No early sex differences found in autism traits among toddlers

Males are more than four times more likely to receive an autism diagnosis than females. But a new study by researchers at UC San Diego School of Medicine has found no clinical differences in autistic traits between the sexes in toddlers when they are first diagnosed with autism. The study was published in Nature Human Behavior on May 26, 2025. The findings have potential implications for early diagnosis and intervention for autistic children.

Between 2002 and 2022, the researchers assessed more than 2,500 male and female toddlers between 12 and 48 months of age. Of these toddlers, 1,500 were autistic, 600 were typically-developing, and 475 were developmentally delayed. The assessments included 19 different measures of language development, social and motor skills, core autism traits such as repetitive behaviors, cognitive skills, and other developmental characteristics. The study also examined social attention using eye tracking technology. All assessments were conducted at a single site – the UC San Diego Autism Center of Excellence – by licensed clinical psychologists.

The researchers found:

• No clinical differences between male and female autistic toddlers on all but one of the metrics. The sole exception was a measure of daily living skill development based on parent reporting, such as getting dressed and feeding themselves, which females scored just slightly higher on than males.
• When clustered into low, medium and high-ability subtypes across the autism spectrum based on robust state-of-the-art methods, there were also no major clinical differences between males and females within these subtypes.
• No clinical differences between the sexes between 12 and 48 months when the researchers followed the trajectory of development in autistic toddlers over time.
• Few sex differences between developmentally delayed toddlers.

A number of previous studies with fewer than 100 participants each have shown differences between the sexes in autistic toddlers. However, the current study is the largest and most comprehensive of its kind to date, and one of very few studies to assess children with autism at a very early age, according to senior author Karen Pierce, Ph.D., professor of neurosciences and  director of the Autism Center of Excellence at UC San Diego School of Medicine.

There is no consensus in the field about whether females are more or less impacted than males, and that is probably because there haven't been really large-scale studies at the earliest ages.  Based on previous small studies, we had anticipated there would be some sex differences. So we were a little bit surprised to find nothing at all."

Karen Pierce, Ph.D., professor of neurosciences and director of the Autism Center of Excellence at UC San Diego School of Medicine

The researchers did, however, find sex differences between typically developing female and male toddlers, with females performing at significantly higher levels than males on more than half of the tests, especially those measuring social skills, language development, and daily living skills.

"This is consistent with the literature; female toddlers seem to develop slightly faster than males in terms of their language ability and their social ability and how well they perform daily living skills – adaptive things for a two-year-old," said Pierce. "Typically developing females are accelerated in their development relative to males."

Pierce says the findings for autistic children of no clinical differences between males and females at the time of first onset of autism lead to two possible conclusions.

"One is that previous studies that report sex differences are wrong, perhaps due to small sample size, sampling bias, limited study measures, or other methodological issues," she said. "An alternative conclusion is that sex differences do not exist at the time of first onset, but instead emerge slowly at later ages, driven by psychosocial factors like socialization or differences in biology that may unfold across development."

To examine this alternative possibility, a high-quality, large-scale study that tracks autistic children from toddlerhood through school age and beyond would be required, according to Pierce.​

Autism is highly heritable, and Pierce says the findings have implications for understanding the development of the condition, enhancing early detection, and improving early intervention. Because the study found that toddlers with autism clustered into scientifically robust subtypes within the autism spectrum rather than by sex, she thinks it may be preferable to focus on these differences instead when seeking to understand clinical heterogeneity and the most appropriate interventions for each subtype, for example.

"If you can improve a toddler's language and communication at the youngest ages possible, then they're going to get their needs met better, and they're going to be able to contribute to society more effectively, because they can do whatever it is that they love to do," said Pierce. "It's really about having every child reach their full potential."

Additional authors of the study include first author Sanaz Nazari, second senior author Eric Courchesne, Sara Ramos Cabo, Srinivasa Nalabolu, Cynthia Carter Barnes, Charlene Andreason, Javad Zahiri, Ahtziry Esquivel, Steven J. Arias, Andrea Grzybowski, and Linda Lopez, all at UC San Diego School of Medicine; and Michael V. Lombardo, at Istituto Italiano di Tecnologia.

The study was funded, in part, by National Institutes of Mental Health (grants R01MH118879, R01MH080134, R01MH10446, R01MH121595, P50-MH081755, R01MH110558, R01DC016385).