Most autistic people have at least one other diagnosis. Research shows that roughly 74% of individuals with autism have at least one co-occurring condition, and a large Swedish population study found that over half have four or more. These aren’t random coincidences. Many of these conditions share genetic roots, neurological pathways, or biological mechanisms with autism, which is why they cluster together so consistently.
ADHD Is the Most Common Co-Occurring Diagnosis
Between 50% and 70% of autistic people also meet criteria for ADHD, making it the single most frequent companion diagnosis. The two conditions share overlapping features in how the brain handles attention, performance monitoring, face processing, and sensory input, but they also have distinct neurological signatures. Brain wave studies show different patterns of electrical activity for each condition, suggesting they are genuinely separate processes that happen to co-exist rather than being different expressions of the same thing.
This overlap creates real challenges for treatment. Stimulant medications that work well for ADHD alone can sometimes produce paradoxical or unwanted effects in autistic people. If you or your child has both diagnoses, treatment often requires more careful adjustment and monitoring than ADHD alone would.
Anxiety and Depression
Anxiety affects roughly 19% of autistic females and 10% of autistic males in clinical samples. Depression is even more common: up to 40% of autistic adults receive a depression diagnosis at some point in their lives. For comparison, adults with a history of depression report autistic traits at a rate of 31%, while adults without depression report them at just 2.6%. The two conditions are measurably correlated, and people with more autistic traits consistently show more depressive symptoms regardless of whether they have a formal autism diagnosis.
These aren’t simply the result of social difficulties, though navigating a world not designed for autistic people certainly contributes. There appear to be shared biological underpinnings, including differences in how the brain processes reward, manages stress hormones, and regulates emotional responses.
Intellectual Disability and Borderline Cognitive Ability
According to 2022 surveillance data from the CDC, about 40% of 8-year-olds with autism have an intellectual disability (IQ of 70 or below). Another 24% fall in the borderline range (IQ 71 to 85), and 36% score in the average or higher range. This means autism spans a wide spectrum of cognitive ability, but intellectual disability remains one of its most common co-occurring conditions, particularly in children who are diagnosed early or who have limited spoken language.
Epilepsy and Seizures
About 22% of autistic individuals develop epilepsy. What often surprises families is the timing: in the majority of cases, seizures begin after age 10, with the average onset around age 13. Some people don’t experience their first seizure until adulthood. This late onset means epilepsy can emerge years or even decades after an autism diagnosis, so it’s worth being aware of the signs (staring spells, unusual repetitive movements, unexplained confusion) even in autistic adults who have never had seizures before.
Sleep Problems
Disrupted sleep is extremely common in autism and goes deeper than just having trouble falling asleep. Research points to fundamental differences in the body’s circadian timing system. Autistic individuals often produce less melatonin, the hormone that signals the brain it’s time to sleep, and show abnormal patterns in cortisol, the hormone that helps regulate the wake cycle. These biological differences lead to difficulty falling asleep, frequent night waking, and irregular sleep-wake schedules. Poor sleep then amplifies daytime challenges with attention, mood, and sensory sensitivity, creating a cycle that can be hard to break without targeted support.
Gastrointestinal Symptoms
Gut problems are a daily reality for many autistic people. In a clinical sample of 308 autistic individuals, the most common issues were constipation (32%), abdominal pain (21%), food refusal (19%), diarrhea (14%), nausea (14%), and bloating (12%). These aren’t minor inconveniences. Chronic GI discomfort can drive increases in irritability, self-injurious behavior, and sleep disruption, especially in individuals who have difficulty communicating their pain. When an autistic person shows an unexplained change in behavior, unrecognized GI distress is one of the first things to consider.
Eating Disorders
The connection between autism and eating disorders has gained significant attention in recent years, particularly in autistic women and girls. In one large clinical dataset, 7% of autistic females had an eating disorder diagnosis compared to just 1% of autistic males. Restrictive eating in autism can stem from sensory sensitivities to texture, temperature, or smell, but it can also overlap with anorexia nervosa, where rigid thinking patterns and a need for control intersect with body image distortion. Eating disorders are one of the conditions most likely to be diagnosed before autism in women, sometimes by many years.
Gender Differences in Co-Occurring Conditions
Autistic females carry a disproportionately higher burden of certain co-occurring diagnoses compared to autistic males. Affective disorders like depression and bipolar disorder affect 19% of autistic females versus 8% of males. Anxiety follows a similar pattern: 19% of females compared to 10% of males. Among those who receive a late autism diagnosis, 26% of females had a childhood mood disorder compared to 13% of males.
This pattern likely reflects both biology and diagnostic bias. Autistic girls are more likely to mask their core autism traits, leading to later diagnosis. In the meantime, the emotional toll of camouflaging may drive higher rates of anxiety, depression, and eating disorders. Many autistic women collect several mental health diagnoses before anyone identifies the underlying autism.
Genetic Syndromes Linked to Autism
Several specific genetic conditions carry a high probability of autism. Fragile X syndrome is the most well-known, but the list also includes tuberous sclerosis, Rett syndrome, Phelan-McDermid syndrome, Timothy syndrome, Prader-Willi syndrome, and Angelman syndrome. Each of these involves mutations or deletions in genes that play roles in brain development and the way nerve cells communicate. Rett syndrome, for example, is an X-linked condition primarily affecting girls that causes developmental regression, seizures, and autistic features. When autism occurs alongside one of these syndromes, understanding the specific genetic cause can sometimes guide more targeted medical monitoring and support.
Immune System and Autoimmune Connections
Autism runs in families with higher rates of autoimmune disease. One study found that 46% of families with an autistic member had two or more relatives with autoimmune conditions. A meta-analysis also identified that maternal autoimmune disease during pregnancy, particularly autoimmune thyroid conditions, modestly increases the odds of autism in offspring. Maternal infections and fever during pregnancy have been implicated as well, likely through the effects of immune signaling molecules that cross the placenta and influence fetal brain development. These findings don’t mean autoimmune disease causes autism directly, but they point to shared immune pathways that may contribute to both.