Sex differences in internalising problem trajectories of autistic and non-autistic youth across childhood and adolescence

Background Autistic children and adolescents experience high rates of internalising problems such as anxiety and depression. In the general population, female children and adolescents typically show higher trajectories of internalising symptoms – but little is known about sex differences among autistic youth. This study examined sex differences in the developmental course of internalising symptoms among autistic and non-autistic youth across childhood and adolescence. Methods Participants included autistic ( n = 573) and non-autistic ( n = 15,945) individuals from the Millennium Cohort Study. Internalising symptoms were assessed via parent-report Strengths and Difficulties Questionnaire at six timepoints (ages 3–17). Latent Growth Curve Modelling (LGCM) was used to model trajectories, and growth factors were regressed onto autism diagnosis, sex, and their interaction. Additionally, separate LGCMs were fitted to autistic and non-autistic male and female groups to describe internalising trajectories. Results Autism diagnosis was associated with higher baseline internalising symptoms and steeper increases over time. Female sex was linked to steeper increases in internalising symptoms, despite slightly lower initial levels. The interaction between sex and autism diagnosis was not statistically significant, indicating no additional combined effect on trajectories. Overall, autistic youth had higher internalising trajectories than non-autistic youth, with group differences emerging earlier in males (age 7) than females (age 11). Conclusion This study showed that that sex differences in developmental trajectories of internalising problems among autistic youth parallel those seen in the general population, but with increased severity.