchcock pick – but he loves the film regardless. He's seen it at least 30 times and he's become a champion of its lasting influence.

Thomson's fascinating new book The Moment of Psycho: How Alfred Hitchcock Taught America to Love Murder, released this week, cuts through conventional wisdom like Bates's blade through a shower curtain.

To him, Psycho was the film that jolted Americans out of their post-war innocence. It helped usher in the 1960s as the decade of social upheaval.

"Most films of the '50s are secret ads for the American way of life," Thomson writes. "Psycho is a warning about its lies and limits."

The film's sex and violence were considered extreme at the time, even though you never see the completely naked form of Leigh's Marion Crane or see her penetrated by the knife of Anthony Perkins' Norman Bates. Aided by Bernard Herrmann's slashing score, Hitchcock used the power of suggestion to shock his audience and to skirt censors.

Film was never the same after Psycho, the London-born Thomson told me this week from his San Francisco home.

"In the long term historically the most decisive change in censorship was not so much with sex and language, it was with violence," he said.

"Psycho in 1960 unleashed a kind of casual violence, when you think of the attitudes then, that was unthinkable at the time."

The strict film censorship rules known as the Motion Picture Production Code, or Hays Code, were still in effect when Psycho arrived in June 1960, opening wide years before Jaws defined the movie "blockbuster." But Psycho was the Code breaker.

It pushed past the Code's rules for the depiction of sex – the film opens with a post-coital scene – and even had the dubious distinction of being the first film to show a flushing toilet, another Code no-no.

"Psycho just seems to say `f--k you' to the Code," Thomson said. "I think all over the world there was a feeling that, `My God, it breaks down as easily as that.' And when censorship broke in the movies, it broke down rather quickly. You can trace it throughout the '60s. Psycho was a key step."

Hitchcock knew exactly what he was doing. He turned 60 in the summer of 1959, when he was in pre-production for Psycho, and he'd been making films for decades. He was tired of dancing to the whims of censors. He determined he was going to take them on, both in America and in his home country Britain, and he also had Hollywood in his sights.

But he was canny enough to use charm and evasion as his weapons. Hitchcock actively courted Geoffrey Sherlock, the chief U.S. censor. Sherlock loved to socialize with filmmakers (so did his British counterpart John Trevelyan) and he was not immune to the lubricating affects of alcohol.

"I think Hitchcock understood this in Sherlock and courted him for few years, and then, I think, used him."

Hitchcock's use of black-and-white film for Psycho allowed him to show blood going down the drain in the shower scene, something that would never have been tolerated in colour.

"I think he was trying to do a lot of things," Thomson said. "He had an instinct to test the audience with a kind of violence that he'd never dared before. I also think he wanted to make a lot of money (Psycho earned more than Rear Window and North by Northwest combined). And I think he was becoming increasingly interested in disturbed psychology. All of those things came into play with this."

What might Hitchcock made of today's torture porn movies like the Saw and Hostel franchises, where death and dismemberment are graphic and constant? In many ways, these films are the spawn of Psycho.

"I suspect he'd be fascinated by it," Thomson said. "If Hitchcock were alive and at his powers, I could imagine him trying it, even. I think it would have dug deep into him. Because I think there's a lot of that in him already."

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The shock of Psycho extended far beyond the shower. Thomson believes the reluctance of many older moviegoers today to go to the pictures began with the horror inside the Bates Motel.

"Psycho changed the temperature of filmmaking ... in my opinion, it has a great deal to do with the loss of audience. One thing more than any other that has kept some people away from the cinema is because they think it's become too violent."

Norman Bates has a lot to answer for, but so does Alfred Hitchcock.