As we’ve discussed in this space before, much of the research related to autism has tended to focus on young boys and men with autism. One reason for this is simple statistics: autism is four to five times more likely to be diagnosed in boys than girls. And the numbers don’t lie, with 1 in 42 boys being diagnosed on the autism spectrum versus 1 in every 189 girls.

So why is autism apparently less likely to be diagnosed in young girls than it is in young boys? One simple reason is that it tends to take longer for girls to be diagnosed, with many girls not being diagnosed until at least elementary school.

According to Dr. Nicole Tartaglia, a developmental-behavioral pediatrician at Children’s Hospital of Colorado in Aurora, “There are a lot of reasons why girls are diagnosed later. The symptoms are usually less significant or evident earlier on, so it takes a little bit longer for people to see the symptoms.”

Not only are the symptoms less evident, they are also often extremely different than those of boys who are on the autism spectrum, making diagnoses more difficult. In the case of Ellie Paul, an 11 year-old girl in Denver, her brother Noah was diagnosed at the age of 4 years old while Ellie wasn’t diagnosed until she was nearly 6, even though she had been tested three times before her positive diagnosis.

Unlike her brother, Ellie always had good eye contact with others and did not display any of the developmental delays that her brother did, having started talking before she was even one year of age. However, it was Ellie’s lack of social interaction that caused her mom to consider that her daughter might be displaying a symptom of autism.

And according to a new study looking at gender differences in children with autism, the later diagnoses is in fact due to the different symptoms that girls with autism display. The study, authored by Dr. Paul Lipkin, director of the Interactive Autism Network at the Kennedy Krieger Institute in Baltimore, looked at data on people with autism using their directory of 50,000 people.

What they found was that in general, girls were diagnosed with autism far later than boys. On average, boys with a pervasive developmental disorder that hinders basic skill development were diagnosed at the age of 3.8 years while girls were generally diagnosed at age 4. The same held true for Asperger’s Syndrome, with boys typically being diagnosed at age 7.1 and girls at age 7.6.

The researchers found that the primary reason for this existed in the different symptoms exhibited by girls, such as their having greater difficulty reading social cues, a symptom that generally takes longer to manifest than the mannerism-related issues more commonly found in boys, such as repetitive behaviors like hand flapping. In boys, the social issues do not tend to appear until they are around the age of 10-15.

According to Lipkin, “These findings suggest that boys’ behaviors are more apparent than girls, with the potential for girls being more difficult to recognize. Since the problems experienced by girls are in social cognition and require social opportunities, they are much more likely to be unnoticed until the elementary school years.”

Thus, the researchers believe that not only do their findings indicate how and why the disorder is diagnosed in girls at older ages, the report also suggests that there is a greater chance for missed diagnoses in girls. Through better understanding of the ways in which children with autism demonstrate symptoms, researchers and advocates alike agree that better observation of girls with autism will lead to a better interpretation of autism overall.