"Pronto?" I lift the phone in our bedroom. "My name is Paolo Baldassarre and I wish to speak to the writer Mr Timothy Parks." It's a breathless, urgent, demanding voice that drags me back 10 years and more.

"Paolo!"

"I'll get Rita," I tell him; his younger sister, my wife.

But he wants to speak to me.

"Next week we'll be seeing each other" he says.

"That's right. I'm looking forward to it"

"I want you to know I've changed. I'm very sorry about what happened."

My last glimpse of Paolo was on the platform at Verona station when I pointed him out to the police. He had announced he was coming to Verona, where I live, to kill me. Shabby, frantic, and obese, he waved three or four bunches of keys as they took him away; he had slipped one round each finger to form a sort of gothic knuckleduster. Shortly afterwards we received a visit from Italy's elite anti-terrorist police. Paolo had written to tell them I was working with Mossad to eliminate Palestinians in Italy. Now, a decade on, I assure him there are no hard feelings. "I'd like you to help get the Anglo-American sanctions against me lifted," he says. "In time for my release, the year after next."

He explains that he needs to purchase an academic book called Symbolic Logic. Only the American publisher is demanding £600 for a hardback edition because of the sanctions against him. He must complete his education before leaving the institute. That is imperative. "As you know my father never let me study," he announces. "He poisoned my orange juice."

I mildly suggest that all this is bullshit. "I bet I can get you a hefty discount on that book, Paolo." He changes tack. For five minutes we enjoy a completely normal brother-in-law conversation. "A presto," I sign off.

Paolo was diagnosed as a paranoid schizophrenic some 25 years ago when he and Rita were studying in the United States, she at UMass Amherst, he at the University of Albany. The previous semester Paolo had written home asking if his mother would come to live with him. Then at a certain point he had stopped responding to either letters or phone calls. This had gone on for months. Finally Rita took the bus to Albany. She found her brother barricaded in his room in a supposedly supervised dorm, ankle-deep in filth, delirious and hallucinating. Nobody had noticed anything.

Sent back from Albany to his family, he began a long series of fruitless, short-lived therapies. He seemed abnormally attached to and at the same time extremely antagonistic towards his parents. Incapable of forming relationships with girls he mailed pornography to his sister and myself. From time to time he disappeared and lived as a bum, practised various kinds of self-mutilation, attempted to buy an Algerian prostitute as a bride, "ran rings", as he boasts, round a number of analysts, and read some of the most advanced books on quantum physics ever published. Or at least, those books were purchased for him, often at considerable expense.

He was for many years a considerable financial burden, emotional strain and even potential physical danger to his parents. When violent he could be forced to stay in hospital and take the neuroleptic drugs that calmed him down. But then the law required he be released and, being well aware of the long-term negative side effects of the drugs, he stopped taking them. Very soon the paranoid delusions and agitation would begin again.

In the end it was only after he had committed a serious crime that he could be imprisoned and forced to take medicine on a regular basis for a long time. This happened seven years ago. Paolo held his mother at knifepoint, then smashed up the family home with a sledgehammer. Since then his parents have visited him fortnightly, first in high-security jails, then in the less restrictive mental institutions. But on New Year's Day 1999 my father-in-law died. Now his wife - we both call her Mamma - who never learned to drive, needs someone else to take her to where Paolo is presently based, way up in the rugged highlands between Genoa and Savona, a place apparently unreachable by public transport.

My mother-in-law arrives in Verona on the train from the family home in Pescara some 300 miles to the south on the Adriatic coast. Going to bed at nine she tells us we must be up at the crack of dawn to set off on the drive west to Genoa. She is irritated when Rita and I refuse to start before 7.30am. In the car there's my mother-in-law, my wife, myself and Lucia, our five-year-old daughter. But the presence of little children was never an obstacle to adult conversation in the Baldassarre family. The old lady begins to talk about her husband's bones. Adelmo was buried in the earth, but in Italy the authorities remove the remains after 15 years in order to recycle the space. "Under no circumstances," Mamma says heatedly, "are those bones ever to be moved into my family tomb. Do you understand that, Rita? I will not have that whoremonger's bones in my family tomb."

Theirs was an embattled marriage. My father-in-law frequently said the happiest times with Mamma were in the car, he driving while she slept. Towards the end they lived in separate apartments on the same decaying family property. Struck by a massive heart attack, having no phone in his apartment, Adelmo staggered over to knock on her windows in the middle of the night. She called an ambulance and went back to bed, believing he had overeaten again. In hospital he didn't wish to see her and she made no attempt to visit. Three days later he was dead.

My wife steers the conversation to Mamma's favourite subject: the possibility that now, after seven years' imprisonment, Paolo will be allowed out for a week in order to come down to visit her in Pescara. Mamma becomes extremely cheerful and excited. There is even a catch in her voice, as of someone looking forward to a week's pure evasion, with a lover perhaps, in any event a bliss beyond which life need never resume again.

The institute houses 40 or so men and women who seem emblematic of all that is run-down and futureless, shuffling vacantly from room to room or standing catatonic in corners. After a few minutes Paolo appears. To my immense relief, apart from the hollow, hunted eyes, the hunched shoulders, and the fact that on a hot day he is wearing at least five layers of shirts, jumpers and jackets, he seems pretty normal. We exchange friendly greetings and even a hug. I had not fully understood, despite my wife's warnings, that the difficult person to deal with during this encounter would not be Paolo, but Mamma. "Look at the make-up she's put on for him," Rita had whispered that morning.

Immediately Mamma is kissing her son, almost on the lips. She disengages, but only to throw out her arms and embrace him again, kiss again. Seconds later she's adjusting the collar of one of his shirts. It's not straight. It's not clean. And his hair's too long. He hasn't been eating enough. Has he eaten? She apologises that we didn't arrive at 10, as she promised. Paolo merely nods amiably, informing us that he's already had his lunch, but that he'll be happy to sit beside us while we eat at our hotel.

Some 30 years ago, in the crazy 60s, a group of psychotherapists concluded that many mental disorders, including schizophrenia, might be related to the family relationships surrounding the sufferers. A form of therapy was developed which involved getting the whole family together and trying to change the way people behaved with each other. Some successes were scored, particularly with anorexia, but any number of sensibilities were offended too. Who wants to hear that they are even indirectly responsible for anything? The feminists ran to the defence of the mothers who were being made scapegoats. Parents' associations complained about public money being spent on such mad ideas. And particularly with chronic schizophrenia, the results of this kind of therapy were disappointing, so it's hardly surprising that by the mid-80s the game was up. The medical profession settled back into its traditional vision that schizophrenia is basically the result of a biological disorder.

So today, if you consult the latest literature, you'll be told that the disorder is partly genetic (though how big a part they can't say), or that it may be due to an abnormally large right brain hemisphere (though many schizophrenics don't have this and many who aren't do), or that it's the result of an unidentified virus contracted in the womb but mysteriously clicking in not when they're cutting the umbilical cord, but 20 years later.

Observe us, then, as we sit down to lunch, a perfectly "normal" family except that 25 years ago one member sadly succumbed to this hypothetical organic anomaly, or gene defect, or virus. The hotel restaurant is empty but the proprietress offers to rustle up tagliatelle and ragout followed by a steak. So far, Paolo's behaviour is exemplary. He is still crazy, already bothering Rita with the story of the hugely expensive Symbolic Logic and international sanctions. But he is calm and pleasant. "No, no tagliatelle for me," he tells the waitress cheerfully. "I've already had an excellent lunch, thanks."

"Bring a plate for him as well," his mother says.

"Mamma," Rita protests. "He says he's eaten!,' she tells the waitress. "Bring a plate for my son," the old lady orders.

"Mamma, no, I've had enough."

"You're thin," she accuses.

Proudly, Paolo begins to explain that over the last two months he's managed to lose 10 pounds.

"Bring tagliatelle for him as well!"

"Mamma, for Christ's sake!" I wade in. "The guy's an adult. He knows whether he wants to eat or not!"

"You understand nothing!" she yells at me. "Nothing! He's starving himself."

But exactly as we gang up against Mamma, Paolo turns his head to the waitress and says, "Yes, do bring me a plate." Then immediately he embraces his mother. He begins to caress her wrists and neck and face. One hand has slipped inside the arm of her short-sleeved dress. She is kissing him. "Mammina," he says. "Mammina. You're all I have left now. There's only us two. Just a few years together,' he whimpers. " Povero, povero ," she says with immense satisfaction at our expense. But as he pulls away from her, he demands: "So, have you brought the money? I want my money."

One of the purposes of these trips is to bring Paolo his monthly spending money.

"We'll have to talk about that later" Mamma says.

"But you can't not give me my money," he whines.

"Tell me about the therapy," I ask when we're back in the car. "Just pills," he says. He names an impressive list of drugs, mostly dopamine-blockers, clearly relishing his expertise. "We're all schizophrenics," he says candidly, "all nuts, so we need these things. Sometimes people do smash the place up, then they get a big injection."

I begin to notice that Paolo's conversation is mostly normal so long as you don't touch on two key subjects: the reasons for his illness or "failure in life", and the possibilities for his future. If this is the effect of a virus, it sure is an odd one.

I ask him if they ever try to reduce the amount of drugs, to see if he still needs them. He says yes. "And how is it?" "Fine." "So why don't they reduce them some more?" "They can't," he says, "because the levels were stipulated in the sentence that condemned me to my imprisonment." "I bet that's not true," I tell him. "I try to get better," he says, "so that I'll be ready to come out in two years' time." I say nothing, because we all know that he can only come out when a psychiatrist decides he's well enough.

I can't help but feel he's engaging me in a sort of teasing game, inviting me to discuss serious questions, then suddenly retreating into fantasy when I poke my nose where I shouldn't. In each case these fantasies have to do with things that blocked him in the past or are preventing him from having any future now. In the past there was his father's perfidy, ordering that he couldn't study, couldn't marry and putting poison in his food. Now there is the court ruling about his level of drug treatment and the Anglo-American conspiracy demanding he pay thousands of dollars for books.

He turns in his seat. "Mammina, Mammina. You're the only one who cares for me."

Mamma smiles and begins to talk about his possible visit to Pescara. "That would be wonderful," he agrees. "So nice. I can see my old room. You will give me the money, won't you, Mammina?"

At the institute again, two big surprises are in store. The first is a bus stopping right outside. And where does that bus come from? From Savona. So it would be possible, after all, for Mamma to take the train to Savona and then the bus to the institute! We discuss the situation. Paolo is extremely sensible. "Why on earth come so often, Mamma?" he asks, "especially when it's so hot. Wait till October."

But Mamma has always insisted that she must visit him once a month if possible, once every two months at least, even though the doctors have suggested that these visits have a negative effect on Paolo.

"You can send the money by post," he suggests. She says she'll think about it. But as soon as Paolo goes off to the bathroom for a moment, she hisses, "What on earth am I supposed to do with him if I come without a car? What am I going to say to him all day?"

Briefly I imagine lovers who don't share the same language and who have been forbidden to embrace. What can they do but, as we did, drive around while others do the talking? That is why Rita and I are here, not because Mamma needed transport. We are chaperones to their mad mismatch.

When Paolo returns Mamma is very businesslike and says it's time to sort out the money. Reluctantly, she gives him what he wants.

The second surprise comes when the psychiatrist finally ushers us in. I'd thought Mamma was going to ask whether Paolo could be granted leave to go down to Pescara. Instead she starts complaining about some detail related to her son's disabled person's pension. She rails about the tangles of Italian bureaucracy. The doctor agrees and seems to think the interview is over.

"But what about the visit to Pescara?" I ask. Mamma leans over to whisper, "He'll never grant it. I asked last time." But I feel that if we don't ask, the interview will have been wasted. "We've been wondering," I say, "if Paolo is well enough to spend a few days with the family in Pescara."

"What do you think, Paolo?" he asks.

Paolo is clearly unhappy. "I'd like to go," he hesitates, "so long as no one touches my room here."

"Of course nobody will touch it," the doctor says. "That's fine then. Let's say, in three or four months' time, shall we?"

Suddenly I see that the doctor knows perfectly well this visit will never happen because neither Mamma nor Paolo wants it. So his words are completely empty. Vaguely he grants what they vaguely ask. Thus he integrates himself perfectly in the way they operate together. They can fantasise about it, without needing to clinch anything. As soon as we're out Rita asks: "Paolo, did they ever suggest you have some therapy with the family?"

Actually, I know she knows the answer to this. We discussed it with her father once. Paolo nods. He's very matter-of-fact. He did two sessions with Babbo, he says, but Mamma wouldn't come.

"That's not true," she exclaims.

"Yes it is," Paolo is mild and matter-of-fact, completely convincing, "I remember you said you didn't want to come."

Mamma is furious. "Do you think I wouldn't have come if it could have been useful for you? I can't believe this!"

Immediately he withdraws and we're back to the Mammina routine. Apparently it is impossible for him to comment openly on his mother's contradictory behaviour, her strange mixture of love and recalcitrance. Or could it be that he actually wants things this way, encourages this behaviour? Certainly there's no question here of any one person being solely to blame, it's more the way each person's behaviour complements the other's that seems so unhealthy.

On the drive back, we start talking about the building work that must be done on the family property. But this only leads Mamma to attack Babbo again. "So why did your father marry me?" Mamma suddenly demands, "if all he wanted was his whores and sluts?" Fortunately little Lucia is asleep. As evening draws on, speeding along this turnpike that my parents-in-law drove up and down so many times, it comes to me that in this family to talk about anything always means to talk about everything, because nothing has ever been resolved between them. The whole family is somehow marooned in the ambiguous behaviour of my father-in-law exactly 50 years ago when he married a woman without apparently wanting to, yet at the same time forever promising that one day there would be the dreamt-of resolution, the whole family together and happy by the sea in Pescara.

It's a mentality that fits perfectly with the contemporary and strictly organic approach to schizophrenia. For the time being the patient can be tranquillized on what are truly very sophisticated drugs. If one has to be marooned somewhere, it's as well to keep the anguish levels low. Anyhow, nothing else can be done and it's certainly no one's fault - unless, that is, we're going to be so primitive as to believe that people can really drive each other mad.

In the 25 years of Paolo's illness no one has suggested to Mamma that she might look for different ways of behaving with her son. No one has suggested that her weirdly intense relationship with him might have anything to do with the unhappy prevarications of her husband. But then, why bother? In the future, when medical research finally gets there, the whole disorder will be cleared up with an appropriate medicine and everybody can go on behaving exactly as they please.

• A longer version of this piece appears in the current issue of Granta magazine: Shrinks, available now in bookshops for £8.99, or direct from Granta for £6.99 or free: Guardian readers can subscribe to Granta for just £21.95 (50% off), and get Shrinks free. Phone or fax Granta for details on FreeCall 0500 004 033. Tim Parks's most recent novel was Destiny (Vintage, £6.99). Details of his work can be found on www.timparks.com.