Psychosis in the 21st century looks something like this: You think your every move is being filmed for a reality television show starring you, and that everyone in your life is an actor.

Or you think you are under intense surveillance by an army of spies, whom you refer to as the "www people," as in the World Wide Web, and they wiretap your furniture and appliances.

Or else you refuse to drink water because you fear that another cup drawn from your faucet will, once and for all, deplete the world's water supply.

Those thoughts are from three case studies of what psychiatrists interested in the intersection of mental illness, culture and society are calling, respectively, Truman Show delusion, Internet delusion and climate change delusion; all of them a window, through madness, into the modern world.

If you have delusions of grandeur in this century, you are probably not Napoleon, but you may be Bill Gates.

The Truman Show delusion, or Truman Syndrome, has drawn attention in recent months, in the United States and Britain, as psychiatrists in both countries describe a small but growing number of psychotic patients who describe their lives as mirroring that of the main character in the 1998 film The Truman Show.

Played by Jim Carrey, Truman Burbank leads a mundane existence in the suburbs, starting from the time he was in the womb, while being filmed for a documentary television show that he cannot escape.

Everyone is in on it, including his wife, and no one will believe Truman when he discovers clues that his life is being chronicled all the time by cameras.

With Internet delusion, patients typically incorporate the Internet into paranoid thoughts, including a fear that the Web is somehow monitoring or controlling their lives, or being used to transmit photographs or other personal information.

The delusions are fueling a chicken-and-egg debate in psychiatry: Are these merely modern examples of classic paranoia fed by the cultural landscape, or is there something about media like reality television and the Internet that can push people over the sanity line?

"Most likely these people would be delusional anyway," said Dr. Joel Gold, a psychiatrist at Bellevue Hospital Center in New York, who said he saw five patients at the hospital from 2002 to 2004 with Truman Show delusion. Gold and his brother, Dr. Ian Gold, the Canada research chair in philosophy and psychiatry at McGill University in Montreal, came up with the term "Truman Show delusion."

"But the more radical view is that this pushes some people over the threshold; the environment tips them over the edge," said Joel Gold, who is a clinical assistant professor of psychiatry at New York University. "And if culture can make people crazy, then we need to look at it."

One way of looking at the delusions and hallucinations of the mentally ill is that they represent extreme cases of what the general population, or the merely neurotic, are worried about. Schizophrenics and other paranoid patients can take common fears - like identity theft because of information transmitted on the Internet, or the loss of privacy because of the prevalence of security cameras to fight crime - and magnify them, psychiatrists say.

"There is the old saying that just because you're paranoid doesn't mean there's not somebody after you," said Dr. Jeffrey Lieberman, chairman of the Department of Psychiatry at Columbia University.

The prevailing view in psychiatry is that a delusion is just a delusion, psychosis is psychosis, and the scenery is incidental. Fear, a sense of persecution and grandiosity are static features of delusional thinking, many psychiatrists say.

During World War II, for example, psychotics might have believed a neighbor was a Nazi. During the Cold War, they might have thought the KGB or CIA was following them. In a post-Sept. 11 world, the persecutor might be Al Qaeda or the Department of Homeland Security.

"Cultural influences don't tell us anything fundamental about delusion," said Vaughan Bell, a psychologist at the Institute of Psychiatry at King's College in London, who has studied Internet delusion.

"We can look at the influence of television, computer games, rock 'n' roll, but these things don't tell us about new forms of being mentally ill," said Bell, who said he had also treated patients who believed they were part of a reality television show.

British psychiatrists, writing in this month's edition of the British Journal of Psychiatry about the phenomenon, called it the Truman syndrome and said they had seen a growing number of patients claiming to be the stars of a filmed reality show.

The Diagnostic and Statistical Manual of Mental Disorders defines a delusion, considered still to be little understood in psychiatry, as, essentially, a false belief that is not grounded in reality and that is held with absolute conviction despite proof to the contrary. The manual lists a caveat that a belief is not delusional if it is something widely accepted by other members of a person's culture or subculture - for example, religious faith. But some psychiatrists say the exception is too vague.

Some experts studying conditions like Truman Show delusion and other culture-bound delusions, which are specific to a time or place, are questioning the premise that culture is only incidental to psychosis, even as a growing body of evidence has pointed to brain abnormalities and other biological causes for illnesses like schizophrenia.

Psychiatrists have studied delusions like turabosis, which is the belief that one is covered in sand, and which has been documented in Saudi Arabia but would be unlikely to occur in, say, North Dakota. Another study found a delusion occurring only in rural West Bengal, India, in which women and men bitten by dogs believe they have become pregnant with puppies.

Joel Gold, who is writing a book about Truman Show delusion with his brother, said that three of the five patients he saw with the condition specifically mentioned the film. He said what distinguishes this delusion from most others is that it involves the patient's entire world, and everything real is unreal.

Psychiatrists say that other movies whose characters are living in an unreal world or being watched by malevolent forces, including The Matrix, Edtv and even the film based on George Orwell's 1984, have come up in conversations with psychotic patients. But the premise of The Truman Show ("What if you were watched every moment of your life?" according to a promotional blurb) is strikingly similar to what patients describe as their own experiences.

Reinforcing their beliefs is the fact that in the movie, Truman is right about being watched and recorded at all times. Every other character is part of the conspiracy.

Since the Golds first presented their findings in 2006, they have learned of about 40 cases of people who say they are experiencing the delusion or have in the past. Sometimes patients contact them directly.

Recently, Joel Gold received an e-mail message from a woman who told him, "It's my show."