What this trial measures

• Change in RespetoAt baseline (i.e., within two weeks prior to the start of intervention); Post-intervention (i.e., within two weeks after completing the intervention); At three-month follow-up

  Two dyadic measures of respeto will be coded using two items from the Joint Engagement Rating Inventory (JERI; Adamson et al., 2020). The Parent Calm Authority (PCA) subscale will measure the extent to which a parent influences the interaction using an assertive and confident yet gentle approach. The Child Affiliative Obedience (CAO) subscale will capture the child's affiliation, deference, and amenability to the parent's direction. These are scored using 7-point Likert scales (where 1 indicates no PCA or CAO, respectively). These measures will be derived from 12-minute naturalistic parent-child interaction video recordings. The interactions will be video-recorded from a wide-angle view as well as from hidden camera eyeglasses worn by the adult to capture the location and direction of the child's the location and direction of the child's eye gaze. The two streams of digitized videos will be time-linked and subsequently coded for measures of respeto.

• Change in Social Communication SkillsAt baseline (i.e., within two weeks prior to the start of intervention); Post-intervention (i.e., within two weeks after completing the intervention); At three-month follow-up

  Change in the raw scores obtained on the Social, Speech and Symbolic composites of The Communication and Symbolic Behavior Scales- Developmental Profile (CSBS-DP; Wetherby \& Prizant, 2002) will be measured. Raw scores for the Social, Speech and Symbolic composites range from 0-64, 0-54 and 0-53 respectively (0 being the worst score). The CSBS-DP is a direct assessment of early social communication designed for children with a functional communication age between 6 months and 2 years.

• Change in Adaptive FunctioningAt baseline (i.e., within two weeks prior to the start of intervention); Post-intervention (i.e., within two weeks after completing the intervention); At three-month follow-up

  Change in the raw scores obtained on the Communication, Daily Living, Socialization, and Motor Development domains of the Vineland Adaptive Behavior Scales, Second Edition (VABS-II; Sparrow et al., 2005) will be measured. Raw scores for these domains range from 0-252, 0-286, 0-224, 0-154 respectively (lower scores being worse outcomes). The VABS-II is a standardized parent report measure of adaptive functioning designed for individuals from birth to age 90.

• Change in Social AttentionAt baseline (i.e., within two weeks prior to the start of intervention); Post-intervention (i.e., within two weeks after completing the intervention); At three-month follow-up

  The amount of time the child initiates eye gaze to the adult's eyes paired with positive affect will be measured (see Rollins et al., 2021 for a detailed description of this measure). This measure will be derived from 12-minute adult-child interaction video recordings wherein the child interacts with an unfamiliar adult during a semi-naturalistic play session. These interactions will be video-recorded from a wide-angle view as well as from hidden camera eyeglasses worn by the adult to capture the location and direction of the child's eye gaze. The two streams of digitized videos will be time-linked and subsequently coded for social orienting.

• Change in Intentional CommunicationAt baseline (i.e., within two weeks prior to the start of intervention); Post-intervention (i.e., within two weeks after completing the intervention); At three-month follow-up

  Intentional communication will be measured using the Early Communication Indicator (EC Indicator; Greenwood et al., 2006). The EC Indicator yields a total weighted communication score weighting gestures and vocalizations as 1, single words as 2 and multiple words as 3. Each utterance will also be coded for the child's attentional focus when communicating (attention to object, attention to adult, coordinated attention to object and adult). This measure will be derived from 12-minute adult-child interaction video recordings wherein the child interacts with an unfamiliar adult during a semi-naturalistic play session. These interactions will be video-recorded from a wide-angle view as well as from hidden camera eyeglasses worn by the adult to capture the location and direction of the child's eye gaze. The two streams of digitized videos will be time-linked, orthographically transcribed at the level of the utterance, and subsequently coded for intentional communication.

• Change in Expressive LanguageAt baseline (i.e., within two weeks prior to the start of intervention); Post-intervention (i.e., within two weeks after completing the intervention); At three-month follow-up

  Expressive language will be measured via the Moving Average Number of Different Words (MANDW) and Mean Length of Utterance in words (MLUw) calculated using the Systematic Analysis of Language Transcripts (SALT; Miller \& Iglesias, 2020) software. MANDW measures expressive vocabulary using a moving window to estimate the number of different words, controlling for variability in length across language samples. MLUw measures combinatorial language by calculating the ratio of main body words to the number of utterances produced. This measure will be derived from 12-minute adult-child interaction video recordings wherein the child interacts with an unfamiliar adult during a semi-naturalistic play session. These interactions will be video-recorded from a wide-angle view as well as from hidden camera eyeglasses worn by the adult to capture the location and direction of the child's eye gaze. The two streams of digitized videos will be time-linked and orthographically transcribed.