Summary: A large, long-term study of more than 2,500 toddlers found no meaningful clinical differences in autistic traits between males and females at the time of early diagnosis. Over two decades, researchers used a wide range of standardized assessments—covering language, motor skills, cognitive functioning, adaptive behaviors, and social attention—to compare boys and girls diagnosed with autism in toddlerhood.
The only consistent difference observed was a small parent-reported advantage for girls in daily living skills. Taken together, the results suggest that sex-related differences in autism are minimal at first symptom onset and may appear later in development or arise from non-biological influences.
Key Facts:
- No early sex differences: Male and female toddlers with autism showed nearly identical clinical profiles across nearly all measures.
- One small exception: Parents reported slightly higher daily living skills for girls with autism.
- Typical development differences: Neurotypical girls outperformed boys in language, social skills, and adaptive behaviors.
Source: UCSD
Context: Although males are diagnosed with autism more than four times as often as females, this comprehensive study by UC San Diego School of Medicine challenges the idea that autistic toddlers display distinct clinical differences by sex at the time they first receive a diagnosis.
Published in Nature Human Behavior on May 26, 2025, the study analyzed data from 2002 to 2022 and evaluated 2,618 toddlers aged roughly 12 to 48 months. The cohort included about 1,500 children diagnosed with autism spectrum disorder (ASD), 600 who were typically developing, and 475 with other developmental delays.
Investigators applied 19 distinct clinical and developmental measures, spanning receptive and expressive language, motor skills, cognitive ability, repetitive behaviors, social interaction, and adaptive functioning. They also incorporated eye-tracking to quantify social attention. All evaluations were performed at a single center—the UC San Diego Autism Center of Excellence—by licensed clinical psychologists, ensuring consistency in testing and clinical ratings.
Main findings included:
- Across 18 of 19 measures, autistic boys and girls were indistinguishable on clinical assessments, including symptom severity, language scores, and eye-tracking measures of social attention.
- Children with autism clustered into low, medium, and high ability subtypes using robust clustering and machine learning approaches; within these subtypes, males and females again showed no clinically meaningful differences.
- Longitudinal tracking from 12 to 48 months revealed no emerging sex differences in developmental trajectories among children with autism during toddlerhood.
- Among children with non-autistic developmental delays, few sex differences were observed.
Previous reports suggesting sex differences in early autism often relied on smaller samples—typically fewer than 100 participants. Senior author Karen Pierce, Ph.D., director of the Autism Center of Excellence at UC San Diego School of Medicine, notes that the field lacked sufficiently large, early-age studies until now. The scale and breadth of this project make its findings particularly informative for early detection and intervention strategies.
Pierce explained two plausible interpretations: either earlier, smaller studies that reported sex differences may have been affected by limited sample size or methodological biases, or sex differences simply do not appear at first symptom onset and instead develop later through social or biological processes that unfold over time. She recommends a large, high-quality longitudinal study following autistic children from toddlerhood into school age to clarify which explanation is correct.
The research team also highlighted that typically developing girls in the sample consistently outperformed typically developing boys on many measures—especially language, social engagement, and adaptive daily living skills—echoing established developmental trends in early childhood.
Given that autistic toddlers clustered into clinically meaningful subtypes rather than by sex, the authors suggest research and clinical efforts may be better focused on subtype-specific characteristics to guide personalized early interventions. Targeting language and communication skills at the youngest ages remains a high priority: improving these domains can help children meet their needs sooner and participate more fully in learning and social environments.
Additional authors include Sanaz Nazari, Eric Courchesne, Sara Ramos Cabo, Srinivasa Nalabolu, Cynthia Carter Barnes, Charlene Andreason, Javad Zahiri, Ahtziry Esquivel, Steven J. Arias, Andrea Grzybowski, Linda Lopez from UC San Diego School of Medicine, and Michael V. Lombardo from Istituto Italiano di Tecnologia. The study received partial funding from the National Institutes of Mental Health (grants R01MH118879, R01MH080134, R01MH10446, R01MH121595, P50-MH081755, R01MH110558, R01DC016385).
About this autism research news
Author: Susanne Bard
Source: UCSD
Contact: Susanne Bard – UCSD
Image: The image is credited to Neuroscience News
Original Research: Open access. “Large-scale examination of early-age sex differences in neurotypical toddlers and those with autism spectrum disorder or other developmental conditions” by Karen Pierce et al., Nature Human Behavior.
Abstract
Large-scale examination of early-age sex differences in neurotypical toddlers and those with autism spectrum disorder or other developmental conditions
Autism spectrum disorder (ASD) is clinically diverse, and debate continues over whether boys and girls show different phenotypes. Identifying sex differences at the age of first symptom onset is essential for improving early detection, clarifying causes, and tailoring interventions. Using the Get SET Early program, the researchers analyzed a cohort of 2,618 toddlers (mean age ~27 months) with cross-sectional, longitudinal, and clustering methods, including statistical and machine learning approaches, across standardized and experimental measures such as eye tracking.
Results showed no significant sex differences in toddlers with ASD on 17 of 18 measures, including symptom severity measured by the Autism Diagnostic Observation Schedule, receptive and expressive language on the Mullen Scales of Early Learning, and social attention measured by the GeoPref eye-tracking test. In contrast, neurotypical girls outperformed boys on multiple measures. Subtyping into low, medium, and high clusters likewise produced minimal sex differences. Overall, findings indicate that phenotypic sex differences are minimal or absent in ASD at first symptom onset.