International classification systems like the ICD 11 and DSM 5 identify a group of conditions described as neurodevelopmental disorders (American Psychiatric Association 2013; World Health Organisation 2022). They share common characteristics that include multifactorial aetiology, early onset of neurocognitive deficits, high heritability indices, a marked overlap between their core symptom domains and a chronic course with impairment generally lasting into adulthood(Stein et al. 2020; Thapar et al. 2017; Thapar & Rutter 2015).

In the ICD-11, the category ‘neurodevelopmental disorders’ includes 8 conditions namely disorders of intellectual development (also referred to as intellectual disability), autism spectrum disorders, attention deficit hyperactivity disorder, developmental speech or language disorders, developmental learning disorders, developmental motor coordination disorder, stereotyped movement disorder, and other neurodevelopmental disorders. The DSM 5 limits itself to 6 conditions that are very similar to those in the ICD-11 namely intellectual disabilities, autism spectrum disorder, attention-deficit hyperactivity disorder, communication disorders, specific learning disorders and motor disorders.

Disorders of intellectual development or Intellectual disability is characterised by deficits in intellectual functioning, deficits in adaptive functioning and onset in the developmental stage, i.e., before the age of 18 years (Jesu et al. 2022). While there are some variations due to a range of socio-economic, culture and methodological factors, its prevalence is estimated to be around 2% (Dunn et al. 2019). Autism spectrum disorders are characterised by deficits in reciprocal social interaction and communication and a restrictive, repetitive pattern of behaviour (American Psychiatric Association 2013; World Health Organisation 2022) and its prevalence is around 1% (Brugha et al. 2011). Neurodevelopmental conditions in general tend to co-occur and there is a suggestion that around 10% of the population will have at least one such condition (Embracing Complexity n.d.).

Crucially, those with intellectual disability and autism spectrum disorder are significantly more likely to have mental illnesses or other mental disorders (Cooper 2020). It has also been suggested these disorders affect not just the way in which psychopathology presents itself, but also the mode of treatments and treatment outcomes (Bhaumik & Alexander 2020). Many of these patients do not just have a lack of equity in terms of access to healthcare, but also a lack of equity in treatment outcomes and many end up in the criminal justice system (Bhaumik & Alexander 2020; Chester et al. 2023). If this situation is to improve, the importance of research that involves close collaboration between health care professionals, patients, family members and carers and experts by experience has been emphasized (Alexander et al. 2021).

This special edition focuses on some key topics in the treatment and community-based rehabilitation of people with intellectual disability or autism spectrum disorders who also have co-existing mental illnesses or other disorders. It brings together a range of perspectives, some of which focus on challenges in meeting the health needs of the population while others explore innovative approaches and support models to overcome the challenges.