The truth about ADHD and autism: how many people have it, what causes it, and why are diagnoses soaring?

It was in the mid 90s that neurodiversity as a formal concept and a rights movement began to emerge. Aided by the internet, autistic people and those with other conditions were able to connect and began sharing their experiences: what they had in common, how their lives differed. A recurring theme was how many felt marginalised, pushed out of a society that embraced only typical ways of being in the world. The phrase “neurological diversity” cropped up in their discussions, which along with “neurodiversity” appeared in magazine articles later that decade.

Neurodiversity has clear parallels with biodiversity. It champions difference and the validity of individuals. It holds that a vaguely defined majority can be described as neurotypical, with brains that operate in a broadly similar way. Others, meanwhile, are neurodivergent, with brains that are built and work somewhat differently.

Neurodivergent people may be diagnosed with a range of conditions, and many co-occur in the same individual. There is no definitive list, but autism (and what was once known as Asperger’s syndrome) and attention deficit hyperactivity disorder (ADHD) are common, as are dyslexia, a learning difficulty that affects reading and writing, and dyspraxia, which manifests as difficulties in movement and coordination.

As a concept and a movement, neurodiversity is reshaping thinking in science, social science and medicine. Neurodivergent people are more involved in the research and in the language used to describe them. Questions are being asked about research priorities, ethics, consent and whether studies will really benefit neurodivergent people.

Interwoven with the concept of neurodiversity is an ongoing shift that embraces a social model of disability. It posits that many of the disabilities neurodivergent people experience are not caused by “something wrong” in the brain, but reflect the friction that arises when a neurodivergent person moves through a neurotypical world. The barriers they face, then, should be reduced, and if possible removed, by making society more accommodating.

“There has absolutely been a change of ethos and tone,” says Professor Francesca Happé of the Institute of Psychiatry, Psychology and Neuroscience, King’s College London. “We no longer talk about curing or treating autism, for example, but we do try and improve the things that go along with it, like anxiety and depression, bad sleep and epilepsy, intellectual disability and language impairment.”

The concept of neurodiversity has been a powerful force at the individual level. Dr Monique Botha, who is autistic and a research fellow in psychology at the University of Stirling, interviewed autistic people for their PhD. One man recounted how some people chose not to vaccinate their children against particular infections for fear of them developing autism. The claim that vaccines drive autism has been vigorously demolished, but the man’s point was that some parents would rather risk their child catching a potentially lethal infection than be autistic.

“It was a real moment of understanding the stigma attached to autism,” says Botha. “One of the most powerful things that neurodiversity has done for neurodivergent people is give us a way of conceptualising ourselves as fundamentally worthy, regardless of how disability is perceived by others.”

About 15% of the global population are said to be neurodivergent. In the UK, at least 1% of people are thought to have an autistic spectrum condition and about 4% have ADHD. Dyslexia, dyspraxia and dyscalculia (difficulty in understanding numbers) sit at about 10%, 6% and 6% respectively. They are regarded as neurodevelopmental conditions: consequences of how the brain forms in the womb or early childhood, and many overlap.

The official numbers weren’t always so high. Rewind the clock 40 years and doctors talked of four, perhaps six, in every 10,000 people being autistic. Today, records suggest 1-3% of the world’s population are autistic.

Diagnoses of ADHD have similarly surged. Those affected tend to have difficulties focusing; they may act impulsively and struggle to sit still. In the UK, diagnoses in secondary school boys have more than doubled in two decades. In adults under 30 there has been a 20-fold rise. More people are being diagnosed for the first time in adulthood.

In the US, the steady rise in autism diagnoses has been described as an “epidemic”, a word best reserved for infectious diseases, and which implies autism itself is rising. For many researchers that doesn’t ring true. The increase, they believe, is down to other factors such as changes in the way autism is defined.

In the 1980s, doctors looked for a “pervasive lack of responsiveness” to others and “gross deficits” in language development. Now they cast a wider net, recording autism on seeing “persistent deficits” in areas such as social communication and interaction, alongside restricted, repetitive behaviours, such as lining up toys, insisting on the same daily schedule and having intense fixations.

Sensory sensitivities feature too, as with ADHD. People can be hypersensitive to bright lights, particular sounds, smells, tastes or textures. Or they might be hyposensitive and seek out certain kinds of stimulation, or struggle to sense when they are hungry, ill or in pain.

But shifts in definition are only one driver. There is a greater awareness of autism today, and doctors do a better job of spotting the condition where it was previously under-diagnosed. This is particularly evident in girls, adults and people of colour. “I think the majority of the rise is due to changes in diagnoses, understanding and awareness,” says Dr Cathy Manning, a psychologist at the University of Birmingham. “It is not necessarily that there are more autistic people than there were before, it’s that we’re now actually diagnosing them and reporting that they have those diagnoses.”

People are missed for all manner of reasons. The stereotype of autism as a male condition (the diagnosis is four times more common in males than females) is one factor. Teachers might notice the boy who constantly walks the playground perimeter while the rest play football, but they overlook the girl who copies everything about a school friend in an effort to fit in. Such masking occurs in both sexes and is damaging in itself.

“Autistic people tell us it’s exhausting and can erode your sense of self,” says Happé, “because you might make friends, but not feel like they are real friends, because they are friends with your mask rather than you.”

Similar forces are at work in ADHD. In the late 90s, diagnosis required onset before seven years old and evidence of “impairment” in several settings. The age has now been pushed up to 12 and children need only show “symptoms” in some settings. Before 2013, ADHD and autism were officially mutually exclusive. If you had one, you could not be diagnosed with the other.

One of the earliest descriptions of autism came in 1943, when Leo Kanner, a psychiatrist at Johns Hopkins University hospital in Maryland, documented “infantile autism”. Kanner published notes on eight boys and three girls who all displayed what he called “autistic disturbances”. One, Donald Triplett, became known as the first person to receive an autism diagnosis. Seen by Tanner at the age of five, Triplett seemed to “live within himself”, learned endless pictures in an encyclopaedia, and delighted in spinning “anything he could seize upon”. Some of his responses were fascinating. Asked to subtract four from 10, Triplett said: “I’ll draw a hexagon.”

Kanner didn’t know the causes. The children he assessed had “come into the world” with an “innate inability” to form the usual social connections, he noted. But he also mused on the significance that all appeared to come from “highly intelligent parents”, very few of whom he considered “really warm-hearted”. For the best part, Kanner added, the closest relatives were “persons strongly preoccupied with abstractions of scientific, literary or artistic nature” and “limited in genuine interest in people”. Three of the marriages he declared “dismal failures”.

While Kanner had hit on autism’s developmental roots, his remarks about parents fuelled damaging assertions that it was caused by “refrigerator mothers”. It led some psychiatrists to recommend “parentectomy”, a word as brutal as it sounds, with children physically removed from their families.

Claims that autism was due to parenting were knocked down in the 1970s. Susan Folstein and Michael Rutter at King’s College London recruited twins where one or both had autism. After interviewing the parents and assessing the children, they wrote one of the most important papers in the field, demonstrating for the first time that autism was largely genetic. Soon after, Lorna Wing and Judith Gould, also at King’s, described an “autism spectrum” after finding a large group of children who didn’t fit Kanner’s narrow definition but displayed repetitive behaviours and had difficulties with social interactions, communication and imagination.

Physicians as far back as the 18th century described various disorders of attention, but today’s understanding of ADHD, which brings inattention and hyperactivity into one diagnosis, was not defined until the 1980s. Doctors look for a persistent pattern of behaviours present before the age of 12 which “interfere with functioning or development”, from having difficulty focusing and acting impulsively to struggling to remain seated when expected.

Studies suggest that some areas of the brain are smaller in ADHD while others are larger; that people have an imbalance in neurotransmitters; and there is greater connectivity between frontal brain regions, which handle attention and control, and deeper structures linked to learning, movement, rewards and emotion.

There’s rarely an identifiable cause of autism or ADHD in a particular individual. Researchers are investigating links between autism and having older parents, a problematic birth and infections in pregnancy. For ADHD, questions are being asked about lead exposure, drinking and smoking in pregnancy, premature birth and brain injury.

Scientists suspect DNA underpins up to 80% of autism and ADHD. While some genetic variants involved are rare and contribute substantially, many more, possibly thousands, will be common and have individually minor effects. These are scattered through the population and come together to different extents in different people, ultimately leading to a diagnosis in some but not others.

For many scientists, understanding the biology of autism, ADHD and other neurodevelopmental conditions means teasing apart the genetics. Autism studies have already highlighted genetic variants that affect neurons and how they communicate. But the research is moving slowly, in part because it can be controversial. Some fear the discovery of gene variants linked to autism will lead to embryo screening and prenatal tests that remove autistic people from society, similar to what happened with Down’s syndrome. Another fear is that doctors may one day seek to prevent, or “cure”, autism through gene editing. As the Autistic Self Advocacy Network stated in 2022: “we do not want “cures” for autism. We want to continue being autistic.”

Morgan James Morgan, a PhD student at the University of Surrey, studies the genetics of autism. He is autistic and has an autistic daughter. “I understand why people are concerned,” he says. “Those concerns are legitimate, but I’m not sure that should stop us doing this whole area of research when it’s clearly so important to understanding what’s going on.”

Morgan says part of his research aims to understand why autism is so varied. “One autistic person can be extremely different from another,” he says. “You could just help people based on what they present with, but it must help to understand why one person is different from another, and it looks like a lot of that is genetically driven.”

It’s not the only area of contention. Some worry that the social model of disability sidelines neurodivergent people who have clear medical needs. “There are groups of parents who feel very left behind because their children are so different from the people who are forerunners of the neurodiversity movement,” says Professor Patricia Howlin, a founding editor of the journal Autism. Up to 30% of autistic people are non-speaking. Some experience epileptic seizures. Others will be fed by a tube directly into their stomach.

“It goes back to the spectrum idea,” says Howlin. “There are autistic people who may be brilliant scientists or artists at one end of the spectrum, and at the other end there are autistic people who need help with almost all their daily activities. There are people who are highly sensitised to anything that suggests you might intervene or treat, whereas others, especially families or carers, say we desperately need intervention and treatment.”

Proper support in education is essential, Happé says. If a teacher doesn’t know a child is autistic, the child might come across as rude when they are just being “autistically direct”. Clear information about changes in activities helps, as do visual schedules, timers that ensure transitions aren’t as jarring, and knowing children’s sensitivities. “Most of the things that work for autistic children are good for all children,” Happé adds.

Improving the environment is key for ADHD too, says Dr Max Davie, a consultant paediatrician in London who has an ADHD diagnosis and is a trustee of ADHD UK. He uses the analogy of a “hungry brain” that is chronically under-stimulated. People need to understand ADHD and know how to respond to it, he says. Building positive relationships is crucial: if a child is crashing around the house, threatening to take away their Xbox could be counter-productive, pushing the relationship on to a confrontational footing. Good brain health is important too, he adds. “Make sure you are sleeping well, that your mood is under control, and get plenty of exercise. It makes your brain healthy and you’ll be better able to cope with your ADHD.”

Autism and ADHD are separate conditions but there is an overlap in the underlying biology, who is diagnosed, the behaviours that manifest, and the care and support that works. Modifying the environment to make it as comfortable as possible is key for both but this is only part of it. “It is also about modifying the attitudes around you,” says Davie. “What helps is people who understand you and can structure and adapt to your needs.”

Where to get help

If you are affected by any of these issues and need support, please refer to:
ADHD UK adhduk.co.uk
ADDitude additudemag.com
National Autism Society autism.org.uk
Autistica autistica.org.uk