A groundbreaking study from the UC San Diego School of Medicine challenges long-held assumptions about sex differences in autism during early childhood. While males are diagnosed with autism spectrum disorder (ASD) at more than four times the rate of females, researchers conducting the largest and most comprehensive assessment of toddlers to date have found no significant clinical differences in autistic traits between the sexes at the time of initial diagnosis. Published in Nature Human Behaviour on May 26, 2025, this extensive research opens new avenues for understanding autism’s developmental trajectory and refining early intervention strategies.
The study evaluated over 2,500 toddlers aged 12 to 48 months, including approximately 1,500 children diagnosed with autism, 600 typically developing peers, and 475 children experiencing developmental delays. This robust sample size marks a significant advance over previous studies, which often involved fewer than 100 participants and produced inconsistent findings regarding sex-based variations in autism. The cohort underwent detailed clinical evaluations covering language acquisition, cognitive capabilities, motor skills, social communication, and core autism features such as repetitive behaviors, all conducted by licensed clinical psychologists at the UC San Diego Autism Center of Excellence. Innovative eye-tracking technology further quantified social attention patterns, providing a multi-dimensional profile of early childhood development in autism.
Contrary to prevailing theories proposing inherent sex differences in autistic presentation, the researchers found remarkable parity between male and female autistic toddlers across nearly all assessed domains. The sole exception involved a slight parent-reported advantage in daily living skills among females, including activities such as dressing and feeding themselves. Moreover, when toddlers were categorized into low, medium, and high-ability subtypes spanning the autism spectrum through sophisticated clustering algorithms, no meaningful sex-specific divergences emerged within these classifications. Longitudinal follow-ups also revealed parallel developmental trajectories for males and females with autism between one and four years of age, suggesting that sex-linked phenotypic differences, if present, do not manifest during this critical early window.
This lack of sex differences amidst toddlers with autism sharply contrasts with the pronounced variations observed between typically developing male and female children in the same age range. Consistent with established developmental neuroscience literature, females without autism exhibited accelerated growth in social skills, language development, and adaptive daily functioning when compared to their male counterparts. These normative sex-related developmental trends underscore the unique nature of the autism phenotype, which appears to be largely independent of sex early in life.
Senior author Dr. Karen Pierce, a prominent neuroscientist and director of the UC San Diego Autism Center of Excellence, emphasizes the significance of these findings. She highlights that prior studies reporting sex differences in autistic toddlers might have been confounded by insufficient sample sizes, methodological biases, and narrow assessment scopes. The new evidence calls for a reconsideration of assumptions about intrinsic biological sex effects in autism at the youngest ages. Alternatively, sex differences may emerge later due to complex interactions between biology, socialization processes, and environmental factors, necessitating longitudinal research to disentangle these influences over time.
From a neurodevelopmental standpoint, these results challenge hypotheses positing fundamentally different autistic phenotypes based on sex-specific neurobiology at disease onset. Instead, the study advocates for conceptualizing autism heterogeneity along dimensions of ability and symptom severity rather than sex categories. This paradigm shift has profound implications for precision medicine approaches aiming to tailor early interventions to the distinct needs of each child, regardless of gender. Enhancing early language acquisition and communication skills remains pivotal for improving functional outcomes and facilitating societal participation.
The employment of eye-tracking metrics to assess social attention also contributes a novel objective tool to autism diagnostics. Quantitative measures of gaze patterns towards social stimuli provide insight into early neurocognitive processes underpinning social communication deficits in autism. The lack of sex differences in such fundamental neural markers further reinforces the notion of a shared developmental pathway during toddlerhood for males and females on the spectrum.
The unprecedented scale and methodological rigor of this study set a new standard for developmental autism research. By encompassing multiple domains of functionality and tracking trajectories longitudinally, the research team circumvented limitations of prior work, providing a comprehensive clinical portrait of sex similarities and differences—or rather, the surprising absence of them. These findings invite a reexamination of diagnostic criteria and intervention frameworks, steering focus away from gender biases and towards the nuanced, individualized profiles of autistic toddlers.
Moreover, the research raises critical questions about when and how sex differences in autism, reported in older children and adults, begin to arise. Are these differences emergent properties of developmental processes, shaped by cultural and biological forces over time? Answering these questions will require longitudinal cohort studies that follow children well beyond toddlerhood, integrating neuroimaging, genetic, and environmental data to chart the unfolding autism phenotype.
Autism’s high heritability underlines the importance of early detection and intervention, as emphasized by Dr. Pierce. Identifying robust subtypes within the autism spectrum based on clinical abilities rather than sex could optimize therapeutic targeting and improve the quality of life for affected children. The goal remains to equip every child—regardless of their sex—with the communication tools and adaptive skills necessary to fulfill their potential and contribute meaningfully to society.
As early intervention programs refine their focus based on emerging evidence, the hope is to mitigate the lifelong challenges associated with autism. This study represents a pivotal step in reshaping the scientific and clinical discourse around autism and sex differences, emphasizing the shared neurodevelopmental trajectories that unite children on the spectrum during their earliest years.
The research was conducted by a collaborative team including first author Sanaz Nazari, senior authors Eric Courchesne and Karen Pierce, and several other experts affiliated with UC San Diego and Istituto Italiano di Tecnologia. Financial support from multiple National Institutes of Mental Health grants underscores the study’s significance within the broader landscape of neurodevelopmental disorder research.
In sum, this landmark investigation dismantles prevailing assumptions about early sex differences in autism, demonstrating parity across a broad range of clinical features in toddlers and highlighting the importance of large, well-powered longitudinal studies. It sets a new course for developmental autism research and offers hope for more inclusive, precise, and effective early diagnosis and intervention strategies.
Subject of Research: Autism Spectrum Disorder, Early Childhood Development, Sex Differences in Autism
Article Title: [Not explicitly provided but can be inferred as:] "No Clinical Sex Differences Found in Autistic Traits of Toddlers at Initial Diagnosis"
News Publication Date: May 26, 2025
Web References:
- Study published in Nature Human Behaviour: https://www.nature.com/articles/s41562-025-02132-6
- DOI: 10.1038/s41562-025-02132-6
References:
- Nazari, S., Pierce, K., Courchesne, E., et al., 2025. (Full citation as per Nature Human Behaviour)
Image Credits: Not provided
Keywords: Autism, Early Diagnosis, Toddler Development, Sex Differences, Neurodevelopment, Autism Spectrum Disorder, Early Intervention
