Like many children, my five-year-old son Euan loves CDs. It's the way he loves them that's unusual. He doesn't just love putting them in the CD player and listening to them: that's old hat, the kind of thing he could do when he was three. It goes further: he loves watching the disc spin in the machine. He particularly loves watching the digital display as it counts up the elapsed time. He carries stacks of CDs around the house and even takes them to bed with him (his movements through the day can be traced through the trail of square plastic cases he leaves in his wake).

Once he's played a CD a few times, he prefers to shuffle through the tracks, listening to just a few seconds before skipping ahead. Finally, for his piece de resistance, he puts the disc in the machine, turns the volume down to zero and sings the songs from memory, keeping strict time with the digital display and including instrumental passages. I can't imagine how Euan would have coped in the days before CDs were invented.

It's a pattern of behaviour that will chime with anyone whose child, like Euan, has been diagnosed with an autism spectrum disorder (ASD).The obsessive, repetitive behaviour, the narrow range of interests, the stubborn refusal to make eye contact, the screaming fits brought on by the slightest change in routine: Euan ticked all those boxes. But what clinched it was the simple fact he wasn't talking.

By the time Euan was three,my wife, Magteld, and I could see he was developing differently from his peers. At birthday parties, while other children charged into the house and played chase with each other, or made for the nearest toy, Euan would find a quiet spot and sit impassively, gazing around the room. When it was time to play pass the parcel, Euan would sit by the CD player watching the music start and stop. For his own party one year, we hung a pinata in the back garden. All the children took eager swipes at the parrot-shaped box to make it yield up its cargo of sweets. All except Euan, who sat in the doorway, watching with a blank expression, not even stirring to pick up the fallen treats.

His nursery, on the south side of Glasgow, began to raise concerns. At first they had nothing but praise: he was never any bother, a contented baby who loved to play on his own. But as time went by it was obvious that his habit of playing alone was symptomatic of something else. One day, one of the nurses was startled to find Euan, then aged two, arranging a deck of cards in numerical order. She swapped two cards in the sequence; he immediately put them back in the right positions. Things were starting to add up.

A few months later, the nursery manager took us to one side and asked if we'd agree to have him assessed by an educational psychologist. "Are you worried about him?" I asked, guardedly. "A wee bit," she replied.

It was the start of two years of agony, heart-break, recriminations and upheaval in our lives,culminating, last July,with his formal diagnosis for autism. We thought we'd reached the end of a long journey; in fact, it was only the start. 'All I understood was that the life I had aspired to - a house, a marriage, a job, children - had turned sour'

One of the hardest aspects of coming to terms with autism is that it is such an elusive concept to understand. It's currently incurable - but, unlike other conditions with that label, such as cancer, there's a furious debate about whether it's appropriate even to be looking for a cure. There is some agreement among scientists that the cause is a combination of genetic and environmental factors, and that the brains of people with autism are somehow "wired differently". In terms of how people with autism think and see the world - why they behave the way they do,with an obsessive,sometimes violent adherenceto routine, and what can be done to help them - the research is still in its infancy.

Simon Baron-Cohen, professor of developmental psychopathology at the University of Cambridge and a leading expert on autism, coined the term "mind blindness" to describe the difficulties that people with autism experience in processing the world. He claims that what has become known as the"triad of impairments" that characterise autism - difficulties with communicating, social interaction and"social imagination"(which includes literal-mindedness and lack of empathy)- stems from a basic inability to read other people's intentions. When I ask him for an off-pat definition of autism, Baron-Cohen adds a fourth element: "It's only if those behaviours or features are interfering with the child's life that it's relevant to think about a diagnosis. Because autism is a medical diagnosis,it shouldn't be given out lightly; it should only be given out if a child or an adult is suffering to some degree."

Baron-Cohen - whose cousin is the comedian Sacha Baron-Cohen,better known as Borat and Ali G - has produced a mountain of research papers, but arguably his most telling contribution is a version of a psychological study known as the Sally-Anne test, which he first carried out with colleagues in the mid-1980s. For this, you need two girls - let's call them Sally and Anne - a marble,and two boxes with lids, one red and one blue. First, ask Sally to put the marble in the red box and close it. Now ask Anne to leave the room. Once Anne is out of earshot, ask Sally to transfer the marble to the blue box. Now turn to your child and ask them:"Which box will Anne look in when she comes back?" By the age of three, children will ordinarily be able to answer "the red box", because Anne was out of the room when Sally made the switch.

Children with Down's syndrome pass this test, too. But Baron-Cohen found that children with autism, even older ones, fail. They cannot think their way into Anne's head.

Autism spectrum disorders are far more prevalent in boys, who outnumber girls with autism by four to one; there is some evidence it might be linked to higher levels of testosterone and is the product of an "exaggerated male brain"."What we know is that the brain is definitely developing in an atypical fashion," says Baron-Cohen."For example, there's some evidence that the brain may be growing too fast in the first two years of life, because a proportion of children with autism have unusually large heads and large brains, although this may normalise by later in childhood. If you use brain-scanning technology you can see differences in the structure and also the function of the brain."

All of which leads on to a question that's dominated my thoughts since Euan was diagnosed: if autism is genetic, how the hell did it get into my son? Two studies in the past five years have discovered that children with autism shared similar distinctive characteristics in their brain structure with their parents and siblings. One found that the amygdala, a region involved in processing emotion, was shrunken not just in children with autism but in their brothers too.

In some ways, it's ironic male genes are copping the blame, because for much of the past century the world of autism care was dominated by a corrosive myth that it was caused by bad mothers.It began when an Austrian psychoanalyst,Bruno Bettelheim, caused a sensation with his book The Empty Fortress, which claimed children with autism had been starved of attention by what he termed "refrigerator mothers". His theories -and his bogus qualifications-had been largely discredited by the time he committed suicide in 1990.

So where might Euan's autism come from? Baron-Cohen has said it is possible to possess mild autistic traits that stop short of meeting the criteria for diagnosis. And when I factor in some of the outlandish stories about the men in my family, or my party trick as a teenager of committing to memory every world record in athletics, including the date, time, venue and performer's name, the list of suspects narrows down to one. And it's not my wife.

Our response to Euan's diagnosis was a common one among families in our situation: grief. As the date for his assessment approached last July, Magteld and I swaddled ourselves in denial. He was "mildly autistic"; he might need a bit of extra help at school; he just needed to be coaxed out of his shell; he would probably grow into a gifted musician.With the right support, he would soon catch up with his peers. All myths, self-comforting delusions, and all about to be crushed by the granite fist of reality. The diagnosis spelled it out: Euan had anautism spectrum disorder, severe communication problems and moderate learning difficulties. He would probably struggle to keep up in a mainstream school. I went home, sat on the stairs and cried.

It might sound like an appalling, selfish and heartless reaction, but it's entirely natural. It works like this: being a parent is an almost primeval competitive sport. Parents chalk up milestones and congratulate themselves when their child appears to be "advanced for their age". So to realise your child is barely able to put two words together while others are fielding complex inquiries about cars or rainbows or horses, or to watch a roomful of squealing three-year-olds enjoying a riotous party game while your child sits and stares at the CD player, is nothing short of heartbreaking.

Tied to grief came guilt: why were we grieving for a child who was still alive, who still needed to be fed and washed and entertained? What right did we have to give up on him? What we slowly came to understand was the child we were grieving for was one who never existed. Grief, in this sense, is a healing process, a way of letting go of the imaginary child you wanted and coming to terms with the real Euan: a boy who might never join in a game of football, and who still hasn't made a telephone call at the age of nearly six, but who is still capable of learning, enjoyment and love.

Of all the myths about autism, perhaps the hardest to break down is the one that portrays people with the condition as automatons, Mr Spock clones devoid of fellow feeling, outcasts in the emotional desert, "empty shells" who need to be cured, restored and made whole.

It's an easy fallacy to fall into,but a fallacy nonetheless, says Baron-Cohen. "There used to be this myth that children or adults with autism didn't have emotions. I think it's now recognised that of course they do; the difficulty for them is understanding other people's emotions. They can feel the same range of emotions, like sadness or hope, but it may not occur to them to communicate it to others."

Evidence is growing, he adds, that people with autism do not perceive less of the world but in fact perceive too much of it. The "intense world"hypothesis,championed by Kamila and Henry Markram of theSwissFederal Institute of Technology in Lausanne, suggests people with autism are hypersensitive and struggle to shut out aspects of the world that aren't relevant to them. It's not that they don't see the wood for the trees: it's that when they stand in a wood they're bombarded from all sides by screeching birds, overpowering smells and a visual chaos of bark, leaves, twigs and sunlight. In such a frightening world,they seek refuge in familiar patterns and routines.

"They may pick up much more detail than the average person, so they may not just see small details but may hear and touch and taste and smell information at a much finer resolution," says Baron-Cohen. "The way we've been studying it is in the context of what we call systemising: children or adults with autism have a very strong drive to understand things in terms of systems or patterns, and when you systemise you really want to get the information the same way over and over again so you can double-check the patterns are the same. That might be watching a washing machine going round and round - that's an example of a mechanical system which a lot of young children with autism like, where the cycle is absolutely the same every time." 'We slowly came to understand we were grieving for a child who never existed'

Six months after Euan was diagnosed, we're starting to reap the benefits. Diagnosis opens many floodgates: speech and language therapists, educational support, access to parenting groups and special-needs services, as well as the all-important disability living allowance.

Euan started school in August. He has the help of a diligent classroom assistant, has learned to read (his fascination with numbers turned out to be not a prodigious maths talent but a fascination with ordering symbols, and he has taken to letters with the same enthusiasm) and threw himself into the Nativity play, where he could indulge his love of singing songs from memory. His noise sensitivity can make it hard for him to sit still in a noisy classroom, but watching the look on his face when he learns to spell a new word is a delight. At the same time,it has become clear mainstream education will eventually become too challenging for him, and next year he will be enrolled in a special-needs school.That's something I couldn't imagine agreeing to even six months ago, but autism is a constant process of reality checks, and this was far from being the hardest.

Our real anxieties are in the future. When Euan was born, we never considered he might not be able to live away from us, or could face moving into an institution when we were too old to care for him. Now we have had to accept these are possibilities, maybe even probabilities. The prospect of him ever being able to manage a job or a bank account seems impossibly remote. His brother, Adam, who is two years younger, has helped him learn to play and share, but Adam needs to grow into an adult in his own right.

Autism might not be curable, but therapies are improving. Professor Aline-Wendy Dunlop, chair of childhood and primary studies at Strathclyde University, says the key is early intervention. "When I think of early intervention, of course I first of all think of young children, but I also think of anyone who eventually has a diagnosis or recognises themselves," she says. "At that point it's absolutely essential there's a follow-through with some kind of support or service or advice."

Dunlop says support services have changed hugely in the 10 years since she launched her postgraduate programme for professionals who work in the autism field."There's far greater awareness. There's still too much of each family, at the point of diagnosis, almost having to reinvent the wheel, but in some areas the practice is very, very good. In other areas it falls down every time, so I suppose it's a bit of a postcode lottery. However, where it works well, professionals are working closely with families, and where professionals and families can have a mutual view of the particular person with autism, you get a good service."

Recently there has been a heated debate about the prospect of prenatal screenings for autism, triggered by Baron-Cohen's warnings of the dangers of doing away with potentially talented people before they were born. When I suggest this risks simplifying the nature of autism, he replies: "The issue isn't about how talented is the person. It's a moral issue about eugenics and the foetus's right tolife,and society recognising that everyone has a value, whether they're talented or not."

People shy away from asking the obvious question about prenatal screening when they know you have a child with autism, but it's the first one we asked ourselves. The answer has to be conditional, but if we'd had Euan screened, if we'd known the full facts about autism, if we'd seen how absorbing it can be to watch his superhuman efforts to make sense of a world whose chaos and disorder is magnified through all his senses, if we'd anticipated the delight he takes in going through the alphabet from A to Z, as he's doing in the picture on pages 12 and 13 - well, if we'd known all that,we wouldn't have had any other thought but to let him live. It's ignorance, not autism, that's the real handicap. EXTRACTS FROM 'AUTISTIC DAD', A BLOG BY GORDON DARROCH May 11, 2008 When the nursery first raised the subject, autism was one of the first things that cropped up in our minds. We considered it, and dismissed it. We were aided in our denial by well-meaning friends and relatives who reassured us Euan couldn't be autistic because he was obviously intelligent, or remarked that he was just a happy, self-sufficient little boy. Neither of these, as I now know, precludes autism.

As perverse as it sounds, it was easier to deny the truth and look for some deficiency in our child-raising techniques than to accept the reality of having to care for an autistic child. Magteld and I blamed ourselves and blamed each other; we tried to shake Euan out of his routines and his closed world, sometimes literally. We waited with growing impatience for his speech to catch up with other children of his age. We felt ashamed every time an adult spoke to Euan and he looked up at them blankly, or when we watched him at birthday parties, standing mutely at the side of the room. His interests became obsessions, sometimes built around meticulous routines, and any attempt to divert from them triggered a storm of protest. The plain truth is, we barely knew our own son.

August 11, 2008 The emergence of Euan's autism was bound up with a period of turmoil in my own life and well-being. It was only once we started to clear the debris that the connection became clear. At the time all I understood was the life I had aspired to - the middle-class dream of a house, a marriage, a steady job and happy, healthy children - had turned sour. Instead we had a child we hardly understood, who frequently left us exasperated as trips to the shops or the park became a series of small battles for his attention.

Things came apart at the seams. When I wasn't feeling a failure as a father, I was querying Magteld's methods. Weeks went by when we hardly spoke. We stopped just short of splitting up; a generation ago, when autism was less readily acknowledged or understood, we certainly would have done.

In rebuilding after an earthquake, many cities reinvent themselves in spectacular ways. Devastation is a catalyst for revival. I feel the same principle can work for families such as ours. Since Euan was diagnosed, we have begun to adjust to the new demands in our lives and been able to look forward without being overcome by fear. The aftershocks will be felt for some time yet, but the foundations are still in place. The challenge is to go back to the drawing board and build anew.

September 23, 2008 Last week I was running around a local park in light rain when a woman came into the park with her son, who looked about 10, and her dog. The mother and dog went off for a walk, the son sat down on a bench with a robot toy and played with it - oblivious to the rain, to the fact his mother had gone for a walk, or to me hurtling round and round like a man being chased by a giant wasp. Presently I saw him walking through the park, unperturbed, staring down at his feet, presumably off to meet up with his mother, looking for all the world as if this was the standard pattern for their days out. And I practically had to restrain myself from approaching the woman and saying: "So, is your child autistic too?"

I could easily add to this list of people I've observed who look somehow out of place: square pegs in the honeycomb of life. The fact is, all of a sudden I've started to see potentially autistic people everywhere I look: on television, in the place where I work, walking the streets. It's like viewing the world through infra-red goggles, or rotating a glass cube containing a disjointed bundle of sticks through 180 degrees to reveal a box-frame. There are hazards here: autism is a difficult condition that takes a team of specialists several weeks to diagnose. There are all sorts of reasons why an unruly teenager might act surly or a young boy might be more interested in his toys than his mother. A friend who also has an autistic son said to me: "You can spot them." And while the autistic radar may not always be spot on, there's no escaping the fact autism has changed the way we look at the world for ever.

December 27, 2008 Christmas can be a good measure of progress in children. This year Euan didn't want to open his presents. That was a step forward from last year, when he didn't want to be in the same room as them. The reason, with hindsight, was simple and a classic piece of autistic logic: we'd talked to Euan about Christmas lunch, cooked the turkey and set the table - then gone into the living room to open presents. And in his mind he must have thought: they've promised me a slap-up feed, so why are they wasting time in the wrong room tearing paper off strange boxes?

This year Euan took no interest in the boxes until someone else had taken off the wrapping. It was striking that his favourite presents were already familiar to him: a game that one of his friends has, and a set of reading books he knows from school.

Euan's progress is erratic, and alinear, but it is progress nonetheless. Every step is a minor triumph. What's heartening is his determination to learn things that don't come naturally to him.

Read Gordon's blog in full at autisticdad.blogspot.com . AUTISM: THE FACTS AND FIGURES

About one in 100 people are thought to have autism. The condition affects 50,000 people in Scotland and a further 150,000 members of their direct families.

A total of 4,551 children in state schools in Scotland receive extra educational support in connection with an autism spectrum disorder (ASD).

Autism is a lifelong disorder , though many with the condition learn to socially interact to some extent, either on their own initiative or through therapy.

ASDs include Asperger's syndrome . People with Asperger's syndrome have fewer problems with speech but may still have difficulties with understanding and processing language, be socially clumsy and behave in an obsessive way.

The average age for diagnosis is five-and-a-half for autism and 11 for Asperger's syndrome.

The latter is often not diagnosed until well into adulthood.

There is no cure for autism . Treatments such as speech and language therapy, dietary interventions, applied behavioural analysis and music therapy have been found to help.

It has been widely speculated that prominent artists and scientists such as Isaac Newton, Albert Einstein, Mozart and Michelangelo had some degree of autism. However, the term has only been used in medicine since it was identifed by Austrian psychiatrist Leo Kanner in 1943.

Carol Evans , National Autistic Society director for Scotland, says: "Autism is a spectrum condition, which means, while all people with autism share certain difficulties, their condition will affect them in different ways. Some people with autism are able to live relatively independent lives, but others may have accompanying learning disabilities and need a lifetime of specialist support."

Source: National Autistic Society. Visit www.nas.org.uk or www.think-differently.org .