How can we explain the rising rate of preschool children diagnosed with autism?

In the wake of a conversation between clinicians and user representatives during a public event at Arendalsuka, Professor Mila Vulchanova raises important questions regarding the ongoing rise in autism spectrum disorder (ASD) incidence in preschool children in Norway.

While there is no dispute that there has been a rise in the rate of preschool children diagnosed with ASD, the explanations vary.

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Vulchanova highlights two possibilities: that the rise is most likely a consequence of widened diagnostic criteria, or, if this is not the case, that these children are mislabelled as autistic due to the predictable consequences of second language acquisition on psychological test performance.

The first point

The first possibility is the most commonly cited explanation, both in international literature and in mainstream media, reiterated just a couple of weeks ago. 

However, despite its popularity, this explanation may lack validity in a Norwegian context. Vulchanova correctly points out that the 2013 DSM-5 provided wider ASD criteria, which were revised and widened again in the 2019 ICD-11 (Kamp-Becker et al., 2024). 

However, the DSM-5 is not the diagnostic framework used in Norway, and Norwegian clinicians are still legally obliged to record ASD according to the ICD-10 until ICD-11 is formally translated and adapted.

Therefore, the discussion of increased prevalence in Norway (which began prior even to the DSM-5), must be made with reference to the ICD-10 framework. While Norwegian health authorities recommend using the DSM-5 diagnostic criteria for ADHD, this is not the case for ASD.

The main point is this: While most preschoolers identified as autistic by the ICD-10 would be identified also by the DSM-5 or ICD-11, ('Severity Level 3'), not all preschool children identified as autistic by the DSM-5 and the ICD-11 would be identified by the ICD-10.

Consequently, the increased incidence in Norwegian preschoolers cannot be mainly due to the publication of the DSM-5.

In fact, the slow adoption of the ICD-11 in Norwegian healthcare may serve as indirect evidence that there has been an actual rise in the behavioural phenotype required to cross the conservative diagnostic thresholds of the ageing ICD-10. Most clinicians working with preschool ASD would agree with this proposition.

The second point

Vulchanova's second question, much less entertained in ASD research, is of paramount importance: Are the adverse effects of bilingualism on psychological test performance falsely construed as evidence of a neurodevelopmental disorder?

This has happened before, and was most certainly the case when 40-60 per cent of European immigrants to the United States were misidentified as intellectually disabled (Goddard, 1917, p. 266), due to a combination of inadequately standardised IQ-tests and a poor understanding of the complex relationship between culture, language experience, and cognitive development (Hakuta, 1986).

Faced with diagnostic rates diverging along demographic and linguistic lines (Keen et al., 2010), ASD researchers may stand to learn a lot from the literature on the relationship between bilingualism and cognitive development, thoroughly explored in the latter half of the last century (e.g. Hamers & Blanc, 2000).

However, language is only part of the equation. Naturally, Norwegian clinicians are well aware that the behaviours necessary to cross the diagnostic threshold of ASD are not simply those associated with delayed language development (Surén et al., 2019), and, besides, the differential diagnosis requires that children are equally delayed in establishing both their first and second language.

But the crux of Vulchanova’s argument still stands: We urgently need to understand how societal factors, such as bilingualism, may impact development in ways that affect performance on the psychological tests used in ASD diagnostics. This is an important point often overlooked in autism research.

References:

• Goddard, H. H. (1917). Mental Tests and the Immigrant. The Journal of Delinquency, 2(5), 243–277.
• Hakuta, K. (1986). Mirror of Language. The debate on Bilingualism. BasicBooks.
• Hamers, J. F., & Blanc, M. H. A. (2000). Bilinguality and bilingualism (2nd ed.). Cambridge: Cambridge University Press.
• Kamp-Becker, I. (2024). Autism spectrum disorder in ICD-11—a critical reflection of its possible impact on clinical practice and research. Molecular Psychiatry, 29(3), 633–638. https://doi.org/10.1038/s41380-023-02354-y
• Keen, D. V., Reid, F. D., & Arnone, D. (2010). Autism, ethnicity and maternal immigration. British Journal of Psychiatry, 196(4), 274–281. https://doi.org/10.1192/bjp.bp.109.065490
• Surén, P., Havdahl, A., Øyen, A.-S., Schjølberg, S., Reichborn-Kjennerud, T., Magnus, P., Bakken, I. J. L., & Stoltenberg, C. (2019). Diagnosing autism spectrum disorder among children in Norway. Tidsskrift for Den Norske Legeforening. https://doi.org/10.4045/tidsskr.18.0960

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