The absence of non-European cohorts in genetic studies of neurodevelopmental and neuropsychiatric disorders severely limits the understanding of their full genetic architecture and undermines implementation of precision medicine. Here, we directly addressed this issue by recruiting African Americans (AfrAms) with autism spectrum disorder (ASD) and analyzing their rare and common genetic variation. We performed both global and local ancestry analyses to characterize the complex patterns of admixture at the individual level and compare genetic factors between European (EUR) and African (AFR) genetically inferred ancestries (GIAs) across multiple cohorts in a total of 38,483 autistic individuals. We showed consistent common variant genetic effect sizes for ASD in EUR and AFR GIAs through genome-wide association studies. We demonstrated the limited transferability of EUR-derived polygenic scores (PGSs) based on polygenic transmission disequilibrium and ancestry partial PGS analysis. We found significant autism association for high-impact rare copy number variants in both GIAs. We identified a set of candidate ASD loci based on rare deletions observed in AFR GIA carriers, including SMC2, DMTN, SORCS1, and ROGDI, and detected a signal for de novo missense variants of predicted low impact in AFR GIA individuals. Finally, we uncovered significant depletion of AFR GIA autistic carriers of rare variants in known associated genes found in EUR cohort studies. These findings are the first to detail common and rare variant genetic contributions to ASD in AfrAms and demonstrate that their involvement in neurodevelopmental and neuropsychiatric disorders' genomic research is essential to advance discovery.

Objective: Callous-unemotional (CU) behaviors and quantitative autistic traits (QATs), heritable domains implicated in social development, are both associated with reduced prosocial behavior and increased aggression at their clinical extremes. However, they are hypothesized to contribute to aggression through separate mechanisms. This study tested whether CU behaviors and QATs exhibited distinct profiles of heritable influences as well as independent contributions to early childhood aggression in a general population sample with enhanced sensitivity to clarify these relationships. Method: Parents of 3-to 4-year-old epidemiologically representative twins ascertained from birth records (N = 113 pairs) completed questionnaires measuring CU behaviors, QATs, and aggression. Correlation coefficients indexed overlap across behaviors. Intraclass correlations were compared between monozygotic and dizygotic twins to characterize relative genetic and environmental influences. Generalized estimating equations tested contributions of CU subdomains, verified via factor analysis, and QATs to aggression. Results: Total CU scores strongly correlated with QATs (r = 0.54) and aggression (r = 0.55), while QATs correlated moderately with aggression (r = 0.38). Among 3 identified CU factors, the uncaring factor strongly correlated with QATs (r = 0.52), while unemotional and callous factors showed small correlations (r = 0.25 and r = 0.16, respectively). QATs, aggression, and all CU factors except the callous factor showed heritable influences. Uncaring and callous factors as well as QATs demonstrated unique and shared contributions to aggression, with the callous factor being moderated by sex. Conclusion: Partially overlapping relations support distinct mechanisms whereby CU behaviors, in particular the callous factor, and QATs contribute to early aggression. In-depth social developmental assessment may enhance personalized intervention for aggression in early childhood. Plain language summary: In this cross-sectional dimensional approach, authors evaluated the extent to which callous unemotional behaviors and quantitative autistic traits showed independent contributions to aggression at age 3 to 4 years based on heritable and environmental influences that set the stage for future outcomes. In 224 preschool-aged twins, callous-unemotional behaviors and quantitative autistic traits were independently associated with reduced prosocial behavior and increased aggression, accounting for almost 30% of the variance in aggression and evidence of some shared genetic influences. Callous-unemotional behaviors accounted for a greater unique contribution to variation in aggression than autistic traits. Callous behavior is also strongly influenced by environmental factors and showed a stronger relationship with aggression in boys than in girls. These results suggest that in early childhood, quantitative autistic traits and callous-unemotional behaviors may represent distinct pathways to aggression. Diversity & Inclusion Statement: We worked to ensure sex and gender balance in the recruitment of human participants. We worked to ensure sex balance in the selection of non-human subjects. One or more of the authors of this paper self-identifies as a member of one or more historically underrepresented racial and/or ethnic groups in science.

Abstract Background According to the most recent U.S. CDC surveillance data, the rise in prevalence of childhood autism spectrum disorder among minority children has begun to outpace that of non-Hispanic white children. Since prior research has identified possible differences in the extent of mate selection for autistic traits across families of different ethnicity, this study examined variation in autism related traits in contemporaneous, epidemiologically ascertained samples of spousal pairs representing Hispanic and non-Hispanic white populations. The purpose was to determine whether discrepancies by ethnicity could contribute to differential increases in prevalence in the current generation of young children. Methods Birth records were used to identify all twin pairs born between 2011 and 2013 in California and Missouri. Families were selected at random from pools of English-speaking Hispanic families in California and Non-Hispanic White families in Missouri. Autistic trait data of parents was obtained using the Adult Report Form of the Social Responsiveness Scale (SRS-2). Results We did not identify a statistically significant difference in the degree of mate selection for autism related traits between Hispanic and non-Hispanic white spousal pairs. However, the degree of spousal correlation observed in this recent cohort was pronounced (on the order of ICC 0.45) and exceeded that typically reported in prior research (on the order of 0.30), surpassing also widely reported estimates for sibling correlation (also on the order of 0.30). Limitations The sample did not allow for a direct appraisal of change in the magnitude of spousal correlation over time and the ascertainments of trait burden were derived from spouse report. Conclusion Across two epidemiologically ascertained samples of spousal pairs representing Hispanic and non-Hispanic white families across two U.S. states (respectively, California and Missouri), the extent of autism-related trait co-variation for parents of the current generation of young children is substantial and exceeds correlations typically observed for siblings. Given the heritability of these traits and their relation to autism risk, societal trends in the degree of mate selection for these traits should be considered as possible contributors to subtle increases in the incidence of autism over time and across generations.

Among the many race-based health disparities that have persistently plagued the US population,1 the disproportionate burden of adverse neurodevelopmental outcomes to Black children affected by autism spectrum disorder (ASD) is particularly devastating given its major lifelong consequences. Recently, in 3 successive reports from the Autism and Developmental Disabilities Monitoring (ADDM) program of the US Centers for Disease Control and Prevention (CDC) (birth cohort years 2014, 2016, and 2018), we and our collaborators reported that although the prevalence of community-diagnosed ASD had equalized for Black and non-Hispanic White (NHW) children in the United States, there has persisted a pronounced racial disparity in the proportion of ASD-affected children with comorbid intellectual disability (ID), on the order of 50% for Black children with ASD vs 20% for White children with ASD.2 Here, we provide data to support the following: much earlier diagnosis is possible; early diagnosis alone is not likely to close the ID comorbidity disparity; and judicious efforts over care as usual are necessary to ensure that Black children have access to timely implementation of developmental therapy, for which we observed promising associations with improved cognitive and adaptive outcomes in our sample.

Adverse experiences superseding a child's capacity to sustain regulation of emotion and adaptive function are theorized to constitute "toxic stressors" when they induce a deleterious biological response within an individual. We ascertained presumptive parameters of toxic stress among 164 low-income infants and toddlers (ages 4-48 months) from 132 families enrolled in Early Head Start (EHS). We randomized a subset of these families into a pilot intervention arm of parenting education (the Incredible Years, TIY), which supplemented the EHS curriculum. Official report child abuse and neglect (CAN) and child behavior were serially ascertained over the course of the study. We observed relatively low associations among maternal depression, CAN, caregiver-child relationship quality, hair cortisol, and adverse child behavioral outcomes. Moreover, despite poverty and the high prevalence (51%) of CAN in this sample, the frequency of clinical-level internalizing and externalizing behavior among the children did not exceed that of the general population, by their parents' report. The pilot supplementation of EHS with TIY improved attendance in group meetings but did not significantly reduce adverse behavioral outcomes or CAN. This study revealed marked independence of standard indices of toxic stress (child maltreatment, maternal depression, caregiver emotional unavailability) which have been presumed to be risk factors for the development of psychopathology. That they were weakly inter-correlated, and only modestly predictive of child behavioral outcomes in this EHS sample, caution against presumptions about the toxicity of individual stressors, highlight the importance of ascertaining risk (and compensatory influences) comprehensively, suggest buffering effects of programs like EHS, and demonstrate the need for greater understanding of what parameterizes resilience in early childhood.

To explore racial disparities for African Americans with autism in diagnosis and access to care, and familial and socioeconomic correlates of elevated rates of comorbid ID. BrightcoveDefaultPlayer10.1542/6158618575001PEDS-VA_2019-3629Video AbstractOBJECTIVES:African American (AA) children affected by autism spectrum disorder (ASD) experience delays in diagnosis and obstacles to service access, as well as a disproportionate burden of intellectual disability (ID) as documented in surveillance data recently published by the US Centers for Disease Control and Prevention. Our objective in this study was to analyze data from the largest-available repository of diagnostic and phenotypic information on AA children with ASD, and to explore the wide variation in outcome within the cohort as a function of sociodemographic risk and specific obstacles to service access for the purpose of informing a national approach to resolution of these disparities.METHODS:Parents of 584 AA children with autism consecutively enrolled in the Autism Genetic Resource Exchange across 4 US data collection sites completed event history calendar interviews of the diagnostic odysseys for their children with ASD. These data were examined in relation to developmental outcomes of the children with autism and their unaffected siblings.RESULTS:The average age of ASD diagnosis was 64.9 months (49.6), on average 42.3 months (+/- 45.1) after parents' first concerns about their children's development. The relationship between timing of diagnosis and ASD severity was complex, and ID comorbidity was not predicted in a straightforward manner by familial factors associated with cognitive variation in the general population.CONCLUSIONS:These findings document significant opportunity to expedite diagnosis, the need to further understand causes of ID comorbidity, and the necessity to identify effective approaches to the resolution of disparities in severity-of-outcome for AA children with autism.

Impairment in reciprocal social behavior (RSB), an essential component of early social competence, clinically defines autism spectrum disorder (ASD). However, the behavioral and genetic architecture of RSB in toddlerhood, when ASD first emerges, has not been fully characterized. We analyzed data from a quantitative video-referenced rating of RSB (vrRSB) in two toddler samples: a community-based volunteer research registry (n = 1,563) and an ethnically diverse, longitudinal twin sample ascertained from two state birth registries (n = 714). Variation in RSB was continuously distributed, temporally stable, significantly associated with ASD risk at age 18 months, and only modestly explained by sociodemographic and medical factors (r2 = 9.4%). Five latent RSB factors were identified and corresponded to aspects of social communication or restricted repetitive behaviors, the two core ASD symptom domains. Quantitative genetic analyses indicated substantial heritability for all factors at age 24 months (h2 ≥ .61). Genetic influences strongly overlapped across all factors, with a social motivation factor showing evidence of newly-emerging genetic influences between the ages of 18 and 24 months. RSB constitutes a heritable, trait-like competency whose factorial and genetic structure is generalized across diverse populations, demonstrating its role as an early, enduring dimension of inherited variation in human social behavior. Substantially overlapping RSB domains, measurable when core ASD features arise and consolidate, may serve as markers of specific pathways to autism and anchors to inform determinants of autism's heterogeneity.

The characterizing features of autism spectrum disorder (ASD) are continuously distributed in nature; however, prior twin studies have not systematically incorporated this knowledge into estimations of concordance and discordance. We conducted a quantitative analysis of twin–twin similarity for autistic trait severity in three existing data sets involving 366 pairs of uniformly-phenotyped monozygotic (MZ) twins with and without ASD. Probandwise concordance for ASD was 96%; however, MZ trait correlations differed markedly for pairs with ASD trait burden below versus above the threshold for clinical diagnosis, with R 2 s on the order of 0.6 versus 0.1, respectively. Categorical MZ twin discordance for ASD diagnosis is rare and more appropriately operationalized by standardized quantification of twin–twin differences. Here we provide new evidence that although ASD itself is highly heritable, variation-in-severity of symptomatology above the diagnostic threshold is substantially influenced, in contrast, by non-shared environmental factors which may identify novel targets of early ASD amelioration.

The preponderance of causal influence on total population attributable risk for autism is polygenic in nature, but it is not known how such liability engenders the development of the syndrome. In 348 epidemiologically ascertained toddler twins, we explored associations between autistic traits and three robust, highly heritable predictors of familial autism recurrence: variation in attention, motor coordination, and parental autistic trait burden. We observed that these predictors—despite collectively accounting for over one third of variance in clinical recurrence—are genetically independent in early childhood, and jointly account for a comparable share of inherited influence on early reciprocal social behavior in the general population. Thus, combinations of what are otherwise discrete, inherited behavioral liabilities—some not specific to autism—appear to jointly mediate common genetic risk for autism. Linking genetic variants and neural signatures to these independent traits prior to the onset of the development of autism will enhance understanding of mechanisms of causation in familial autistic syndromes. Moreover, ongoing biomarker discovery efforts will benefit from controlling for the effects of these common liabilities, which aggregate in individuals with autism but are also continuously distributed in “controls”. Finally, early inherited liabilities that participate in the early ontogeny of autistic syndromes represent parsimonious intervention targets for polygenic forms of the condition, and represent candidate trans-diagnostic endophenotypes of potential relevance to a diversity of neuropsychiatric syndromes.

Deficits in reciprocal social behavior are a characterizing feature of autism spectrum disorder (ASD). Autism-related variation in reciprocal social behavior (AVR) in the general population is continuously distributed and highly heritable—a function of additive genetic influences that overlap substantially with those which engender clinical autistic syndromes. This is the first long-term prospective study of the stability of AVR from childhood through early adulthood, conducted via serial ratings using the Social Responsiveness Scale, in a cohort-sequential study involving children with ASD, other psychiatric conditions, and their siblings ( N  =   602, ages = 2.5–29). AVR exhibits marked stability throughout childhood in individuals with and without ASD.

Although widely conceived as distinct conditions, higher-functioning autism spectrum disorder (ASD) and schizoid personality disorder (schizoid PD) share similar clinical symptomatology. This study explored the relationship between the two disorders by collecting extensively validated measures of autistic trait burden (Social Responsive Scale, Second Edition) and schizoid PD affectation (Diagnostic Interview for Genetic Studies) from clinically ascertained verbal males with and without autism ages 12 to 25 years (N = 72) via parent, teacher, and self-report. Although only a small minority of adolescents with ASD met full diagnostic criteria for schizoid PD, participants with ASD endorsed a continuous distribution of schizoid PD traits that reflected a pronounced pathological shift in comparison with those in the control group, with one half of ASD males experiencing three or more Diagnostic and Statistical Manual of Mental Disorders, 4th Edition schizoid PD criterion items "often" or "almost always." Results suggest significant amplification of schizoid PD trait burden in adolescents with ASD. ASD-specific interventions should be considered for patients with schizoid PD with premorbid histories of ASD.

ASD is characterized primarily by early-onset deficits in reciprocal social behavior (RSB). Autism-related variation in RSB (AVR) is highly heritable and continuously distributed in the general population, with ASD representing the pathological tail of that continuum. Although the lifelong stability of ASD has been established, the course of AVR across its entire range of variation has yet to be examined. Therefore, this cohort-sequential longitudinal study was undertaken to evaluate the stability of AVR over the course of childhood and early adulthood across the entire range of variation in which it manifests itself in nature.

Deficits in reciprocal social behavior are a characterizing feature of autism spectrum disorder (ASD). Autism‐related variation in reciprocal social behavior (AVR) in the general population is continuously distributed and highly heritable—a function of additive genetic influences that overlap substantially with those which engender clinical autistic syndromes. This is the first long‐term prospective study of the stability of AVR from childhood through early adulthood, conducted via serial ratings using the Social Responsiveness Scale, in a cohort‐sequential study involving children with ASD, other psychiatric conditions, and their siblings (N = 602, ages = 2.5–29). AVR exhibits marked stability throughout childhood in individuals with and without ASD.

Reciprocal social behavior (RSB), an early-emerging capacity to engage in social contingency-which is foundational for both social learning and social competency-is hypothesized to be disrupted in autism spectrum disorder (ASD). The ability to quantify the full range of RSB during the toddler period, when core symptoms of ASD often arise, is pivotal for evaluating early risk for ASD, characterizing social development, and tracking response to early interventions. However, important parameters of variation in RSB-especially prior to the development of verbal language-can be nuanced and difficult to characterize using questionnaire-based methods. To address this challenge, we developed a system for measuring quantitative variation in RSB in toddlers (ages 18 - 30 months) that incorporated not only standard questionnaire data from caregivers but also a novel set of video-referenced items, through which a respondent compares the behavior of a subject to that observed in a short video of a young child manifesting a highly competent level of social communication. Testing of this measure in a general population sample of twins confirmed that both the video-referenced items and the RSB Total Score (video-referenced items plus non-video-referenced items) displayed unimodal, continuous distributions, strong internal consistency, marked preservation of individual differences, and extremely high heritability. In addition, video-referenced items were particularly sensitive to quantifying incremental changes in social communication, a major element of RSB, over the course of early childhood development. Scores on the vrRSB clearly differentiated children with and without ASD and these data comprise an initial validation of this promising method for quantifying early RSB-cross-sectionally, over time, and as a function of early intervention.

We previously demonstrated that a video-referenced rating of reciprocal social behavior (vrRSB), a novel toddler measure in which caregivers rate their child’s level of RSB compared with a typically developing video anchor, shows strong psychometric properties and marked impairments of RSB in toddlers with ASD. Here, we compare heritability of total RSB scores at ages 18 and 36 months; examine longitudinal stability of RSB scores from 18 to 36 months of age, when ASD frequently manifests; and estimate 18-month heritability of social visual engagement (SVE), a quantitative eye-tracking measure that has been shown to predict ASD in infancy.

Background Recent studies have indicated that quantitative autistic traits (QATs) of parents reflect inherited liabilities that may index background genetic risk for clinical autism spectrum disorder (ASD) in their offspring. Moreover, preferential mating for QATs has been observed as a potential factor in concentrating autistic liabilities in some families across generations. Heretofore, intergenerational studies of QATs have focused almost exclusively on Caucasian populations—the present study explored these phenomena in a well-characterized Hispanic population. Methods The present study examined QAT scores in siblings and parents of 83 Hispanic probands meeting research diagnostic criteria for ASD, and 64 non-ASD controls, using the Social Responsiveness Scale-2 (SRS-2). Ancestry of the probands was characterized by genotype, using information from 541,929 single nucleotide polymorphic markers. Results In families of Hispanic children with an ASD diagnosis, the pattern of quantitative trait correlations observed between ASD-affected children and their first-degree relatives (ICCs on the order of 0.20), between unaffected first-degree relatives in ASD-affected families (sibling/mother ICC = 0.36; sibling/father ICC = 0.53), and between spouses (mother/father ICC = 0.48) were in keeping with the influence of transmitted background genetic risk and strong preferential mating for variation in quantitative autistic trait burden. Results from analysis of ancestry-informative genetic markers among probands in this sample were consistent with that from other Hispanic populations. Conclusions Quantitative autistic traits represent measurable indices of inherited liability to ASD in Hispanic families. The accumulation of autistic traits occurs within generations, between spouses, and across generations, among Hispanic families affected by ASD. The occurrence of preferential mating for QATs—the magnitude of which may vary across cultures—constitutes a mechanism by which background genetic liability for ASD can accumulate in a given family in successive generations.

This study examined sensory responsiveness in unaffected siblings of children with autism spectrum disorder (ASD) and associations between sensory responsiveness and social severity. Sensory Profile Caregiver Questionnaires and Social Responsiveness Scales were completed by parents of 185 children between age 4 and 10.95 years. Significant differences were found between participants with ASD and controls, and between participants with ASD and unaffected siblings for all sensory quadrants and domains, but not between controls and unaffected siblings. Social responsiveness scores were significantly correlated with scores from most sensory profile categories. Sensory responsiveness as an endophenotype of ASD is not indicated from these findings; however, studies with larger numbers of unaffected siblings and controls are needed to confirm the null hypothesis.

ImportanceRecent reports have demonstrated a higher incidence of autism spectrum disorder (ASD) and substantially elevated autistic trait burden in individuals with neurofibromatosis type 1 (NF1). However, important discrepancies regarding the distribution of autistic traits, sex predominance, and association between ASD symptoms and attentional problems have emerged, and critical features of the ASD phenotype within NF1 have never been adequately explored. Establishing NF1 as a monogenic cause for ASD has important implications for affected patients and for future research focused on establishing convergent pathogenic mechanisms relevant to the potential treatment targets for ASD.ObjectiveTo characterize the quantitative autistic trait (QAT) burden in a pooled NF1 data set.Design, Setting, and ParticipantsAnonymized, individual-level primary data were accumulated from 6 tertiary referral centers in the United States, Belgium, United Kingdom, and Australia. A total of 531 individuals recruited from NF1 clinical centers were included in the study.Main Outcomes and MeasuresDistribution of ASD traits (Social Responsiveness Scale, second edition [SRS-2], with T scores of ≥75 associated with a categorical ASD diagnosis); attention-deficit/hyperactivity disorder (ADHD) traits (4 versions of Conners Rating Scale, with T scores of ≥65 indicating clinically significant ADHD symptoms); ASD symptom structure, latent structure, base rate derived from mixture modeling; and familiality.ResultsOf the 531 patients included in the analysis, 247 were male (46.5%); median age was 11 years (range, 2.5-83.9 years). QAT scores were continuously distributed and pathologically shifted; 13.2% (95% CI, 10.3%-16.1%) of individuals scored within the most severe range (ie, above the first percentile of the general population distribution) in which the male to female ratio was markedly attenuated (1.6:1) relative to idiopathic ASD. Autistic symptoms in this NF1 cohort demonstrated a robust unitary factor structure, with the first principal component explaining 30.9% of the variance in SRS-2 scores, and a strong association with ADHD symptoms (r = 0.61). Within-family correlation for QAT burden (intraclass correlation coefficient, 0.73 in NF1-affected first-degree relatives) exceeded that observed in the general population and ASD family samples.Conclusions and RelevanceThis study provides confirmation that the diversity of mutations that give rise to NF1 function as quantitative trait loci for ASD. Moreover, the within-family correlation implicates a high degree of mutational specificity for this associated phenotype. Clinicians should be alerted to the increased frequency of this disabling comorbidity, and the scientific community should be aware of the potential for this monogenic disorder to help elucidate the biological features of idiopathic autism.

Objective To examine the distribution of quantitative autistic traits (QATs) in an independent neurofibromatosis type I (NF1) sample, the relationships between QAT, sex, and attention deficit hyperactivity disorder (ADHD) symptomatology, and to explore evidence for QAT mutational specificity within families. Study design Age-appropriate versions of the Social Responsiveness Scale, second edition and the Conners Adult ADHD Rating Scales were completed for 103 patients with NF1 from the Washington University Neurofibromatosis Center. Results Patients with NF1 exhibited a pathologically shifted unimodal distribution for QAT. Forty-four percent of the subjects exhibited a QAT burden at or above 1 SD from the population mean; 13% scored at or above the extreme first percentile of the general population distribution. Elevations in ADHD symptomatology exhibited a distinct bimodal distribution; however, mean ADHD index scores were equivalent in patients who had been diagnosed in the community with ADHD compared with those who had not. We observed striking within-family associations for QAT, reflected by an Social Responsiveness Scale, second edition intraclass correlation of 0.77 in pairings of first degree relatives with NF1. Conclusions Impairments in reciprocal social behavior and attention affect a large proportion of patients with NF1 throughout life and are often clinically unrecognized. Further exploration of genotype-phenotype correlation is strongly warranted for the purpose of gaining insights into mechanisms by which specific mutational variations in the NF1 gene may influence autistic trait severity.

BACKGROUND:Reciprocal social behavior (RSB) is a developmental prerequisite for social competency, and deficits in RSB constitute a core feature of autism spectrum disorder (ASD). Although clinical screeners categorically ascertain risk of ASD in early childhood, rapid methods for quantitative measurement of RSB in toddlers are not yet established. Such measurements are critical for tracking developmental trajectories and incremental responses to intervention.METHODS:We developed and validated a 20-min video-referenced rating scale, the video-referenced rating of reciprocal social behavior (vrRSB), for untrained caregivers to provide standardized ratings of quantitative variation in RSB. Parents of 252 toddler twins [Monozygotic (MZ) = 31 pairs, Dizygotic (DZ) = 95 pairs] ascertained through birth records, rated their twins' RSB at two time points, on average 6 months apart, and completed two developmental measures, the Modified Checklist for Autism in Toddlers (M-CHAT) and the MacArthur Communicative Development Inventory Short Form (MCDI-s).RESULTS:Scores on the vrRSB were fully continuously distributed, with excellent 6-month test-retest reliability ([intraclass correlation coefficient] ICC = 0.704, p < .000). MZ twins displayed markedly greater trait concordance than DZ twins, (MZ ICC = 0.863, p < .000, DZ ICC = 0.231, p < .012). VrRSB score distributions were highly distinct for children passing versus failing the M-CHAT (t = -8.588, df = 31, p < .000), incrementally improved from 18-24 months, and were inversely correlated with receptive and expressive vocabulary on the MCDI-s.CONCLUSIONS:Like quantitative autistic trait ratings in school-aged children and adults, toddler scores on the vrRSB are continuously distributed and appear highly heritable. These ratings exhibited minimal measurement error, high inter-individual stability, and developmental progression in RSB as children matured from 18-24 months, supporting their potential utility for serially quantifying the severity of early autistic syndromes over time and in response to intervention. In addition, these findings inform the genetic-environmental structure of RSB in early typical development.