[Posted on 20 March, 2018 by Sebastian Lundström]
Autism has traditionally been much more prevalent among men than women. The male-to-female autism ratio varies depending on a number of factors such as IQ: this ratio seems to be much lower at lower IQ levels than at higher levels. How credibly a study can answer the question “What is the male-to-female autism ratio?” is dependent on how the study is structured. For example, the ratio is influenced by whether the study uses registry data/public records (i.e. includes those who are already connected to health care services) or if it is a prospective study (includes everyone, regardless of connection to health care services). In cases where the study is based on registry data, one must consider the age group of those examined (girls/women are usually diagnosed later than boys/men) and when the people in question were diagnosed (autism prevalence has increased significantly over the last decades and research indicates that girls who today likely would have been diagnosed with autism were given other diagnoses ten or more years ago). As for prospective studies, the respective rates (and thus the ratio) are influenced by for example what kind of instrument has been used to score autism/autistic traits (screening or clinical instruments). Prospective studies generally produce more even rates among boys and girls than passive studies do.
A recently published (Loomes et al., 2017) meta-analysis (a study including already existing studies) analysed data from 54 different studies with over 13 million participants. The authors weighted the various studies differently depending on how they were structured (for example, a study with a low number of participants was given less weight than one with a high number of participants). The point of doing a meta-analysis is to provide a larger basis for examination and to statistically correct for the potential strengths/weaknesses of the studies included (e.g. registry study compared with prospective study) in order to achieve the most reliable estimate possible. The study reported that the male-female autism ratio was 3:1, which is to say that there are three times as many men as there are women diagnosed with autism. This is more even than the 4:1 ratio reported in the diagnostic manuals, but still a considerable overrepresentation of men.
The reasons behind the overrepresentation of men in autism are unclear. It has been suggested that women may need to pass a higher pathological threshold in order to exhibit symptoms, meaning that being born with two X chromosomes could be a protective factor. Another suggested explanation has been that women with an autism spectrum disorder are harder to detect, that they “fly under the radar”, for example by exhibiting less pronounced stereotypies and repetitive behaviours. Another reason that women are examined to a lesser extent than men may be that they exhibit fewer (or less pronounced) behaviours that might elicit diagnostic assessment, such as hyperactivity or aggressive outbursts. In conclusion, there are potential reasons to believe that autism might manifest differently in men and women. The studies that formed the basis for the establishment of the previous diagnostic criteria (Diagnostic and Statistical Manual of Mental Disorders – fourth edition, DSM-IV) included approximately 650 individuals, of which around 1/6 were girls. As such it is easy to imagine that the diagnostic criteria actually reflect a form of autism that is more common or distinctive among men, and which might not take female manifestations of autism into account. It is difficult to estimate how much this affects clinical work. The clinician’s experience and knowledge regarding child psychiatry are of course very consequential to how they think about and interpret behaviours, and whether this process should be handled differently for boys than for girls. However, it is easy to imagine that scoring scales based on DSM criteria are more prone to measuring a more male-oriented variant of autism. Studies at the GNC in Gothenburg have shown that girls have a response profile somewhat different to that of boys, and a revised version of the Autism Spectrum Screening Questionnaire has been designed with the specific aim of identifying girls with autism at an earlier stage.
The question of whether boys can be compared with girls and vice versa, is a recurring issue in the field of child psychiatry, one which is often discussed but seldom researched. In other fields of medicine, this is uncontroversial, e.g. when measuring blood pressure, different thresholds are used for different genders. Although this blogpost won’t produce any answers to this question, perhaps some preliminary results might encourage further consideration of this subject. In an ongoing research project, we identified a little over 400 people who had been diagnosed with autism. These individuals had also responded to a questionnaire that continuously measured autism, ADHD, oppositional-defiant symptoms and learning problems. When boys were compared with girls, boys showed a significantly higher degree of all conditions compared to girls. When we made statistical adjustments to the scales so that girls were compared with girls and boys were compared with boys, while implementing the same basic scale for both genders, the results were different. Girls with a diagnosis had significantly higher average values on all scales compared to the boys. This indicates that girls with a diagnosis actually have, quantitatively speaking, greater difficulties than boys with a diagnosis, but that this fact never comes to light if girls are compared with boys. In case the preliminary results hold up, they would indicate that scoring scales might need different threshold values for boys and girls, simply in order to avoid comparing apples and oranges.