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.

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.