“…Specifically, we aimed to identify: (1) differences in subscales on the Infant Behavior Questionnaire (IBQ; Rothbart and Derryberry, 1981 ) at 6 and 12 months and the Toddler Behavior Assessment Questionnaire-Revised (TBAQ-R; Rothbart et al, 2003 ) at 24 months between children who are diagnosed with ASD by age 5 (IL-ASD) and children who are not diagnosed with ASD by age 5 (IL-N), (2) if individual subscales on the IBQ ( Rothbart and Derryberry, 1981 ) at 6 and 12 months and the TBAQ-R ( Rothbart et al, 2003 ) at 24 months could predict internalizing and externalizing total problem behaviors as measured by the Child Behavior Checklist (CBCL; Achenbach and Rescorla, 2000 ) at age 5; and (3) if person-centered temperament profiles, created from the temperament questionnaires at 6, 12, and 24 months using latent profile analysis, could predict internalizing and externalizing total problem behaviors at age 5. Because siblings with ASD have lower scores on developmental measures, such as the Mullen Scales of Early Learning ( Mullen, 1995 ) compared to siblings without ASD ( Longard et al, 2017 ; Patterson et al, 2021 ), and Mullen scores are related to temperament profiles ( Garon et al, 2022 ), we controlled for developmental influences on the relationship between temperament and problematic behavior by including the Receptive and Expressive Language subscales of the Mullen in our analyses. We hypothesized that: (1) children who are diagnosed with ASD by age 5 (IL-ASD) would have higher rates of negative emotionality and lower rates of positive emotionality on the temperament measures at 6, 12, and 24 months compared to the IL-N group, (2) individual subscales on the temperament measures and temperament profiles would predict internalizing and externalizing problem behaviors at age 5, and (3) developmental ability, as measured by expressive and receptive language, will affect the relationship between temperament and problem behavior.…”