A groundbreaking large-scale study conducted at the University of California San Diego School of Medicine has challenged long-standing assumptions about sex differences in autism spectrum disorder (ASD) during early childhood. While males have been historically diagnosed with autism at rates exceeding those of females by a factor of four, this new research reveals a striking similarity in autistic traits between male and female toddlers at the time of their initial diagnosis. Published in Nature Human Behaviour in late May 2025, the findings have profound implications for the future of diagnosing and understanding autism in its earliest stages.
The researchers undertook an extensive evaluation of over 2,500 toddlers aged between 12 and 48 months over two decades, from 2002 to 2022. Among these participants, 1,500 toddlers were diagnosed with autism, 600 were typically developing, and 475 exhibited developmental delays unrelated to autism. Utilizing a comprehensive battery of 19 assessments, the team meticulously measured language development, social communication, motor skills, cognitive function, and core autistic behaviors including repetitive actions. Notably, state-of-the-art eye tracking technology was employed to assess social attention -- an advanced technique that allows direct quantification of how infants and toddlers attend to social stimuli, a key marker in autism.
Across this vast and varied dataset, the UC San Diego team uncovered an unexpected absence of clinically meaningful differences in autistic symptoms between males and females at diagnosis. The only exception emerged in a subtle advantage for females in daily living skills, such as self-care routines like dressing and feeding, as reported by parents. Even when the autistic toddlers were stratified into robustly derived subtypes of low, medium, and high ability based on objective metrics of functioning, no significant sex divergence was found within these groups. Furthermore, longitudinal tracking of developmental trajectories from infancy through toddlerhood reaffirmed consistent symptom profiles between the sexes.
These results stand in contrast to earlier smaller studies, many with fewer than 100 participants, which suggested sex-based differences in autism traits. The UC San Diego study is unprecedented not only in sample size but also in thoroughness, being one of the few investigations to analyze autism at such a formative age by licensed clinical psychologists in a controlled research environment. Karen Pierce, Ph.D., a leading neuroscientist and director of the Autism Center of Excellence at UCSD, explained that the team was somewhat surprised by the homogeny between sexes given prior expectations grounded in limited earlier findings.
Interestingly, the investigators detected marked sex differences among typically developing toddlers, with females outpacing males on more than half of the developmental tests -- particularly those assessing social competence, language acquisition, and adaptive behaviors. This acceleration in typically developing females relative to males aligns with a broad body of developmental psychology literature. Such contrast between typical development and autism within the same cohort underscores that sex differences in general early childhood development do not necessarily translate to differences within autism.
The absence of detectable sex differences in toddlers newly diagnosed with autism offers two intriguing lines of interpretation. One possibility is that previous smaller studies, which reported sex differences, may have been influenced by limitations such as sampling bias or insufficient analytical granularity. Alternatively, it is conceivable that sex differences in autism do not manifest at initial onset but instead develop progressively over time due to complex interactions among biological factors and psychosocial influences including socialization patterns and environmental exposure.
Pierce and colleagues emphasize the need for longitudinal, high-powered studies that follow autistic children from toddlerhood through school age and beyond to clarify whether and how sex-related differences emerge later. Such research would help disentangle transient developmental phenomena from enduring sex-specific variations in symptom expression, treatment response, and functional outcomes.
The study also offers a nuanced view of autism heterogeneity, highlighting the utility of subtyping autistic individuals based on specific behavioral and cognitive profiles rather than sex. This shift towards biologically and clinically meaningful subtypes promises more targeted intervention strategies tailored to the underlying neurodevelopmental profiles of autistic children. By improving core skills such as language and communication at the earliest possible ages, clinicians and families can facilitate better adaptive functioning and social integration, optimizing each child's potential.
Autism's strong genetic underpinnings add urgency to refining early detection paradigms. These findings suggest that early screening and intervention protocols might benefit from focusing on nuanced symptom clusters discovered through sophisticated algorithmic methods rather than employing normative sex-based expectations. Enhanced diagnostic precision can inform personalized therapeutic approaches, ultimately increasing the efficacy of interventions and supporting lifelong outcomes.
Pierce remarked, "If you can enhance a toddler's communication abilities at the youngest ages, they will be better equipped to express their needs, engage socially, and contribute meaningfully to society." This emphasis on maximizing individual developmental trajectories resonates with contemporary views emphasizing personalized medicine in neurodevelopmental disorders.
Among the study's co-authors are Sanaz Nazari and Eric Courchesne, who contributed to the detailed analysis alongside a multidisciplinary team including Sara Ramos Cabo, Srinivasa Nalabolu, Cynthia Carter Barnes, and others at UC San Diego, as well as Michael V. Lombardo from the Istituto Italiano di Tecnologia. The research was supported by multiple grants from the U.S. National Institutes of Mental Health, underscoring its significance and the priority it holds within the scientific community.
In sum, this landmark study calls for a reevaluation of prevailing assumptions about sex disparities in autism during the critical window of early childhood diagnosis. It reframes clinical and research perspectives, advocating a focus on individual developmental profiles and trajectories rather than binary sex differences. The findings pave the way for innovative clinical practices and raise pivotal questions to guide future investigations into the intricate biology and development of autism spectrum disorder.
Subject of Research: Autism spectrum disorder in toddlers; sex differences in early childhood autism traits
Article Title: No Sex Differences in Early Autism Traits Among Toddlers at Diagnosis: Evidence from a Large-Scale Longitudinal Study
News Publication Date: May 26, 2025
Web References: https://www.nature.com/articles/s41562-025-02132-6; http://dx.doi.org/10.1038/s41562-025-02132-6
References: Nazari S., Courchesne E., Pierce K., et al. (2025). No Sex Differences in Early Autism Traits within a Large Cohort of Toddlers. Nature Human Behaviour. DOI: 10.1038/s41562-025-02132-6
Image Credits: University of California San Diego School of Medicine
Keywords: Autism, early diagnosis, toddlers, sex differences, developmental trajectories, language development, social attention, eye tracking, neurodevelopment, personalized intervention