Autism in young adulthood : etiology, health outcomes and sex differences

Autism is a spectrum of neurodevelopmental conditions characterized by differences and difficulties in social communication, behavioral flexibility, and sensory processing. The progressive understanding of autism, recognizing it as a lifelong and heterogeneous condition, together with research priorities voiced by the autistic community, has underscored the need to better understand the continuity of autism and associated mental and physical health challenges beyond childhood and into adulthood. The reconceptualization and the growing recognition of the experiences of autistic females has also emphasized the importance of understanding sex-specific healthcare needs to adequately support autistic individuals of all sexes.

This thesis uses nationwide Swedish registers to enhance the understanding of etiological factors underlying autism into young adulthood, as well as the mental and physical health of autistic females and males compared to nonautistic individuals and each other.

In Study I we estimated the heritability of a clinical autism diagnosis and the etiological stability of autistic traits from childhood to adulthood. Heritability estimates of the clinical diagnosis were high in childhood and adulthood. Genetic influences shared across different ages contributed to the moderate stability of autistic traits into adulthood, although we also observed new genetic influences emerging over time. The findings highlight the importance of genetic factors on autism across development into adulthood.

Study II investigated sex differences in psychiatric diagnoses preceding autism diagnosis and their stability, indicated through continued specialist care needs within five years after autism diagnosis. Compared to autistic males, autistic females were more likely to receive various psychiatric diagnoses prior to autism diagnosis with persistent difference over a ten-year period. Preceding diagnoses were associated with a later age at autism diagnosis in females compared to males. The stability varied between conditions but was higher in females for most conditions. This study demonstrates the persistently increased vulnerability of autistic females to be diagnosed with potentially persistent psychiatric conditions from an early age and underscores the need for better understanding the fluctuating nature of mental health problems in autistic individuals over time.

In Study III we explored sex differences in psychiatric diagnoses and hospitalizations in autistic compared to nonautistic young adults. Both, autistic females and males were at increased risk for various psychiatric diagnoses and hospitalization due to different psychiatric conditions compared to nonautistic individuals of the same sex. Compared to autistic males, autistic females were more likely to experience mental health problems and be hospitalized for different conditions. The findings emphasize the profound mental health support required by autistic individuals, notably autistic females, in the transition to adulthood across care levels and consequently the need for clinical monitoring and tailored support.

Study IV examined the association between childhood somatic conditions and psychiatric conditions in young adulthood and vice versa (childhood psychiatric conditions and adult somatic conditions). Childhood conditions, except for epilepsy, were generally associated with a risk increase across different adult mental and physical health problems in autistic individuals. However, risk increases were smaller than what was observed among nonautistic individuals, pointing to an already increased risk in autistic individuals, particularly for mental health problems. Similar patterns were observed for autistic females and males, and for individuals with and without intellectual disability, though there were differences in absolute risk between these groups.

Together, the findings from this thesis enhance the understanding of etiological underpinnings of autism continuity into adulthood, highlighting the importance of genetic factors. Moreover, they illustrate the extensive mental and physical health challenges among autistic young adults, which appear to influence each other. In addition, our findings identify autistic females as a group with particularly elevated care needs, apparent at different time points and across secondary care levels. In conclusion, these insights underscore the importance of tailored interventions and long-term support strategies to address the complex and evolving needs of autistic females and males.

List of scientific papers

I. Martini, M. I., Butwicka, A., Du Rietz, E., Kanina, A., Rosenqvist, M. A., Larsson, H., Lichtenstein, P. & Taylor, M. J. (2024). Age effects on autism heritability and etiological stability of autistic traits. Journal of Child Psychology and Psychiatry. 65(9), 1135-1144.
https://doi.org/10.1111/jcpp.13949

II. Martini, M. I., Kuja-Halkola, R., Butwicka, A., Du Rietz, E., Kanina, A., Brikell, I., Chang, Z., Larsson, H., Lichtenstein, P., Bölte, S., Happé, F. & Taylor, M. J. Sex differences in psychiatric diagnoses preceding autism diagnosis and their stability post autism diagnosis. [Manuscript]

III. Martini, M. I., Kuja-Halkola, R., Butwicka, A., Du Rietz, E., D'Onofrio, B. M., Happé, F., Kanina, A., Larsson, H., Lundström, S., Martin, J., Rosenqvist, M. A., Lichtenstein, P. & Taylor, M. J. (2022). Sex differences in mental health problems and psychiatric hospitalization in autistic young adults. JAMA psychiatry. 79(12), 1188-1198.
https://doi.org/10.1001/jamapsychiatry.2022.3475

IV. Martini, M. I., Kuja-Halkola, R., Butwicka, A., Du Rietz, E., Kanina, A., Larsson, H., Liu, S., Brikell, I., Chang, Z., Lichtenstein, P. & Taylor, M. J. The longitudinal bi-directional association between psychiatric and somatic conditions in autistic individuals. [Manuscript]