Project Summary/Abstract Puberty is among the most complex developmental transitions over the human lifetime. The timing (age of onset) and tempo (rate of change) of puberty can significantly and negatively impact physical, psychosocial and physiological functioning, especially in females. Autism spectrum disorder (ASD) is characterized by impaired reciprocal social communication and poor adaptation to change; thus, the onset and course of puberty marks a pivotal transition. In a series of cross-sectional studies, findings from the study team have shown: 1) advanced pubertal timing for female youth with ASD compared to typically developing females (i.e., breast development, menses) using rigorous assessment of pubertal staging; 2) distinct camouflaging behaviors in females with ASD when socially interacting with peers; 3) higher depressive symptoms in early adolescents with ASD, especially females; 4) sex-based differences in neural socioemotional processing in youth with ASD; and 5) elevated stress and arousal in youth with ASD compared to same-age TD peers that increases with age and pubertal development. Consistent across these studies is the unique profile of females with ASD and the potential deleterious impact of early puberty in this vulnerable, understudied population and developmental window. While the previous and ongoing pubertal development research is compelling, at the start of enrollment, 80% of the 10-year old females with ASD had already entered puberty compared to 29% of the TD group; therefore, the true onset of puberty could not be determined. Moreover, it was based on a relatively small sample of females with ASD 10-13 years (N = 35). Thus, a comprehensive exploration of pubertal maturation in a large sample of younger girls with ASD is needed. The overarching goal is to systematically examine the precise onset, tempo and course of pubertal, psychosocial and physiological development in females with ASD using a multimodal, multimethod, accelerated longitudinal design. A large sample of females with ASD (n=120) and TD (n=120) spanning 6 to 15 years will be followed over four years: Cohort 1 enrolled at 6-years (follow 6-9), Cohort 2 enrolled at 8-years (follow 8-11), Cohort 3 enrolled at 10-years (follow 10-13) and Cohort 4 enrolled at 12-years (follow 12-15). The aims will address three key areas. Aim 1: Pubertal development - will examine the timing, tempo and course of puberty based on physical development (Tanner stage (TS), body mass index (BMI), height, linear growth velocity (LGV)), hormones (Estradiol, Luteinizing Hormone (LH)), and menstrual cycle. Aim 2: Psychosocial profiles - will examine reciprocal social communication (CASS) and internalizing symptoms (anxiety and depression) at the onset and over the course of pubertal development. Aim 3. Physiological characterization - will simultaneously examine social functioning at the level of the central nervous system (EEG hyperscanning), HPA axis (cortisol) and peripher...