Any psychiatrist will tell you that comorbidity in mental health is the norm. Comorbidity between autism spectrum disorders (ASD) and attention deficit hyperactivity disorders (ADHD) is no exception. The statistics are unequivocal: 30% to 80% of individuals with ASD also meet criteria for ADHD, and 20% to 50% of individuals with ADHD also meet criteria for ASD (Rommelse NN et al, Eur Child Adolesc Psychiatry 2010;19:281–295). And yet, surprisingly, the DSM-IV does not “allow” for this co-occurrence. Even if individuals meet criteria for both conditions they cannot receive both diagnoses, despite what this might offer in terms of treatment and education. The logic of this is that ASD includes ADHD, so the ADHD is part of, or a result of, the ASD and does not need to be considered separately.

However, as Bob Dylan once put it, “The times they are a-changin.” The proposals for the forthcoming DSM-5 criteria are not short of changes: DSM-5 will allow both ASD and ADHD to be diagnosed in the same individual. To many, this is an important change that will mean that the criteria accept the “natural” state of affairs, which is that ASD and ADHD occur together.

So, the statistics reveal considerable ASD-ADHD comorbidity, and the powers that be (that is, www.dsm5.org) are acknowledging it in the next round of diagnostic criteria. But what causes this high overlap? And what can it teach us about these conditions?

Clinically, ASD and ADHD are conditions with many similar characteristics. Both are developmental conditions, meaning that they begin in infancy or childhood and persist across development. Both show a considerable male bias. Both include problems with social functioning and can occur in individuals at all levels of the IQ spectrum.

Intriguingly, we know from twin studies that ASD and ADHD are two of the most highly heritable complex disorders in psychiatry (Ronald A & Hoekstra RA, Am J Med Genet B Neuropsychiatr Genet 2011;156B:255–274; Asherson P & Gurling H, Curr Top Behavior Neurosci 2012;9:239–272). But we also know that neither ASD nor ADHD is 100% heritable; part of the risk for these conditions comes from the environment.

Knowing that two conditions are highly heritable, however, does not tell us why they co-occur. A specific type of analysis, known as multivariate twin model-fitting, is needed to assess the degree to which two conditions or sets of traits have the same genetic influences. Crucially, we now have converging evidence from large twin studies in Sweden, Australia, the US, and the UK that ASD and ADHD show considerable overlap in the genetic influences involved (Ronald op cit).

These studies compared how alike one twin’s level of autistic behaviors were with the other twin’s level of ADHD behaviors. If the same genes cause both ASD and ADHD, it is expected that in identical twin pairs (who share all of their genes), the level of autistic behaviors in one twin and the level of ADHD symptoms in the other twin will be very similar, whereas less similarity between autistic and ADHD behaviors would be expected in fraternal twins because they do not share all their genes, but they do share their environment.

The results—which come from studies of both children and adults—show that at the genetic level, ASD and ADHD aren’t so different. It has become clear from these findings that a significant proportion of the genetic influences for one also influence the other. This overlap contrasts to other common comorbidities of ASD, such as anxiety, that do not seem to share genetic influences to the same degree.

But what about the role of the environment in ASD and ADHD? Both ASD and ADHD have been associated with prenatal maternal stress (Ronald A et al, Frontiers Develop Psychology 2011; http://bit.ly/1151mga), and pre-, peri- and postnatal complications (Kolevzon A et al, Arch Pediatr Adolesc Med 2007;161:326–333; Thapar A et al, J Child Psychol Psychiatry 2013;54(1)3–16), suggesting there might be some shared environmental risk factors. But with any “environmental risk factor,” careful consideration is needed as to whether it operates independently of genetic effects. Rather than dichotomizing nature and nurture, we need to think of them as co-dependent. For example, a genetic problem that contributes to ASD may also contribute to perinatal complications.

However tempting as it may be, do not take all of this evidence too far. There is also significant evidence that ASD and ADHD are, and should remain, conceptualized as distinct conditions. Note that twin studies indicate that while considerable genetic overlap exists between the two diseases, they also show that there are genetic influences that are specific to ASD and ADHD individually. Some environmental risk factors appear also to be specific, such as smoking during pregnancy (repeatedly shown to be a risk factor for ADHD in offspring but not ASD) and higher paternal age (evidence to date suggests this is a risk factor for ASD but not ADHD) (Gabis L et al, Pediatr Neurology 2010;43:300–302). Most notably, the core behavioral signs of these conditions remain distinct.

As neuroscience advances, we begin to see how ASD and ADHD compare at the cognitive and brain level, too (Rommelse NN et al, Neurosci Biobehav Rev 2011;35:1363–1396). It is crucial to consider how specific symptoms within ASD and ADHD may interact and be related to one another. Any clinician who works with individuals with ASD and ADHD will be aware of the huge heterogeneity of symptom manifestation between individuals. It is also likely that different cases of ASD and ADHD will have different underlying causes.

Finally, this research field is guilty of conducting mainly cross-sectional studies, despite the developmental nature of these conditions. Some exciting new longitudinal research suggests that traits characteristic of ASD and ADHD actually influence one another across childhood (Taylor M et al, Psychological Med 2012; Nov 16: online ahead of print).

Changes in the DSM that allow both ASD and ADHD to be diagnosed in the same individual will provide researchers with a better ability to understand how these two disorders exist independently, comorbidly, and how they interact with one another.

Just as Bob Dylan concluded, “As the present now will later be past … For the times they are a-changin’,” so we can hope that with more research, and feedback from the DSM revisions, the relationship between ASD and ADHD will be better understood and lead to improved outcomes and treatment for individuals with these conditions.