Caregivers receiving the JASPER intervention will undergo weekly virtual 60-minute sessions for 10 weeks conducted via Zoom.

The content of the intervention includes teaching and implementation of contingent responding and specific strategies to provide a high-quality response to children's communication, and play behaviors. Caregiver training is completed through review of pre-recorded videos of the caregiver-child dyad each week. The first half of each session will be dedicated to reviewing previous content and feedback on the play recording. The following half will consist of new material. The coaching support includes contingent responding, the parent is also taught to use specific strategies to provide a rich, high-quality response to children's communication, and play behaviors.

Caregivers receiving the 10 weekly psychoeducational curriculum modules will be provided with the resources via secure email and encouraged to engage with the material a self-directed manner.

The psychoeducational modules will include written information about child development, communication and social interaction skills, behavioral principles for managing challenging behavior and strategies for teaching new skills. No direct caregiver-child mediated coaching will be provided. The content of the modules will include information about developmental milestones in this age group for children with DS, behavior management strategy recommendations, positive parenting materials, and materials on increasing engagement through play.

JASPER is a therapist and caregiver-mediated intervention that (1) targets the foundations of social communication, (2) uses naturalistic behavioral strategies to increase the rate and complexity of social communication and (3) includes caregivers as implementers of the intervention to promote generalization across settings and to ensure maintenance. This intervention is individualized and centered around two key developmental domains critical for social communication function: joint engagement and joint attention.

Joint engagement and joint attention skills will be coded from Caregiver-Child Interaction (CCX) video recordings. The 15-minute CCX videos measure the amount of time that caregiver and child engage during a play session. The frequency of the child's engagement states will be coded using an Engagement State Coding Scheme to measure proportion of video during which child is maintaining Joint Engagement (shared engagement in a play routine).

Social communication skills will be coded from Caregiver-Child Interaction (CCX) video recordings. The 15-minute CCX videos measure the amount of time that caregiver and child engage during a play session. A Social Communication Coding Scheme will measure the child's social communication skills. This coding scheme includes various social communication items and "Yes" or "No" codes to measure social communication skill development over time. Increasing answers of "Yes" indicate growth in social communication skills.

The frequency with which caregivers use behavioral strategies will be collected from a weekly caregiver Behavioral Strategy Log.

Language, socialization, play, problem behavior, and adaptive skills measured by a parent interview about development using the Vineland Adaptive Behavior Scales (VABS-III). Behavior frequency is measured on a range of 0=Never to 1=Sometimes to 2=Usually.

Socialization and play skill development measured by the Play Assessment Questionnaire. The Play Assessment Questionnaire includes "Yes" or "No" items about play skills, symbolic play types and routines, and play level. This caregiver questionnaire measures play skill development over time. Increasing answers of "Yes" indicate growth in play skills.