Inclusion Criteria:

   - Diagnosis of Autism Spectrum Disorder (ASD) based on clinical interview and Diagnostic
   and Statistical Manual, 5th edition (DSM-5) and confirmed using the Autism Diagnostic
   Interview Revised (ADI-R) and Autism Diagnostic Observation Schedule, 2nd edition
   (ADOS-2) or Childhood Autism Rating Scale (CARS-2); Diagnostic and Statistical Manual,
   5th edition (DSM-5), and expert clinical opinion

   - Boys and girls between 2 years and 3 years and 11 months

   - Ability to participate in the testing procedures to the extent that valid standard
   scores can be obtained

   - Stable treatment (e.g., applied behavior analysis (ABA), speech therapy, psychotropic
   medication(s) or biomedical intervention(s) for at least 1 month prior to baseline
   measurements with no anticipated changes during study participation

   - Developmentally delayed with Mullen Scales of Early Learning (MSEL) or Developmental
   Profile (DP-4) composite score below 85 (1 Standard Deviation below the mean)

   - Availability of at least one English-speaking parent who can consistently participate
   in parent training and research measures.

Exclusion Criteria:

   - Current or lifetime diagnosis of severe psychiatric disorder (e.g., bipolar disorder,
   etc.)

   - Presence of active medical problem (e.g., unstable seizure disorder or heart disease)

   - Child primary language other than English

   - Previous adequate trial of Pivotal Response Treatment

   - More than 15 hours per week of in-home applied behavior analysis (ABA).