When understanding the differences between autism spectrum disorder and attention- deficit hyperactivity disorder (ADHD) it is vital to acknowledge the history of conceptualizing these two conditions. Until recently, individuals could not receive both an Autism and ADHD diagnosis. The Diagnostic and Statistical Manual of Mental Disorders - 4th Edition (DSM-IV), utilized before 2013, did not allow for this comorbidity clinically. The publication of the DSM-V in 2013 allowed individuals to be diagnosed with both conditions. This change reflected the acknowledgement of overlap between Autism and ADHD based on more recent literature from psychological, neurological, and genetic studies. Current research indicates there’s roughly a 50% comorbidity rate between Autism and ADHD, meaning 50% of individuals with one have the other as well, depending on which study/article is reviewed.
These two neurodiverse diagnoses do indeed share some overlapping symptomology and risks. For instance, those diagnosed with either autism or ADHD are more likely to experience sleep problems, have lower academic performance and employment rates, and exhibit problems with executive functioning. The recognition of many similarities and increase in dual-diagnoses since the 2013 DSM change has lead to two emerging theories:
Some diagnosing clinicians continue to provide either only one diagnosis, meaning in many cases clinicians misdiagnose people with ADHD instead of ASD - or vice versa. This is partly due to confusion, and in some cases disagreement, over the 2013 DSM change and general perspective shift to approaching neurodevelopment. Clinician bias is an unfortunate reality - some will provide an ADHD diagnosis instead of ASD because of the stigmas that continue to surround autism.
Nevertheless, given that the current diagnostic criteria views autism and ADHD as independent conditions it is important to understand what those differences are, and how they manifest. The following is an overview of some variations in commonly discussed overlapping symptoms between autism and ADHD:
Both individuals with autism and ADHD experience difficulties with social interactions and maintaining friendships. Notably, this is part of the diagnostic criteria for Autism and not for ADHD. The only specific criteria mentioned in the DSM that relates to social interactions for ADHD is “talks excessively”.
It’s true, however, that inattentive behaviours associated with ADHD, such as overtalking and/or zoning out, can often lead to missed social cues. For example, a child with ADHD may be seen violating the rules of an organized game because they’re not interested, and find something more entertaining in their reach; a child with Autism is unlikely to violate rules they understand. Therefore, their lack of engagement is likely from not comprehending the rules, or the autistic individual even questioning them.
Another key behaviour often discussed is eye contact. Individuals with ADHD may not keep eye contact because of daydreaming, or visual distractions in their perceptual field. For individuals on the spectrum, it can be uncomfortable, and for some may even be painful. Those on the spectrum often do not feel the need for eye contact to experience human connection.
The relationship between sensory issues and having some sort of neurodiverse diagnosis - be it Autism, ADHD, BPD, OCD, etc. - is well known, and arguably common knowledge within this population. In simple terms, the brain is notably wired differently in neurodiverse populations. This correlates to variations in processing overall, and virtually all human experiences are from the combined processing of the various senses. Many individuals who are neurodiverse and experience sensory differences often experience sensory difficulties in some scenarios, as well. Variations in sensory processing range from hyper to hyposensitive, meaning over or under sensitive to certain stimuli.
Those with ADHD are more prone to being hypersensitive, especially to auditory and visual stimuli - anything that is distracting, in essence. This may outwardly appear as being overwhelmed by the complexity of noise, such as multiple people talking at once, and having a meltdown. Sometimes it’s as simple as seeing something that distracts from the original objective. Many individuals with ADHD, for instance, may find themselves jumping from task to task, completing each one partially, until finished. This is frequently seen in scenarios such as cleaning one’s house.
Autistic individuals are prone to these types of sensory issues as well, but may be even more sensitive, or as mentioned, in some scenarios less. Hyposensitivity is less commonly experienced in ADHD compared to autism, but not undocumented. In addition to the auditory and visual differences experienced, individuals on the spectrum are more prone to tactile, olfactory and proprioception variations as well. However, difficulties with auditory processing may relate more to the intensity or frequency of the sound; being too loud or too high pitched. Others that are sensory-seeking, or hyposensitive, may want to participate in activities such as listening to music on full blast. As for visual stimuli and processing in autistic individuals, an increased sensitivity to the brightness or frequency of light is more common. A notable amount of individuals on the spectrum also have epilepsy, which is frequently triggered by overstimulation of the visual field.
First aspect to acknowledge, before discussing stimming behaviours in neurodiverse populations, is that stimming is a universal human behaviour. One does not have to have any neurodiverse diagnosis to participate in and enjoy stimming behaviours. Stimming is thought to be an observable way the body (and therefore, brain) aims to regulate input of sensory information. Some examples of stimming behaviors include fidgeting, tapping and/or picking at one’s skin, hair or nails, etc. These are behaviours frequently exhibited when someone is anxious. However, these behaviours are observed in ADHD individuals more frequently than the general population.
Autistic individuals are likely to stim more frequently in comparison to ADHD counterparts, and certainly more so than the general population. Stimming is a repetitive action, and repetition is a key diagnostic feature for Autism. The difference with autistic individuals is often the length and severity of the stimming. Moving one’s whole body, as opposed to fidgeting with a pen for a brief moment.
One common stimming behaviour often exhibited in autistic individuals is defined as the concept of echolalia - or the verbal repetition of something. For many, this can be a phrase they heard from a friend, or perhaps a line from a TV show. For others, this is simply a sound. Other unique stimming behaviours - again, focusing on the repetition - have been noted in autistic individuals. For instance, repeating the same song for hours, days or even weeks on end. One autistic female in her early 20’s reports continuous and repeated sniffing of candles and fragrances (5*). Anything that stimulates the senses is stimming. As mentioned, the type of action and severity of repetition often indicate whether behaviour is more likely ADHD, Autism, or simply the human experience.
The concept of executive function (EF) continues to be defined as we learn more about the processes of the brain. In general, however, EF refers to the ability to begin and follow through with a task in all it’s steps, or parts. A seemingly straightforward task such as cleaning the kitchen, for example, is not as straightforward as just “clean the kitchen” - it’s unloading the dishwasher, then loading it, taking out the trash and then wiping down all the counters.
EF is an area where arguably all neurodiverse individuals struggle and therefore, it’s harder to pinpoint the exact differences between neurodiverse populations. Nevertheless, both those with ADHD and Autism have issues with initiating and completing certain tasks. For individuals with ADHD it’s likely the task is simply not engaging, and therefore, they are more vulnerable to distractions. Autistic individuals, in comparison, are more likely to withdraw from a task they are not engaged over overwhelmed, and revert to a familiar activity, than replace it with doing something new like ADHD counterparts.
HYPERFIXATIONS & SPECIAL INTERESTS
The short and simplistic perspective is that individuals with ADHD are more prone to hyperfixations whereas individuals on the spectrum are more likely to have special interests. This is because hyperfixations usually arise from those with ADHD having generally lower levels of dopamine in their system; hyperfixations produce an increase of dopamine for a short time period of over a few days or weeks until the individual loses interest. With ADHD, the loss of interest is often abrupt, instead of gradual. When the interest is gone those with ADHD are more likely to find something new in their reach to produce another surge of dopamine.
Individuals with autism, on the other hand, are known to submerge themselves continuously in activities or topics they enjoy. This type of interest is viewed under the umbrella of repetitive behaviors and thus, special interests. Individuals with autism revert back to the same activity or interest to a notably greater extent than any other population. Some autistic individuals have one special interest, where others have a handful. Interestingly, due to the rise of individuals being diagnosed with both ADHD and autism since 2013, new research is coming forward indicating that individuals with a dual ADHD/ASD diagnosis often rotate through special interests in a hyperfixation pattern.
Considering the 2013 change of the DSM from versions IV to V (4 to 5), it’s likely there are a subset of individuals who have a dual diagnosis of autism and ADHD that only have received or are aware of only one of these diagnoses impacting their lives. Therefore, it’s likely that some diagnosed with ADHD may actually be on the autism spectrum, or vise-versa, especially if diagnosed before 2013. Both ongoing clinician bias, as well as unclear defining of overlapping symptoms, continue to contribute to the confusion between neurodiverse diagnoses, especially ADHD and autism. Recommendations for future research include expanding data, and general knowledge, of clinician bias when looking to diagnose between, or both of these conditions. In order to support or challenge either of the two aforementioned hypotheses, it is essential to conduct further studies by analyzing neural patterns and networks, observable behaviour, and utilize non-invasive neuroimaging methods to better understand the relationship between ADHD and autism.
Lastly, it’s vital to remember that when discussing human behaviour, individual differences are the core. Therefore, especially when it comes to understanding the unclear overlap between neurodiverse individuals, remember that no two are alike just as no two human experiences are the same.
1. Cheyette, B. & Cheyette, S. (2020). Making sense of Sensory Overload in Autism and ADHD. Psycholology Today. Link: https://www.psychologytoday.com/gb/blog/1-2-3-adhd/202004/making-sense-sensory-overload-in-autism-and-adhd
2. Lau-Zhu, A., Fritz, A. & McLoughlin, G. (2019). Overlaps and distinctions between attention deficit/hyperactivity disorder and autism spectrum disorder in young adulthood: Systematic review and guiding framework for EEG-imaging research. Neuroscience and lBiobehavioral Reviews. Link: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6331660/
3. Leitnert, Y. (2014). The Co-Occurence of Autism and Attention Deficit Hyperactivity Disorder in Children - What do we Know? Frontiers in Human Neuroscience. Link: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4010758/
4. McCarthy, C. (2015) Is it ADHD - or Autism? Harvard Health Publishing. Link: https://www.health.harvard.edu/blog/is-it-adhd-or-autism-201510278462
5. Alex Pearson, M.A.
Laina K. Townsend, MSc