Autism is a neurodevelopmental disorder, meaning it originates in differences in how the brain develops and is wired from early life. Officially called autism spectrum disorder (ASD), it is classified in the Diagnostic and Statistical Manual of Mental Disorders (DSM-5) alongside other conditions that affect brain development, such as ADHD and intellectual disability. The word “spectrum” reflects the wide range of ways autism can present, from people who need minimal day-to-day support to those who need substantial help with basic activities.
What “Neurodevelopmental” Actually Means
Calling autism a neurodevelopmental disorder distinguishes it from mental health conditions that emerge later in life, like depression or schizophrenia. Neurodevelopmental disorders are rooted in the way the brain forms during fetal development and early childhood. In autism, one key difference involves synaptic pruning, the process by which the brain trims excess connections between nerve cells. During typical development, a burst of new synapses forms in infancy, and roughly half of those connections in the outer brain are pruned away by late adolescence. Research from Columbia University found that in children with autism, only about 16 percent of those connections were pruned during the same period. The result is a surplus of synapses, which appears to be driven by an overactive growth-signaling pathway that interferes with the brain’s ability to clear out old or damaged cellular material.
These structural differences help explain why autism isn’t something a person develops or catches. It’s present from the earliest stages of brain formation, even if signs don’t become obvious until a child faces social demands that exceed their capacity, sometimes not until school age or later.
The Two Core Feature Areas
A diagnosis of autism requires persistent differences in two broad areas: social communication and restricted or repetitive behaviors. Both must be present, though how they look varies enormously from person to person.
Social Communication Differences
This first area covers three specific types of difficulty, all of which must be present for a diagnosis. They include trouble with back-and-forth social exchange (for example, only initiating conversation when needing help rather than for social connection), differences in nonverbal communication (reduced eye contact, limited use of gestures, or difficulty reading body language), and challenges with building and maintaining relationships (such as difficulty adjusting behavior across different social settings or limited interest in peers).
These aren’t simply shyness or introversion. A child might not respond to their own name, rarely point out things they find interesting, or struggle to take conversational turns. In adults, it can look like missing unspoken social rules or finding it exhausting to navigate group interactions.
Restricted and Repetitive Behaviors
The second area requires at least two of four types of repetitive patterns. These include physical repetitive movements like hand flapping, body rocking, or lining up objects in rows. They also include a strong need for sameness, where even small changes to routines or environments cause significant distress. Intensely focused interests are another hallmark, such as a deep preoccupation with train schedules, specific dates, or nonfunctional parts of objects like buttons. Finally, unusual sensory responses qualify: overreacting to certain sounds or textures, seeming indifferent to pain, or being fascinated by lights and movement. Sensory differences are common enough that they were formally added to the diagnostic criteria when the DSM-5 was published in 2013.
Why It’s Called a “Spectrum”
Before 2013, what we now call autism spectrum disorder was split into several separate diagnoses. A child might receive a label of autistic disorder, Asperger’s syndrome, pervasive developmental disorder, or childhood disintegrative disorder. These conditions shared overlapping symptoms, and the boundaries between them were blurry. Many people didn’t fit neatly into any single category, which sometimes meant they received no diagnosis at all and missed out on early support.
The DSM-5 merged all of these into one diagnosis, autism spectrum disorder, and introduced a three-level support system instead. Level 1 (“requiring support”) describes individuals who can function independently in many areas but have noticeable difficulty initiating social interactions and may struggle to redirect from fixed interests. Level 2 (“requiring substantial support”) involves marked delays in verbal and nonverbal communication, limited ability to form social relationships even with help, and repetitive behaviors obvious to a casual observer. Level 3 describes the highest support needs. This framework acknowledges that autism is a single condition with a wide range of presentations, not a collection of separate disorders.
Genetics Play a Major Role
Autism is one of the most heritable neurodevelopmental conditions. Twin and family studies estimate its heritability at 70 to 90 percent, with some analyses pushing above 90 percent. That doesn’t mean a single “autism gene” exists. Hundreds of genetic variations contribute, including small changes in individual genes and larger deletions or duplications of chromosomal material. About 20 percent of people with autism have chromosomal deletions involving one or more genes. In cases where there’s no family history of autism, roughly 10 percent show detectable genetic differences when tested at the chromosomal level, and these individuals are more likely to carry rare spontaneous mutations not inherited from either parent.
Environmental factors during pregnancy, such as certain infections, medications, or complications, can also contribute, but they typically interact with existing genetic vulnerability rather than causing autism on their own.
How Common Autism Is Today
CDC data from 2022 found that about 1 in 31 eight-year-olds in the United States had an autism diagnosis, a prevalence of 32.2 per 1,000 children. Among four-year-olds, the rate was 29.3 per 1,000. Boys are diagnosed 3.4 times as often as girls, though growing evidence suggests that girls are underdiagnosed because their symptoms often present differently and may be masked by learned social strategies.
Rising prevalence numbers largely reflect broader diagnostic criteria, increased awareness, and better screening, not necessarily a true increase in how many people are autistic.
Conditions That Commonly Co-Occur
Autism rarely exists in isolation. Up to 85 percent of children with autism also have at least one co-occurring psychiatric condition, with ADHD, anxiety, and depression being the most common. Epilepsy affects 25 to 40 percent of people with autism, compared to 2 to 3 percent of the general population. Gastrointestinal problems, including chronic constipation, reflux, and abdominal pain, are reported in as many as 85 percent of individuals with autism depending on the study.
These co-occurring conditions aren’t part of autism itself, but they are so common that clinicians typically screen for them during evaluation. For many autistic people, managing these accompanying issues has as much impact on daily quality of life as addressing the core features of autism.