A note about terminology and focus

Before elaborating this alternative viewpoint, we make some notes on the terminology and focus of our editorial. First, with ASD we refer to high-functioning autism (that is, autism without intellectual disability – formerly known as Asperger’s syndrome), since this is the kind of autism that is frequently misdiagnosed as (borderline) personality disorder, particularly in females.^{Reference Allely, Woodhouse and Mukherjee4} Second, by female/male, we mean people who were assigned female/male sex at birth (although it is worth noting that most researchers do not distinguish between gender and sex and no research on transgender individuals exists in this line of inquiry, at the time of writing). Finally, although most research in this area has focused on 'borderline' personality disorder, in this editorial we focus on its dimensional counterpart for at least two reasons. First, borderline personality disorder is so heterogenous that it falls on the border of virtually all other psychiatric conditions, not just ASD.^{Reference Tyrer and Mulder5} Second, accruing evidence suggests that borderline personality disorder can be subsumed under a general personality disorder spectrum (much like Asperger’s can be subsumed under the autistic spectrum).^{Reference Tyrer and Mulder5} Thus, focusing on the broader personality disorder spectrum, as opposed to borderline personality or any other putative personality 'type', presents a fruitful conceptual shift. With these notes being made, we move on to examine the two alternative viewpoints on the interface of ASD and general personality pathology.

Misdiagnosis thesis

According to the first viewpoint, cases of high-functioning ASD can sometimes be misdiagnosed as (borderline) personality disorder, given the inherent overlap of the two conditions (particularly in females).^{Reference May, Pilkington, Younan and Williams1,Reference Allely, Woodhouse and Mukherjee4} Since being first expressed in the early 2000s, this argument has been supported by diagnostic asymmetries in the two conditions. In particular, the relative underdiagnosis of ASD (and overdiagnosis of ‘borderline’ personality disorder) in females versus males has prompted several researchers to argue that ‘female autism’ could be conflated with (borderline) personality disorder.^{Reference May, Pilkington, Younan and Williams1,Reference Allely, Woodhouse and Mukherjee4} Indeed, compared with male autism, female autism is less well-defined, more camouflaged and has a higher likelihood of being misdiagnosed (with borderline personality disorder being among the main misdiagnoses).^{Reference May, Pilkington, Younan and Williams1,Reference Allely, Woodhouse and Mukherjee4}

The misdiagnosis thesis clearly bears merit and is supported not only by diagnostic asymmetries but also by notable phenomenological overlaps across the two conditions (e.g. in relational and emotional instabilities).^{Reference May, Pilkington, Younan and Williams1} Nevertheless, extreme variations of this viewpoint may perhaps better be avoided since they risk promoting the notion that ASD and personality disorder can never co-occur, something that cannot be justified by recent clinical findings.

Comorbidity thesis

Indeed, in light of recent findings revealing genuine comorbidities of ASD and personality disorder,^{Reference Allely, Woodhouse and Mukherjee4} researchers have argued that personality disorder and autism can often present together.^{Reference Lugnegård, Hallerbäck and Gillberg2} Unlike those endorsing the misdiagnosis perspective, proponents of this viewpoint emphasise points of convergence, rather than divergence, of the two phenotypes and attempt to shed light on their clinical presentation. Two notable findings can be reported from this literature. First, epidemiologically, although the comorbidity rate of the two disorders matches their population estimates (1–5%), the prevalence of personality disorder traits (such as instability of self, emotional reactivity and impulsivity) in those with ASD is higher than the prevalence of ASD features (such as alexithymic or systemising tendencies) in those with (borderline) personality disorder.^{Reference May, Pilkington, Younan and Williams1,Reference Lugnegård, Hallerbäck and Gillberg2} Second, clinically speaking, in comorbid cases, the neurodevelopmental symptoms of ASD are considered primary and the personality features secondary (since they could stem from autistic neurodivergence).^{Reference May, Pilkington, Younan and Williams1,Reference Allely, Woodhouse and Mukherjee4}

In this sense, the personality disorder phenotype can be thought to be closer to the ASD one (than vice versa). In light of this observation, the comorbidity and misdiagnosis theses can be fruitfully united.

The autistic and personality disorder spectra

Given that the comorbidity and misdiagnosis accounts are not mutually exclusive (and both may be endorsed in conjunction by most clinicians), they could be conjoined by embracing recent advances in the field of personality disorder.

First, contemporary classification systems (including both ICD-11, with its new personality disorder classification, and DSM-5, with its alternative model of personality dysfunction) diagnose personality disorder in dimensional, not categorical, terms.^{Reference Tyrer and Mulder5} ICD-11, in particular, highlights that personality disorder falls on a spectrum of severity that ranges from personality difficulty (in the subsyndromal case) to mild, moderate and severe personality disorder.^{Reference Tyrer and Mulder5}

Second, maladaptive personality traits (including the ICD-11 ones and their DSM-5 counterparts) are omnipresent in various psychopathologies, particularly neurodevelopmental ones.^{Reference Anckarsäter, Stahlberg and Larson3}

Finally, although somewhat controversial, the diagnosis of personality disorder can now be made at any age once a pattern of personality pathology has been demonstrated for at least 2 years.^{Reference Tyrer and Mulder5} This is in keeping with the new lifespan approach to diagnosis that is being followed in both ICD-11 and DSM-5. The option of making an assessment of personality pathology across severity and age groups allows many relational psychopathologies (particularly neurodevelopmental ones such as ASD or attention-deficit hyperactivity disorder) to be characterised by levels of (at least) personality difficulty, which if recognised during early development can be addressed before major personality disorder becomes manifest.^{Reference Lugnegård, Hallerbäck and Gillberg2,Reference Anckarsäter, Stahlberg and Larson3}

Given these points, we argue that some level of personality disorder is intrinsic to ASD, but not vice versa. In other words, certain personality problems (that is, maladaptive ways of relating to oneself and others) are inherent to the autistic phenotype; but autistic traits are not inherent to those with personality disorder. This conclusion is supported by (a) epidemiological findings indicating high rates (48–68%) of personality difficulties in those with ASD (but not vice versa)^{Reference May, Pilkington, Younan and Williams1,Reference Lugnegård, Hallerbäck and Gillberg2} and (b) developmental findings showing that personality difficulties are a common clinical sequela of neurodivergence.^{Reference Anckarsäter, Stahlberg and Larson3}

This conclusion unifies the abovementioned perspectives by highlighting that even though neurodiversity could give rise to maladaptive personality traits (potentially birthing a comorbid personality disorder),^{Reference Anckarsäter, Stahlberg and Larson3} (sub)threshold personality pathology could also dominate the clinical picture (yielding a misdiagnosis of ASD as personality disorder).^{Reference May, Pilkington, Younan and Williams1}

Clinical and research implications

Based on the evidence synthesis above, several research and clinical implications may follow. First and foremost, ASD may be considered as a differential diagnosis of personality disorder, particularly for females, for whom ASD is currently under-recognised.^{Reference Allely, Woodhouse and Mukherjee4} Although this point may seem intuitive clinically, it is worth reiterating because personality problems are often a consequence of other issues, including complex trauma, social deprivation and, of course, neurodiversity.^{Reference Tyrer and Mulder5} Examining these other issues (and often formulating them as primary) is crucial for an accurate personality diagnosis (particularly when the medical history of a patient is unknown).^{Reference Tyrer and Mulder5}

This point leads to the second implication of the reviewed evidence above: a more scientifically accurate and clinically helpful way of conceiving the interface of ASD and personality disorder is in dimensional, not categorical terms. That is, researchers should focus on identifying the personality traits, not personality ‘disorders’, that are most likely to ensue in people with ASD. For instance, affective volatility, relational instability, behavioural rigidity and detached relationships are found in many psychiatric conditions, including ASD.^{Reference Anckarsäter, Stahlberg and Larson3,Reference Tyrer and Mulder5} Examining how and why these, as well as related, traits proliferate in some, but not all, individuals with ASD could help clarify the intimate link between neurodiversity and personality psychopathology.^{Reference Lugnegård, Hallerbäck and Gillberg2,Reference Anckarsäter, Stahlberg and Larson3}

Finally, in light of this intimate link, the treatment of ASD could be extended to interventions that have been constrained to personality disorder. This includes environment-based interventions (which focus on altering patients’ environments rather than their lifelong illnesses)^{Reference Tyrer, Tarabi and Bassett6} but also dialectical behavioural interventions (which focus on teaching coping skills and acceptance practices). Although thought to be inapplicable to people with autism, such interventions are increasingly found (in modified forms) to be useful.^{Reference May, Pilkington, Younan and Williams1}