Early intervention for toddlers on the autism spectrum (formally, autism spectrum disorder / ASD) can lead to developmental progress in a range of domains including language, communication, social skills, and cognitive abilities (Dawson et al., 2010; Sandbank et al., 2020, 2023). To date, most empirical studies and systematic reviews have focused on group-level outcomes, with only a small number of studies exploring whether (or which) child-level factors predict treatment response, an evidence gap initially highlighted over a decade ago (Vivanti et al., 2014). Given substantial heterogeneity in autism presentation across individuals, it stands to reason that particular intervention elements or approaches might vary in terms of “fit” (Lai et al., 2020) across children with varying patterns of strengths, learning needs, and preferences, and at different developmental stages. For example, Koegel and colleagues proposed that “interventions may be more appropriate for subpopulations of individuals with ASD depending on their verbal levels at intake” (2020, p. 2958). The heterogeneity of presentations across the autism spectrum, together with the pressing need to provide evidence-based supports as soon as first signs emerge, establishes the importance of examining factors that predict response to intervention in the toddler years. Moreover, understanding child-level factors that predict which children will benefit most from which approaches may be informative for clinical triage processes, program planning, and advocacy (see Lord et al., 2022).

Amongst baseline child-level characteristics, cognition (IQ), adaptive functioning, and autism characteristics have been demonstrated to predict response to intervention across several studies (e.g., Frazier et al., 2021; Grzadzinski et al., 2023; Koegel et al., 2020; Landa, 2018; Perry et al., 2011; Smith et al., 2015; Vivanti et al., 2014). Pre-intervention language skills or related communication abilities also hold promise as predictors, but findings to date have been mixed. One challenge with interpretation across studies is the use of different measures that capture diverse aspects of communication. Communication entails a broad array of skills involved in sharing information with another person, including through expressive language (use of words, signs, and gestures to express oneself), receptive language (understanding another person’s message), and other modes of sharing information such as facial expressions, gaze, or tone of voice. Following Koegel et al. (2020) the term “communication-related skills” will be used in the current paper to refer broadly to language and other forms of communication. However, when discussing particular research findings or measures, the relevant specific terminology will be used wherever this level of detail is available.

Varied associations have been reported between communication-related skills and treatment response in autism intervention research. Namely, some studies report associations in a positive direction, wherein individuals who begin with stronger skills make larger gains (e.g., Casenhiser et al., 2011 – total language; Tager-Flusberg and Kasari, 2013 – spoken words; Virues-Ortega et al., 2013 – expressive language), whereas others report the opposite (e.g., Siller et al., 2013 found a larger intervention response for children with lower expressive language skills at baseline). Other studies have failed to demonstrate such an association (Green et al., 2010; Hardan et al., 2015) or have identified complex interactions between variables. For example, Fossum et al. (2018) identified a profile predictive of positive response to intervention that included preschoolers with high expressive language ability, positive affect, and appropriate toy contact, combined with reduced social avoidance and repetitive vocalizations. Thus, although baseline communication-related skills have potential as informative predictors, consistent patterns have yet to be firmly established. Moreover, prediction of outcomes in the younger years remains under-studied, resulting in an evidence gap with respect to interventions for toddlers.

Naturalistic developmental behavioral intervention (i.e., NDBI; Schreibman et al., 2015) approaches, which have gained traction over the past decade, provide some insights into the role of communication-related skills in early intervention with toddlers. A solid body of evidence demonstrates group-level developmental progress across studies that use NDBI approaches (e.g., see Sandbank et al., 2020, 2023; Song et al., 2025, for evidence syntheses). In efforts to understand which children are best served by NDBI models, some researchers have examined outcomes specifically for young children who enter intervention with relatively limited expressive language abilities. Two recent studies (Gengoux et al., 2019; McDaniel et al., 2020) report on outcomes from autistic children aged 2–5 years who also presented with expressive language delays (i.e., with scores 1–3 standard deviations below the mean, depending on the child’s age, as measured on the Preschool Language Scale (PLS-5); Zimmerman et al., 2011). In both studies, children participated in a 12-week NDBI, called the ‘Pivotal Response Treatment Package’ (PRT-P) which is an in-home program delivered by therapists, with associated parent training. Group-level increases were reported for children’s word use and vocal responsiveness immediately or 12 weeks following program completion (Gengoux et al., 2019; McDaniel et al., 2020, respectively), when compared to controls receiving community treatment as usual. Although these findings demonstrate positive outcomes for young children with delayed language, the predictive power of different language profiles was not examined.

Barrett et al. (2020) similarly examined outcomes from a 6-month PRT-based NDBI program (‘Pivotal Response Intervention for Social Motivation’ or ‘PRISM’ intervention) with children aged 1.5–4.5 years. Compared to waitlist controls, children in the treatment condition demonstrated gains in social responsiveness to their caregivers and increased spoken language (mean length of utterance). Notably, the intervention was found to be particularly effective for a small subgroup that the authors defined as “minimally verbal” based on having used fewer than 30 functional spoken words during an initial play interaction – this subgroup experienced particularly large increases in responsivity and word use following the intervention, demonstrating a relative advantage for children starting with fewer spoken words. These findings mirror the earlier findings by Siller et al. (2013), wherein young children with baseline expressive language abilities below a 12-month level exhibited the largest expressive language gains in response to a caregiver “capacity building” program designed to foster coordinated play between caregiver and child (‘Focused Playtime Intervention’; FPI). Taken together, these findings suggest that NDBI approaches may be particularly well-suited to children at relatively early stages of expressive language development.

The studies discussed above report on NDBI approaches that are delivered by therapists with some involvement from family caregivers (i.e., concomitant parent training is a cornerstone of NDBI models; Smith et al., 2015). However, in some NDBI models, the intervention is delivered exclusively by a family caregiver (usually a parent) who is trained to implement the intervention strategies – such a difference might uncover unique predictors. These caregiver-mediated intervention (CMI) approaches often focus on toddlers under 3 years old (see Zhao et al., 2025, for a review), with some studies enrolling children as young as 1 year of age (Kasari et al., 2010; Rogers et al., 2012, 2019; Stahmer et al., 2020; Yoder et al., 2021), allowing for exploration of early developmental factors. Across studies, CMI approaches appear to be acceptable and feasible (e.g., see Pellecchia et al., 2024, for a review), and they yield group-level gains in various aspects of toddler development and skill acquisition (e.g., responsiveness to caregiver, adaptive and social communication skills; see review by Zhao et al., 2025). However, as with studies in older children, the examination of individual predictors is scant in this younger age range. Moreover, “language-level” groupings that are conceptualized in studies of older children are not likely to be applicable to toddlers. For example, using a cut-off of <30 functional spoken words to identify a “minimally verbal” subgroup may not be meaningful in the first 2 years of life because “nearly all toddlers younger than about 18 months of age are NV [non-verbal] or MV [minimally verbal] … However, one could argue that limited verbal ability during the toddler years is meaningfully different from that during later childhood due to the developmental trajectory of language” (Koegel et al., 2020, p. 2958). As such, the extent to which emerging communication-related abilities predict treatment response in autistic toddlers is particularly understudied and remains poorly understood. Extrapolating from older samples may not yield accurate information to guide clinical decisions about who will benefit from which interventions and when. Direct examination of predictors in the toddler years has become possible with the rise of interventions developed for autistic toddlers.

One such CMI program, the Social ABCs, was developed specifically for use with toddlers (≤ age 3 years) with emerging or confirmed autism, and is delivered exclusively by a family caregiver. The standard version of the program entails 12 weeks of individual parent-coaching (tapering over time) supported by a Parent Manual. The program entails coaching caregivers in the use of 10 specific strategies to enhance child social attention and vocal communication directed to the parent, as well as positive affect-sharing between caregiver and child. Each caregiver receives live, in-the-moment coaching while interacting with their toddler, an approach that has been identified to be particularly effective (referred to as “real-time” “triadic intervention” in Sone et al., 2021). Research findings, across pilot, randomized control trial, and community implementation studies, demonstrate that participation in the Social ABCs yields increased use of enhancing strategies by caregivers (i.e., implementation fidelity) and group-level gains in toddlers’ early social communication skills (i.e., vocal responsivity, social orienting; see Brian et al., 2016, 2017, 2022a, for further details). An abbreviated, 6-week, group-based adaptation of the program has also demonstrated positive findings from a large pilot evaluation (Brian et al., 2022b). An attempt to identify predictors did not yield any informative family- or treatment-level factors, including marital status, number of languages spoken in the home, frequency of English spoken in the home, which caregiver was coached, their ethnicity, occupation, educational attainment, or the number of coaching sessions attended (note that all had attended a minimum of 5 sessions; Brian et al., 2022b). The predictive role of children’s communication-related skills before starting the program has not been explored.

The current study entailed secondary analysis of data from a randomized control trial (RCT) evaluating the Social ABCs with an attention training co-intervention (Clinicaltrials.gov identifier NCT03215394). Primary intervention outcomes paralleled previous findings (Brian et al., 2017, 2022a), revealing significant group-level gains in caregiver fidelity (i.e., strategy use), and toddler responsivity, smiling, and social orienting (Brian et al., 2025). Toddlers were assessed prior to program entry (at baseline; BL) using one domain of a standardized direct clinical assessment measure, a standardized parent interview of children’s everyday communication, and a parent questionnaire of vocabulary (measures described below). Video-coded data were used to characterize the following outcomes: Caregiver implementation fidelity (primary parent outcome), and toddler responsivity (primary toddler outcome), initiations, orienting toward caregiver, and social smiling. The change scores (post- vs. pre-intervention) for these target skills were used analyses.

Intervention research has only begun to understand the extent to which a child’s early developmental proficiencies might influence their response to treatment, with particular knowledge gaps in the toddler years, which are often characterized by rapid developmental changes. This paper contributes to the field by identifying communication-related predictors of outcomes in response to a caregiver-mediated intervention for toddlers with emerging autism.

The largest effects emerged for caregivers’ implementation fidelity. Specifically, toddlers’ baseline communicative functioning (i.e., their everyday application of both receptive and expressive abilities) predicted their caregivers’ mastery of program strategies. Consistent with hypotheses, this was an inverse association, meaning that toddlers with lower communication functioning at program entry had caregivers who made larger gains in strategy use while participating. One possibility for this finding is that, when interacting with a toddler who experiences greater communication challenges, some caregivers may have had more difficulty knowing how to engage them prior to learning the Social ABCs, thus leaving room for substantive growth of skills with intervention. Conversely, learning the program strategies may be easier with children who are at earlier stages of communication development. For example, it may be more straightforward to focus on core techniques such as building clear opportunities for toddler responding than to implement the strategies in the context of elaborate activities such as imaginary play, which may be the situation for toddlers with more advanced communicative functioning. Another plausible explanation is that caregivers may be particularly motivated to learn the enhancing techniques when they notice greater developmental delays in their young children. We have yet to demonstrate an association between caregiver motivation (or “buy-in”) and their program uptake, but such an association appears to be consistent with findings reported by Schertz et al. (2020) regarding increased self-efficacy for caregivers when enabled to take a lead role in their child’s learning. These associations warrant careful examination in future studies and may be particularly relevant in the emerging discourse about pre-symptomatic intervention (e.g., see Grzadzinski et al., 2021). We have added a measure of caregiver buy-in to our ongoing work which will allow us to begin exploring this phenomenon.

Among toddler outcomes, increased responsivity to caregivers had the strongest predictors. Significant predictors emerged for caregiver-reported indices of toddlers’ receptive (communication and words understood) but not expressive abilities. These predictions were in a positive direction, meaning that higher baseline receptive proficiencies predicted steeper gains in responsivity, with toddlers’ comprehension of words accounting for almost one-quarter (24%) of the variance in outcomes. It was contrary to our hypotheses that these associations were restricted to toddlers’ receptive abilities, particularly because the intervention, at least nominally, targets toddlers’ expressive (vocal) communication-related skills (along with social attention and affect-sharing), and most related work has focused on expressive abilities predicting outcomes. However, receptive abilities may be associated with other ‘learning readiness’ skills such as child attention (and/or joint attention; Bottema-Beutel, 2016), motivation, and participation, any of which might be essential components of a child’s responsiveness to intervention. To our knowledge, this is the first intervention study with autistic toddlers to demonstrate the predictive power of receptive (rather than expressive) communication-related skills.

Finally, the trend toward a positive association between baseline word comprehension (interacting with age) and toddler smiling, warrants some discussion. The full model accounted for 13% of the variance but failed to meet our stringent threshold for significance. Such a pattern suggests that older toddlers, with larger receptive vocabularies, tended to show greater increases in smiling during the intervention. Our interest in smiling is motivated by findings from longitudinal research showing that, as early as 12 months of age, toddlers who subsequently receive an autism diagnosis express less social smiling than their non-autistic peers (Zwaigenbaum et al., 2005). This is relevant given that positive emotions play a fundamental role in language development (Hohenberger, 2011), social learning, and early relationship-building (Seehagen et al., 2024). Increased smiling during the course of the program may also be an indication that the child is enjoying the interaction, which would support the program’s emphasis on building learning opportunities through activities and interactions that are motivating for the child (Brian et al., 2016). The importance of fostering positive affect is underscored by previous findings that high positive affect, combined with other factors, predicted greater response to intervention for autistic preschoolers (Fossum et al., 2018). In the current study, the full mediation model approached significance, warranting further attention in future work to more fully understand the nature of the associations between toddler communication-related skills and positive affect.

The current findings contribute to an evolving understanding of how early communication-related skills impact response to intervention. Findings revealed significant predictions for caregiver fidelity and toddler responsivity, based on toddlers’ baseline skills, but these occurred in opposite directions. Moreover, findings also failed to show associations for several variables. Notably, expressive communication-related skills, as measured in the current study, did not significantly predict any of the toddler outcomes. As such, findings do not support the use of a toddler’s expressive abilities to guide clinical decisions about their inclusion or exclusion in the program. The current sample included toddlers from a wide range of abilities including those with expressive language and communication skills at a 4-month level and no words spoken or understood. Half the sample (34/67 = 50.1%) began the program with an expressive language age equivalent at or below a 12-month level (nine of whom started the program at or below a 6-month level). Notably, however, the sample also included toddlers with strong communication-related skills at study entry (i.e., expressive language at a 36-month age-level, age-appropriate communication skills, and close to 400 words understood and used). In light of this wide range of abilities across participants, the limited prediction of outcomes other than fidelity and responsivity, together with the lack of association between expressive abilities and any of the child outcomes, suggests that the program is a good fit for toddlers across a wide range of abilities.

It should not be surprising that young children with diverse strengths and needs benefitted from the program in different ways. For example, receptive communication positively predicted responsivity, but trended in an inverse direction for social orienting, suggesting that toddlers with lower communication abilities may have benefitted relatively more from the non-verbal aspects of the program. The ability to orient one’s attention to a social partner is arguably a necessary element of joint attention, which is a well-established marker of early autism and a predictor of later development across various indices of language (Mundy et al., 1990; Kasari et al., 2012; Bottema-Beutel, 2016). Adamson et al. (2019), for example, showed that “coordinated joint engagement,” which entails the toddler explicitly attending to the caregiver during a shared activity, is a stronger predictor of expressive language development than is joint attention. In that study, toddlers who were not yet using spoken words had particularly notable challenges with joint engagement, and the acquisition of first spoken words was associated with gains in joint engagement (i.e., “Moving from not talking to talking seemed to kindle a developmental transformation of joint engagement,” Adamson et al., 2019, p. 16). This appears to be a reciprocal relationship, with joint engagement establishing the opportunity for ongoing language learning. Although the current design did not allow for such analyses, it is possible that toddlers beginning the Social ABCs with lower communication proficiencies needed first to establish social orienting before other learning could be optimized. As a field, intervention research will benefit from acknowledging that a unitary outcome may not be a reasonable expectation for all participants, recognizing the value of personalized treatment pathways and individualized outcomes.

The issue of measurement has been identified as a priority in intervention research (Grzadzinski et al., 2023). In the current study, different measures were used to capture varied aspects of communication-related skills, including a caregiver interview that measured toddlers’ everyday communication, a parent-report questionnaire to capture toddlers’ word use and comprehension, and a direct clinical assessment of expressive language. Unexpectedly, the parent-reported measures had more predictive power than the clinical assessment measure selected for the study. This highlights the importance of obtaining information from multiple sources when identifying children’s learning strengths and needs. However, caution is warranted against the exclusive use of parent-report measures, in light of evidence that caregivers may be inclined to overestimate the receptive language proficiency of their child due to the child’s use of contextual, non-verbal cues that commonly occur in interactions with caregivers (Luyster et al., 2008). Such cautions may be particularly relevant in the context of CMIs, where caregivers cannot be blinded to treatment condition and thus may be particularly vulnerable to an expectancy effect or unconscious bias (e.g., detection bias [Sandbank et al., 2023], or effort justification bias).

Some, but not all, indices of toddlers’ pre-intervention communication-related skills were informative for predicting response to the intervention. Specifically, lower (expressive and receptive) communication development in toddlers predicted larger gains in caregivers’ strategy use. Conversely, higher baseline (receptive but not expressive) skills predicted greater gains in toddlers’ responsivity. Predictions occurred in both positive and inverse directions, highlighting the complexity of the relationships between baseline proficiency and outcomes. Starting the program with relative developmental advantages appeared to predict greater gains in toddlers’ vocal responsivity (with a parallel trend for social smiling), whereas starting with relatively lower proficiency appeared to be an advantage for caregivers’ learning, and may also have subtly impacted toddlers’ social orienting. The clinical measure selected for this study did not have any explanatory power, but other clinical measures may be useful for capturing different information.

The current findings may be helpful in informing clinical triage decisions and personalizing intervention approaches. Our results support the use of the Social ABCs with a wide range of toddlers with autism or early signs thereof, with no consistent evidence that the program systematically favors toddlers at either end of a wide range of ability. Clinicians should feel confident in referring toddlers with autism, or related social communication and behavioral differences, to caregiver-mediated early interventions that support social communication development, regardless of the child’s communication-related abilities (whether high or low) at program entry.

Future research will benefit from further exploring the complex interactions among child-, caregiver-, program-, and system-level factors that predict treatment outcomes, with the goal of developing individualized treatment pathways that are informed by evidence. The toddler period is a key stage in development, during which strengthening foundational social communication skills can enhance long-term outcomes (e.g., Pickles et al., 2016; Warlaumont et al., 2014). This highlights the importance of finding the best “fit” between each toddler’s developmental needs, their family and community characteristics, and the type, timing, and intensity of intervention and supports that they receive.