Along with Sheepshead Bay, where the losers swam with the fishes, and Sparks Steak House, where Paul Castellano never made it for dinner, one of New York's top attractions for the mafia tourist used to be Vincent "Chin" Gigante. For many years, Chin could be seen taking his daily walk, unshaven and mumbling, shuffling around the streets of Greenwich Village on the arm of his brother, the priest. Until 1996, when a judge ruled him mentally competent to stand trial, Gigante used madness as a defence for almost 30 years.

He was convicted of racketeering in 1997. Next week he goes back to court on charges including obstruction of justice, and prosecutors hope to see him do time for using his crazy act to evade justice all those years. Since Gigante's antics kept him out of jail for so long, prosecutors are naturally sceptical when other mobsters attempt to get their sentences reduced by pleading mental illness.

But are claims of mania always faked for the benefit of the jury, or is mental illness an occupational hazard of organised crime?

Gigante, 74, has been boss of the Genovese organised crime family of New York since the mid-1980s. It was an extraordinary reign, characterised by duplicity: he lived in two homes, with two families - with his wife and their five children in New Jersey, and with his mistress and their three children in a town house on the exclusive Upper East Side. For 30 years he kept up his crazy act: his public face drooling and uncomprehending, his private demeanour (observed by an FBI agent perched uncomfortably on a ledge behind his house) domineering and sharp.

Gigante first sought psychiatric help in 1966 when he was under investigation for bribery. Subsequent indictments found him hiding under the blankets in various psychiatric wards. At his 1997 trial, a crowded courtroom witnessed an impressive performance; the reporter from the New York Daily News was fascinated and appalled: "He twitched and he trembled. His lips quivered and his arms shook. He played with his ear; he rubbed his chest. He shook his head, stroked his chin and scratched himself. And as the judge and opposing lawyers spoke to each other, Gigante talked to himself."

Over the years, legal brains have spent many hours poring over volumes of psychiatric "assessments" which claimed Gigante was suffering from schizophrenia and various forms of dementia. Not all the doctors who examined him were cynical, one prosecutor admits: "He definitely fooled a number of those people."

For Gigante, now serving a 12-year sentence and facing a good deal more prison time, the act must have worn a little thin. As one of his former soldiers, Fat Tony Salerno, put it: "If he gets pinched, all those years in that fucking asylum would be for nothing."

Since Gigante checked in for tests, one North Carolina prison hospital has seen a steady stream of convicted mobsters coming in for psychiatric evaluation. Joey "Flowers" Tangorra, boss of the Luchese family, was awaiting trial on loansharking and racketeering charges when he was seen by prison psychiatrists, complaining of panic attacks and severe depression. His lawyers were hoping for a lenient sentence, but since he was facing a life sentence, prosecutors retorted that they would be worried about his mental state if he weren't depressed.

After he pleaded guilty to a murder conspiracy, Bonanno captain Joe Benanti's lawyer tried to obtain a lenient sentence on the grounds that Benanti "will almost definitely" develop "primary degenerative dementia" within five years. Are these mobsters merely malingering, or are their symptoms sign of some deeper trouble?

Jerry Capeci, whose Gang Land column on New York's organised crime scene is required reading for lawyers and wiseguys alike, says: "Gangsters and wiseguys are weak human beings, afflicted with the same ailments as you and me. I don't think anybody is faking mental illness to try and beat their case - certainly not to the same degree as Gigante. It's not a manly thing to do."

One of David Chase's problems as creator of The Sopranos was how to make Tony a credible mob boss who commanded power and respect, while being obsessed with his mother. The problem was solved by making his mother more of a mobster by nature than her son. She is mafia through and through, despising weakness and conspiring to kill her own son because he is seeing a therapist. "I bet he talks about me," she whines.

The Sopranos, and to a lesser degree, the films Analyze This and Analyze That, show that the harsh realities of life in the mafia can put a great strain on the mind - killing your best friend, seeing your father murdered, these things can give a man the jitters. According to analyst Dr Philip Ringstrom, anxiety and depression go with the territory. "Unstable individuals tend to be drawn into the mafia, or else they are part of a family raised in that culture. It's an extremely stressful job. You're not only looking outside the family for enemies, you're also looking inside: whoever you just hugged and kissed could be looking to kill you. It's a de facto paranoid system."

After his arrest on racketeering and extortion charges, lawyers for no-neck mobster Carmine Agnello suggested his criminal behaviour was the result of an impulse disorder which prevented him from understanding the nature of his actions. These actions reportedly included firebombing a rival scrap-metal merchant's workshop and beating an ex-employee with a phone. According to a psychologist's report, Agnello's manic phase manifests itself not only in threatening behaviour, but also in gestures such as throwing money out of the window.

Not everyone is convinced. "They're big on psychologists' reports," says one New York prosecutor. "They've all been watching The Sopranos too much."

One thing is for sure: any mobsters faking mental illness had better let their bosses know it's all an act. "There is no doubt in my mind," says Capeci, "that Gigante is as sane as they come. This is the best proof that he is perfectly sane: when the mob becomes convinced that one of its members is losing his mental acuity, just to prevent him from disclosing secrets - either to a doctor or the law - they whack him. It happened to Willie Moretti back in the 1950s. If Gigante were nuts, 200 wiseguys wouldn't be listening to his every demand and order; they would kill him. As it is, the most influential wiseguys in the metropolitan area still do whatever he tells them to do."

But sometimes it's hard to distinguish the act from the real thing. Consider the fate of Chicago mob enforcer "Mad Sam" DeStefano, whose love of torture bordered on the psychotic. Indicted for threatening a witness, DeStefano claimed he was too ill to attend his trial in 1973. When the judge refused to believe him, Mad Sam duly showed up in the courtroom in pyjamas, on a stretcher borne by his tough guys, and bellowed through a loudhailer. The judge may have thought he was faking it, but the mob decided Mad Sam was truly demented: before his next court appearance, he was killed at close range with a double-barrelled shotgun.