If you have ever wondered why one diagnosis rarely shows up alone, you are not imagining things. ADHD, autism, OCD, Tourette syndrome, anxiety, sensory processing differences, specific learning difficulties and depression frequently co-occur in the same person. This is not a coincidence, and it is not a sign that something was diagnosed incorrectly - it reflects real, overlapping biology.
Shared genetic architecture
A landmark analysis by the Cross-Disorder Group of the Psychiatric Genomics Consortium, published in Cell in 2019, examined genetic data from hundreds of thousands of people and found substantial genetic correlation across eight major psychiatric and neurodevelopmental conditions, including ADHD, autism, obsessive-compulsive disorder, Tourette syndrome, major depression, schizophrenia and bipolar disorder. In practice, this means many of the same genetic variants that raise the likelihood of one of these conditions also raise the likelihood of several others. Twin and family studies point in the same direction: relatives of someone with autism have a higher-than-average likelihood of ADHD, OCD or tics, even without autism themselves.
What this looks like in real life
Someone might struggle with auditory processing while also carrying anxiety that spikes in loud environments, or manage ADHD alongside a sensory integration difference that makes certain textures or sounds genuinely unbearable. A child with Tourette syndrome frequently also meets criteria for ADHD or OCD - large clinical studies estimate that the majority of children with Tourette syndrome have at least one co-occurring condition. Developmental coordination disorder (dyspraxia), specific learning difficulties like dyslexia or dyscalculia, and giftedness can layer on top of any of the above, since none of these categories were designed to be mutually exclusive.
Why this changes the conversation
Understanding this overlap shifts the goal away from finding one clean label and toward building a fuller, more accurate picture of how a person's brain actually works. Most neurodivergent people live somewhere in the overlap between several circles rather than inside just one. That has practical value: a clinician aware of the ADHD-autism-OCD-Tourette cluster is less likely to stop investigating after the first diagnosis, and more likely to notice a second or third condition that would otherwise go unaddressed.
Getting a fuller picture of your own overlap
None of this replaces a full clinical evaluation, but free self-screening tools can help you describe what you are noticing before that conversation. Our ADHD screening, Autism AQ-10, OCD screening, anxiety (GAD-7) and sensory profile tests each look at one piece of the picture, and taking more than one can reveal the kind of overlap described above. For more on the shared biology between ADHD and autism specifically, see our article on AuDHD.
Sources: Cross-Disorder Group of the Psychiatric Genomics Consortium, "Genomic Relationships, Novel Loci, and Pleiotropic Mechanisms across Eight Psychiatric Disorders," Cell (2019); clinical literature on Tourette syndrome comorbidity rates. Educational summary - not a diagnosis or medical advice.