According to autism expert Dr. Judith Gould, Asperger's syndrome may be massively underdiagnosed in girls, perhaps because they hide their symptoms better.
Conventional thinking says that Asperger's and other autism spectrum disorders are four to sixteen times as common in boys as girls. But Gould thinks a more likely ratio is 2.5 boys to every girl. She says the underdiagnosis is caused by "a stereotyped view of what Asperger's is, which is based entirely on how boys present with the condition." Tony Attwood, author of The Complete Guide to Asperger's Syndrome, agrees, saying:
Boys tend to externalise their problems, while girls learn that, if they're good, their differences will not be noticed. Boys go into attack mode when frustrated, while girls suffer in silence and become passive-aggressive. Girls learn to appease and apologise. They learn to observe people from a distance and imitate them. It is only if you look closely and ask the right questions, you see the terror in their eyes and see that their reactions are a learnt script.
Getting clinicians to see this "terror" is especially important, because, according to Attwood, "undiagnosed Asperger's can create devastatingly low self-esteem in girls." He even says that "in my experience, up to 20% of female anorexics have undiagnosed Asperger's."
Doctors Are 'Failing To Spot Asperger's In Girls' [The Guardian]