One of the leading theories is that girls are protected from genetic disorders because of their extra X chromosome. The best evidence for this came in a 2014 study published in American Journal of Human Genetics in which researchers reported that girls with neurodevelopmental disorders actually have more genetic abnormalities—mutations and deletions of genetic material–than similar boys, but the conditions do not show up as readily. What this data seem to suggest is that the genetic threshold for ASD may be lower for boys than girls.
Take, for example, a 2012 study in the UK of 15,000 twins which compared symptoms to clinical diagnosis. The researchers found that for a girl to receive an actual autism diagnosis they needed to exhibit more behavioral problems, or significant intellectual disability or both to be diagnosed. More mild cases in girls were being ignored or given other labels (e.g. ADHD).
Another large study from 2013 in the Netherlands found that girls with milder symptoms (i.e. Asperger’s syndrome) were diagnosed, on average, two years later than boys with similar severity.
