Marina Sarris

Date Revised: May 31, 2023

One of the first clues that Sarah was different was her walking. She did not walk until she was 20 months old, about 9 months later than her twin sister.

“My pediatrician told me, ‘Oh, don’t worry. They all walk eventually,'” recalls Sarah’s mother, Alycia Halladay. But those words did not reassure her. “It wasn’t just late walking. There were other clues there,” she says.

About two years later, Sarah was diagnosed with autism, a social communication condition that is not known to cause late walking. But new research suggests that Sarah’s story is not unusual ─ at least for girls on the autism spectrum. Researchers examining the differences between autistic boys and girls found that girls are more likely to walk later than boys, according to their parents.¹

They also found other differences that could shed light on one of autism’s mysteries: Why are girls diagnosed later ─ and less often ─ than boys? Boys outnumber girls by almost 4 to 1, and boys are diagnosed, on average, four months earlier than girls.

Researchers who made those discoveries used a relatively new tool: data from the largest collection of autistic girls and women ─ participants in an autism study called SPARK.

Orphans in the Autism World

Girls were considered “orphans” in the world of autism research, as psychologist Ami Klin, Ph.D., once said. Researchers could not find enough girls to include in autism studies. As a result, much of what we know about autism comes from research on boys.

This male focus carried over into our perceptions of autism. Some professionals would use examples geared toward boys when screening for autism. They might ask parents if their child had an intense interest (a sign of autism) in motors or trains, rather than dolls or animals. Some researchers wondered if autism was being missed in some girls, particularly those with milder traits.

The SPARK autism study has helped broaden the focus on females by enrolling 31,000 of them, along with 92,000 males, since 2016. Researchers now have data on an unprecedented number of autistic girls and women, whom they can also invite to join their studies.

In two recent studies, researchers analyzed data from more than 3,500 girls and 14,000 boys in SPARK.

One research team found that SPARK parents noticed delays in their children’s development around the same age, but what they noticed often differed by the child’s sex.¹ Parents of girls were more likely to report motor delays, such as late walking, while the parents of boys were more likely to report speech delays, repetitive behaviors, and loss of skills.

This difference may be important to diagnosis. Motor delays are not a trait of autism, while speech delays and repetitive behaviors certainly are. As the researchers point out, a boy’s language delays may make his autism diagnosis “more straightforward” to a doctor or psychologist. But a girl’s motor delays may hinder or delay her autism diagnosis.¹

What Does it Mean When a Baby Sits Up or Walks Late?

Motor delays are common in babies and toddlers, according to the American Academy of Pediatrics. Many children catch up on their own, but for others, the delays may be symptoms of cerebral palsy, overall developmental delay, or a condition affecting the nervous system or muscles.²

Babies often walk around their first birthdays, before they speak, on average. “So we may be seeing delays in girls a bit earlier in development, before we’re usually thinking about autism,” explains RUSH Medical College instructor Emily F. Dillon, Ph.D., a psychologist who led one of the two studies using SPARK data.

It’s Not Just Walking

Sarah’s motor delays – walking 8 months later than average – were the most obvious, but she also had speech and social delays, says her mother, Alycia Halladay, Ph.D., chief science officer of the Autism Science Foundation.

In fact, Sarah scored in the autism range on a screening tool before she turned 2, Halladay says. But that did not speed up her journey to an eventual autism evaluation and diagnosis.

Whether or not motor delays are an early clue of autism, Halladay says, they can complicate toddlers’ development by limiting their ability to explore their world. “If you’re not walking on time, then that affects the way you interact with your environment and that can cascade into social interaction issues,” says Halladay, who joined SPARK with Sarah.

But that’s not the only difference among autistic girls and boys that researchers have found using SPARK data.

Anxiety, ADHD, and Autism

The girls were more likely to have anxiety, and less likely to have attention-deficit/hyperactivity disorder, than the boys.³ That is also true for girls who don’t have autism.

In general, people with autism, regardless of sex, are more likely to have anxiety and some conditions that affect mental health or learning than people who do not have autism.

Having another diagnosis may delay getting an autism diagnosis for both boys and girls, according to a study of 25,000 children ages 6 to 18 in SPARK. Those with anxiety disorder or ADHD, especially girls, were diagnosed with autism later than children who had autism only. That was particularly true of girls with both anxiety and ADHD. They received their autism diagnoses at age 7, on average, while boys with the same two conditions were diagnosed with autism at 6.³

By comparison, boys and girls who have autism only were diagnosed an average of two to three years earlier, at age 4.

Late Diagnosis, Late Services

A later autism diagnosis means that those girls could miss out on services that are geared to autism, from early intervention in the toddler years to social communication therapies in elementary school and beyond.

Even the average four-month delay in autism diagnoses between girls and boys overall may have an impact, Dillon says. “It may mean the difference between getting early intervention services, which are wonderful if given at the right time, or not getting them,” she says. Children with developmental delays are eligible for early intervention from birth until their third birthday, and for preschool special education services after that.

How Might Having Anxiety and ADHD Delay an Autism Diagnosis?

Sometimes another diagnosis can “overshadow” autism. Overshadowing occurs when doctors or teachers believe that a child’s autism traits are actually signs of another diagnosis they already have. “Children can have social and communication problems, but it’s attributed to their anxiety or ADHD,” explains neuropsychologist Ericka Wodka, Ph.D., who led the study of how those conditions affect children in SPARK.

“A child with ADHD can be impulsive and may not appreciate personal space; they can be inattentive and miss social cues. Also, if a child has anxiety, they can be withdrawn socially,” says Wodka, clinical director of Kennedy Krieger Institute’s Center for Autism and Related Disorders in Maryland. If professionals assume a girl’s anxiety and ADHD are causing her social problems, that could delay an evaluation for autism.

An Unexpected Finding for an Autism Symptom

Although motor delays, anxiety, and ADHD are not symptoms of autism, Wodka’s team found an unexpected difference in a core autism trait: repetitive and restrictive behavior. These behaviors include lining up objects, flapping hands, insisting that routines be the same, and repeating certain words over and over.

Unlike previous studies, Wodka’s study found that 6- to 11-year-old autistic girls have more repetitive behaviors than boys. That was especially true for the girls who also have ADHD and anxiety.

Other studies have come to the opposite conclusion: girls have fewer or less obvious repetitive behaviors than boys, which could explain why they might escape diagnosis.^4,5

The difference may be due to the tools used to measure repetitive behaviors. Wodka’s team used a different tool, one that measures these behaviors in more depth, than some other studies have used. “Other research suggests that girls report more insistence on sameness, compulsive behaviors like hoarding, as well as self-injurious behavior such as pulling hair and scratching themselves,” Wodka says. The measure her team used may have picked up more of these behaviors than those used in other studies, she says.

Teenage girls in SPARK, however, were similar to their male counterparts in repetitive behaviors. If repetitive behaviors are soothing, as some believe, then perhaps older girls have begun to use other ways of coping, Wodka says.

Are Girls and Women Hiding Some Autism Traits?

Could teen girls be hiding their repetitive behaviors, too? Some research suggests that by adulthood, women are more likely to hide various autism traits than males, which is called camouflaging.

Dillon says she worked with autistic preteen and teen girls who hid repetitive and self-soothing behaviors. For example, one girl ripped up toilet paper in the bathroom, and another banged her head in private, where no one would notice, she says.

Other research of SPARK teens suggests that girls are more likely to try to hide their autism by focusing on the social impressions they make on others.⁶

Mary Lipiec, a SPARK participant who was diagnosed at age 7, says she believes many women can camouflage their autism. “Generally, in our society there’s a difference in the way we socialize boys and girls, and I think because of the way we socialize young girls, they learn to mask their symptoms in ways boys don’t always catch on to,” she says. “That is one of many ways girls can fall through the cracks.”

Wodka and Dillon say that they hope their research into sex differences in autism will help improve our ability to diagnose and understand girls on the spectrum. “These differences are small and subtle, but it should be a reminder for us to just think a little harder and ask a few more questions a little earlier for girls than we might do otherwise,” Dillon says.

Interested in joining SPARK? Here’s what you should know.

Photo credit: Dominique Stilletti

How do the needs of autistic girls and women differ from those of boys and men? Watch this important video to learn more.

References

1. Dillon E.F. et al. J. Autism Dev. Disord. 53, 1305-1318 (2023) PubMed
2. Noritz G.H. et al. Pediatrics 131, e2016-e2027 (2013) PubMed
3. Wodka E.L. et al. Clin. Neuropsychol. 36, 1069-1093 (2022) PubMed
4. Kaat A.J. et al. J. Child Psychol. Psychiatry 62, 97-106 (2021) PubMed
5. Mandy W. et al. J. Autism Dev. Disord. 42, 1304-1313 (2012) PubMed
6. Jorgenson C. et al. J. Autism Dev. Disord. 50, 4344-4355 (2020) PubMed