With autism diagnoses rising more and more parents are plunged into a battle to understand the condition and find their child the right treatment, writes Louis Theroux.

Joey Morales-Ward is a 13-year-old kid who lives in suburban New Jersey. He likes playing on his computer, making books that he illustrates himself, and drawing in coloured chalk on his parents' front drive.

Joey also has violent tantrums on a daily basis, which often involve him hitting himself, punching holes in the walls all through the house, and assaulting his mother, leaving her bruised and shaken.

Joey has been diagnosed with autism.

People with autism vary widely in terms of their symptoms. Some are above average intellectually, though many are below average and struggle in mainstream schools.

Commonly, people on the autistic spectrum have trouble with social interaction - using speech, recognising emotions (their own and other people's), body language. They also often have repetitive behaviours and routines and can appear locked in their own worlds.

Find out more Louis Theroux - Extreme Love: Autism is on BBC Two at 21:00 BST on Thursday 19 April Or catch up later via iPlayer

For reasons that aren't fully understood, diagnosis rates for autism have gone steadily upward in America in recent years. New Jersey is at the forefront of the trend. Latest figures put the autism rates among boys in New Jersey at one in 29 (rates for girls tend to be much lower).

Despite its increasing levels of diagnosis, autism is still poorly understood. Indeed, it is not clear if the real rates of autism are climbing. Some say there are more cases due to improved detection, or, some believe, an overly expanded set of criteria.

In the popular mind, the condition is forever linked to the Dustin Hoffman character in Rain Man, an autistic savant whose idiosyncratic behaviour - obsessive routines, strange vocal mannerisms - was offset by a host of "savant" abilities. He could memorise a phone book and beat the casinos in Las Vegas.

In fact, savant abilities are rare among those with autism.

For my part, my interest in the condition stemmed from an interest in the unique nature of the relationship between parents and their diagnosed kids.

Image caption Parents of autistic children can find it a struggle to cope

Raising a child on the autistic spectrum presents a very demanding, though often rewarding, set of challenges.

As a father of two young boys, who are in psychiatric parlance "neuro-typical", I know first-hand how hard it can be when your four-year-old refuses to eat his vegetables or goes through weird phases of waking every few hours; the tantrums over certain clothes and the squabbles over who was playing with what first.

But raising a child with autism puts my stresses in the shade.

Carol has a cot next to Joey's bed where she sleeps most nights, to stop him getting out of bed and wandering around.

Children with autism sometimes sleep erratically into their teens. In terms of sleepless nights, many parents of diagnosed children remain in a kind of "newborn" mode for 10 or 15 years.

Language can develop incredibly slowly, or barely at all. Even sometimes, when the communication skills are there, an autistic child may seem to have no interest in communicating.

There can also be tantrums and outbursts.

It's not always clear what is causing a tantrum. It might be that a kid's playtime has been refused or brought to an end, but it might be something more obscure - a thought or a memory.

Nor is it always clear how best to handle a tantrum once it's started. When I first met them, Carol would lie on top of Joey to stop him from smashing up the place, sometimes in tandem with her husband Tadeo who would pin down his legs.

Autism in popular culture Perhaps the most famous movie depiction of autism is in Rain Man

But Kim Peek - on whom the film is based - was not autistic

- on whom the film is based - was not autistic The tendency of movies and TV to focus on savants - who represent a small proportion of people with autism - has been noted

UK Chancellor George Osborne criticised for calling Gordon Brown "faintly autistic"

There have been concerns over "autism" being used as a term of abuse

A few weeks later, she implemented a new regime of giving him boxing gloves to soften his self-inflicted blows and keeping him in his room until the tantrum had blown over.

As a TV presenter, the subject of autism also put me in a tricky position. I had to figure out how to get to know children, some of whom could only speak a handful of words, and whose way of interacting socially was very different to the ones I was used to.

But this, in a way, was the point - that I should get a little glimpse of the strains, and the pleasures, of having a relationship with someone diagnosed with autism.

On the positive side, kids on the spectrum can make massive strides in their progress, in rare cases losing the diagnosis entirely.

With its high rates of autism, New Jersey is home to some of America's best services, including a remarkable school, the Developmental Learning Center in Warren, NJ. The DLC Warren lavishes resources on the 250 or so kids who go there, almost all of them diagnosed with autism. The teacher/student ratio is about 1/1.5.

One of the children I met, Nicky Ingrassia, had been non-verbal until the age of six, and yet was now highly articulate, not to mention curious and humorous. Nicky had progressed to the point that he was being moved to a more mainstream school.

But Nicky's level of progress is not the rule.

Media playback is unsupported on your device Media caption Nicky Ingrassia turns the tables on Louis Theroux

Just as typical was the story of the Englehard family.

Josephine Englehard's son Brian was eight when he burned down the family house. As he grew older, he began assaulting Josephine, often when she refused him certain items of food. Sometimes he chased her around the house and pulled her hair out in clumps.

After one particularly violent incident Josephine called the police. Brian was sent to a psychiatric hospital. From there, he moved to a group home where he still lives, aged 20.

Brian spends Saturday and Sunday back in the family home. One Saturday I went with Josephine as she picked him up. Having heard so much about Brian's tantrums, I was a little nervous about meeting him.

But over the course of the afternoon, using body language and a little bit of speech, Brian and I seemed to strike up a bit of a rapport. I found Brian outgoing, mischievous, and - especially after everything I'd heard about autism - surprisingly interested in me.

Josephine told me that, although it had been a huge wrench moving Brian out of the house, he was now much calmer and seemingly much happier - a change she partly put down to the effect of the correct use of psychiatric medication.

The autism spectrum Autism and Asperger syndrome are part of a range of related developmental disorders

The disorders are described as on a spectrum

Symptoms include problems with social interaction including lack of understanding and awareness of other people's emotions and feelings

Impaired language and communication skills

Unusual patterns of thought and physical behaviour including making repetitive physical movements

Source: NHS Choices Autism and Asperger, BBC

Sure enough, by the end of the visit, in the early evening, it was Brian who volunteered that he wanted to go back. In the car on the drive to the home, we listened to some merengue music on a Latin radio station, and the two of us grooved together sedately in the backseat.

Joey's future remains uncertain.

Carol says she is praying for a miracle for him, that he will somehow emerge from his autism.

At the moment he is not on medication. Should his behaviour become even more disruptive as he gets bigger, Carol has resolved to try drugs as a first resort. If this doesn't help, a move to a group home like Brian's is not out of the question.

In the end, I came away from my trip in New Jersey impressed, more than anything else, by the patience and love shown by the parents of the autistic children.

The demands made of parents whose children are diagnosed with autism can be immense.

Though Carol was praying for a miracle, in the course of spending time with her I felt she was performing a small miracle of her own simply by keeping going.

An earlier version of this story incorrectly gave the ratio of teachers to students at DLC Warren as 1.5:1. The correct ratio is 1:1.5.