Story highlights The ratio of boys to girls in autism spectrum disorders is about 5-to-1

Clinicians who aren't experts in diagnosis might miss higher-functioning girls with disorder

Something seems to protect girls from developing autism and other developmental disorders

"Protective" factors could play an important role in pointing toward new treatments, interventions

Autism (now better known as autism spectrum disorder or ASD) is a neurodevelopmental disorder caused, at least in part, by genetic factors.

The disorder usually shows up in infancy, starting with noticeable problems in social communication, with a preference for engaging in repetitive, sensory-oriented behaviors. The severity of these deficits varies greatly among individuals diagnosed with ASD. Recovery is rare, though it is true that some individuals may make substantial progress over time.

The disorder is associated with a high burden of suffering in terms of the limitations it imposes on those affected. There has also been a lot of attention paid to the extra burden and loss of income it represents for parents and the cost to the social, educational and health care systems.

But one aspect of autism that doesn't get a lot of attention is the way the condition manifests differently in girls and boys.

We have known for many years that autism is more common in boys than girls. The disorder appears different in the sexes, and these differences may have important implications for both diagnosis and treatment.

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A striking finding of the recent Centers for Disease Control and Prevention report , showing a 78% increase in cases over the past decade, is that the ratio of boys to girls in ASD is about 5-to-1. That is higher than what is usually reported in other studies, where a ratio of 2-, 3- or 4-to-1 is more common.

There is no adequate explanation for this imbalance in the sex ratio, though it must be recognized it is also true for other developmental disorders of early childhood such as learning disabilities and attention deficit disorder. In contrast, girls appear to be overrepresented in disorders that begin after puberty such as depression and anxiety.

Something seems to protect girls from developing ASD and other developmental disorders. That "something" could be hormone levels in utero, epigenetic factors that turn autism susceptibility genes "on" and "off" during development, or the fact that young girls have in general better social skills than boys and so need a bigger "dose" of what causes ASD to cross that threshold to being impaired. It is also possible that a proportion of girls with mild autistic traits lose those traits early on and so escape detection by 8 years of age (the age of the children in the CDC study).

Whatever the reason, the sex ratio in ASD is perhaps the most consistent finding in the field but perhaps the least understood. Yet it is possible that those "protective" factors, once identified and understood, could play an important role in pointing toward new treatments and interventions that capitalize on those protective factors and so make a real difference to long-term outcomes.

The clinical expression of autism is also different in boys. Generally speaking, girls with autism have greater learning disabilities and more problems academically than boys. The sex ratio in ASD approaches 1-to-1 as the degree of cognitive impairment increases. Conversely, the sex ratio has sometimes been reported to be even greater than 5-to-1 among so-called "higher-functioning" individuals with ASD, though not all studies agree on this point.

An intriguing finding is that perhaps higher-functioning girls with ASD are missed by clinicians who are not experts in diagnosis. There is some evidence that among this subgroup, girls have better social skills than higher-functioning boys with ASD and so are not diagnosed as readily.

Often, the symptoms of ASD appear as extreme shyness or anxiety in girls, masking that they may not be responsive to the social cues of others. And while fixated interests are common in both sexes on the autism spectrum, girls tend to focus on topics such as on ponies, princesses, dolls or drawings -- common passions for non-autistic girls, too. Boys, on the other hand, may become stuck on less typical activities, such as lining up blocks or running sand through their fingers. As a result, doctors may miss that some of their female patients show signs of autism.

Girls on the autism spectrum also engage in fewer repetitive behaviors such as rocking and spinning and less sensory irritability than boys with ASD. Since these signs are subtle, doctors may not recognize the disorder. Diagnosis at a later age reduces the chances of early treatment, which is thought to produce a better outcome.

Girls may be different than boys with ASD in their lived experience of the disability as well. There's increasing concern about peers bullying children with ASD at school and in the community. Some evidence suggests that girls with ASD are bullied less often than boys. They appear to "blend in" more readily with their peer group and are less often the victim of bullying by other girls.

Sex differences in ASD is an important topic, but one that has only recently become the focus of attention. Understanding those sex differences both biologically and experientially holds the promise of improving the long-term outcome of all children with ASD.

Most important, clinicians need to be sensitive to how the disorder shows up in girls so that those affected can receive early intervention as soon as possible.