Autistic-like features and functional somatic symptoms (FSS) frequently co-occur. It remains unknown how autistic-like features and FSS affect each other and develop throughout adolescence. This study examined reciprocal relations between autistic-like features and FSS in adolescence. Participants were 2772 adolescents (52.5% male) from the Tracking Adolescents’ Individual Lives Survey population and clinical cohort. Data from four waves were included, covering the ages between 11 and 19 years. Autistic-like features were measured using the Children’s Social Behavior Questionnaire. FSS were assessed using the Youth Self Report and Adult Self Report, respectively. Using the random intercept cross-lagged panel model, a stable positive, moderately strong between-persons association was found between autistic-like features and FSS. No within-persons reciprocal effects from wave to wave were observed. Secondary analyses revealed a consistent relation with FSS for three different domains of autistic-like features (social and communication behaviors, repetitive behaviors, and self-regulatory behaviors), and highly similar interrelations in a subsample of adolescents with a clinical autism spectrum disorder diagnosis. In conclusion, the co-occurrence between autistic-like features and FSS is stable throughout adolescence. Clinicians working with adolescents with autistic-like features should be alert to the presence FSS, and vice versa. Lay abstract Adolescents with autistic-like features frequently experience unexplained somatic symptoms too, and vice versa. We followed 2772 adolescents for 8 years, starting at 11 and ending at 19 years of age. At four different moments during this time, we asked these adolescents how often they suffered from unexplained somatic symptoms, such as stomachache and dizziness. We asked their parents to what extent the adolescents showed autistic-like features at those four moments too. Then, we assessed whether the relation between autistic-like features and unexplained somatic symptoms stayed the same between 11 and 19 years old. We also looked at whether there was a reciprocal influence. So far, most studies only looked into the relation between autistic-like features and unexplained somatic symptoms at a specific moment in time. It is important to study how this relation develops over time in adolescence, so we can improve treatment for burdening co-occurring symptoms. In our sample, adolescents who experienced many autistic symptoms also experienced many unexplained somatic symptoms. This relation stayed the same over time. There was no reciprocal influence, so higher autistic-like features did not contribute to higher unexplained somatic symptoms, or the other way around. The findings of this work tell us that in adolescents with autistic-like features it is important to be alert to the presence of unexplained somatic symptoms, and vice versa.

Functional somatic symptoms, i.e., physical complaints that cannot be sufficiently explained by an objectifiable biomedical abnormality, become increasingly more prevalent in girls than in boys during adolescence. Both parents and adolescents report more functional somatic symptoms in girls, but their reports correspond only limitedly. It remains unknown whether parent-adolescent discordance contributes to the higher symptom prevalence in girls. This study investigated parent-adolescent discordance in reported functional somatic symptoms throughout adolescence, examined the longitudinal association of parent-adolescent discordance with symptom prevalence in early adulthood and focused on sex differences in these processes. Participants included 2229 adolescents (50.7% female) from four assessments (age 11 to 22 years) of the TRAILS population cohort. Parents and adolescents reported significantly more symptoms in girls than in boys during adolescence. Variance analyses showed that throughout adolescence, parents reported fewer symptoms than girls self-reported and more than boys self-reported. Regression analyses using standardized difference scores showed that lower parent-report than self-report was positively associated with symptom prevalence in early adulthood. Polynomial regression analyses revealed no significant interaction between parent-reported and adolescent self-reported symptoms. Associations did not differ between boys and girls. The findings show that lower parent-reported than self-reported symptoms predict future symptom prevalence in both sexes, but this discordance was more observed in girls. The higher functional somatic symptom prevalence in girls might be partly explained by parental underestimation of symptoms.