High functioning autism, now clinically called Level 1 autism spectrum disorder, results from a combination of genetic differences, brain development patterns, and environmental influences during pregnancy. There is no single cause. Instead, multiple factors converge to shape how the brain forms and organizes its connections, producing the social communication differences and repetitive behaviors that define the condition. About 1 in 31 children in the U.S. are now identified with autism spectrum disorder, making it one of the most common neurodevelopmental conditions.
What “High Functioning Autism” Means Today
The term “high functioning autism” is still widely used in everyday conversation, but it no longer appears in clinical guidelines. The DSM-5, which psychiatrists and psychologists use to diagnose mental health conditions, folded older categories like Asperger’s disorder into a single diagnosis: autism spectrum disorder. Severity is now described in three levels. Level 1, “requires support,” is the closest equivalent to what people mean by high functioning autism. It describes someone with noticeable social communication challenges and restricted or repetitive behaviors who can generally manage daily life with some support but without the intensive assistance that Levels 2 and 3 require.
The causes of Level 1 autism are the same as the causes of autism more broadly. What determines where someone falls on the spectrum is still not fully understood, but it likely involves the specific combination, timing, and severity of the genetic and environmental factors involved.
Genetics Play the Largest Role
Autism is one of the most heritable neurodevelopmental conditions. Twin studies consistently show that if one identical twin has autism, the other has a significantly elevated chance of having it too. But the genetics are complex. There is no single “autism gene.” Instead, hundreds of genes contribute small amounts of risk, and different combinations produce different outcomes in different people.
One major area of research focuses on spontaneous genetic changes, called de novo mutations, that appear in a child’s DNA but aren’t inherited from either parent. A large-scale genomic study published in Cell analyzed over 59 million genetic variants across families with a single autistic child and found that affected children carried more gene-disrupting mutations than their unaffected siblings. These mutations disproportionately affected genes active in the developing fetal brain, particularly in a region called the striatum, which is involved in learning, motivation, and habit formation. Children with multiple spontaneous mutations were especially likely to have disrupted genes tied to brain development.
Beyond spontaneous mutations, inherited genetic variants from parents also contribute. Many of these are common in the general population and individually have tiny effects, but in certain combinations they can shift brain development enough to cross a diagnostic threshold. This helps explain why autism often runs in families, with parents or siblings showing subclinical traits like a preference for routine or difficulty reading social cues, without meeting criteria for a diagnosis themselves.
How the Brain Develops Differently
The genetic differences associated with autism don’t operate in the abstract. They change how the brain physically builds itself, particularly during fetal development and early childhood. One of the clearest findings involves synaptic pruning, the process by which the brain eliminates unnecessary connections between neurons to create more efficient circuits. In typical development, specialized immune cells in the brain called microglia “eat” excess synapses during critical windows of early life. In autism, this process appears to go wrong.
Animal studies have shown that when microglia are reduced or impaired during early postnatal development, synaptic pruning fails and autism-like social behaviors emerge. Brain tissue from autistic individuals shows increased density of dendritic spines, the tiny structures where neurons connect to each other, consistent with too many synapses surviving into later development. The result is a brain with more connections but less organized ones, which may explain the sensory sensitivity, difficulty filtering social information, and intense focus on specific interests that characterize Level 1 autism.
Microglia also maintain the balance between excitatory and inhibitory signals in the brain. When that balance tips, particularly in the prefrontal cortex (the region responsible for social decision-making and flexible thinking), it can produce the cognitive and social profile associated with autism. This excitatory/inhibitory imbalance is one of the most consistent neurobiological findings in autism research.
Structural brain differences have also been documented. Autistic children, particularly those with co-occurring anxiety, tend to have a slightly larger right amygdala, the brain region central to processing threats and emotional reactions. One longitudinal study in Biological Psychiatry found that autistic children with standard anxiety disorders had right amygdala volumes roughly 5 to 6 percent larger than neurotypical peers at two different time points during childhood.
Prenatal Environmental Factors
While genetics set the stage, environmental exposures during pregnancy can influence whether and how autism develops. These factors don’t cause autism on their own, but they can increase risk in genetically susceptible individuals.
Maternal immune activation is among the best-studied environmental contributors. When a pregnant person’s immune system mounts a strong inflammatory response, whether from infections like rubella or cytomegalovirus, prolonged fever, or chronic inflammatory conditions, the resulting flood of immune signaling molecules can cross the placenta and alter fetal brain development. Gestational and pregestational diabetes also fall into this category, as they create a pro-inflammatory state that affects the developing nervous system.
Certain chemical exposures during pregnancy are linked to increased autism risk as well. The anticonvulsant valproic acid and the drug thalidomide have the strongest evidence. Heavy exposure to air pollution (specifically fine particulate matter), pesticides, heavy metals, and significant alcohol use during pregnancy have also been associated with higher rates of autism in offspring. For newer chemicals of concern like phthalates and bisphenol A, the evidence remains inconsistent.
Parental Age
Older parental age at conception is a well-established risk factor. A study using CDC data found that various combinations of parental ages above 35 increased autism risk by 21 to 65 percent compared to younger parents. The relationship works in both directions: for mothers under 35, the father’s increasing age raised risk, and for fathers under 35, the mother’s increasing age raised risk. The effect is strongest when both parents are older.
The likely explanation is that older parents accumulate more spontaneous mutations in their eggs and sperm over time, particularly fathers, whose sperm cells divide continuously throughout life. Each division introduces a small chance of a new copying error in DNA, and those errors can land in genes important for brain development.
Epigenetics: Where Genes and Environment Meet
Epigenetics offers a framework for understanding how environmental factors translate into lasting changes in brain development without altering the DNA sequence itself. Epigenetic modifications are chemical tags that attach to DNA or to the proteins that package it, turning genes up or down like a dimmer switch. In autism, these modifications appear to follow a U-shaped pattern: both too much and too little activity in these chemical tagging systems can disrupt neurodevelopmental pathways.
For example, valproic acid exposure during pregnancy causes excessive activation of certain genes by adding chemical tags to their packaging proteins. Lead and bisphenol A exposure alter a different tagging system that controls which genes are silenced. Even gut bacteria may play a role. Metabolites produced by intestinal microbes interact with the same epigenetic machinery, and differences in these metabolites have been linked to autism.
What makes epigenetics particularly relevant to Level 1 autism is that these modifications can produce a wide range of severity depending on their timing, location, and extent. A mild epigenetic shift affecting social cognition genes during a specific developmental window could produce the subtle but real social difficulties seen in Level 1 autism, while more extensive changes might contribute to higher support needs.
Why No Two Cases Are Alike
The reason autism presents so differently from person to person is that there are many paths to the same destination. One person’s autism may be driven primarily by inherited genetic variants from both parents. Another’s may stem from a handful of spontaneous mutations that disrupted brain connectivity in a specific way. A third person’s profile may reflect a genetic predisposition amplified by prenatal infection or chemical exposure. The combinations are nearly endless, which is why autism is described as a spectrum rather than a single condition.
For Level 1 autism specifically, the contributing factors are often more subtle and distributed. Rather than a single dramatic genetic event, the cause is frequently a collection of small genetic nudges, possibly combined with mild environmental influences, that shifted brain development just enough to produce noticeable differences in social communication and behavioral flexibility. This is also why many people with Level 1 autism aren’t identified until adolescence or adulthood, when social demands exceed their capacity to compensate.