Why are boys far more likely to develop autism than girls?

It is not difficult to demonstrate the fact that there are more boys with autistic spectrum disorders than girls. Hans Asperger originally believed that

 no girls were affected by the syndrome he described in 1944, although clinical evidence later caused him to revise this statement. In Kanner's 1943 study of a small group of children with autistic syndrome there were four times as many boys as girls; and in their much larger study of Asperger syndrome in mainstream schools in Sweden in 1993, Ehlers and Gillberg found the same male to female ratio of 4:1. The ratio of male to female clients in NAS adult services is approximately 3:1.

In epidemiological research Wing (1981) found that among people with high-functioning autism or Asperger syndrome there were as many as fifteen times as many males as females. On the other hand, when she looked at individuals with learning difficulties as well as autism the ratio of boys to girls was closer to 2:1. This would suggest that, while females are less likely to develop autism, when they do they are more severely impaired.

It is difficult to explain why the sexes should be affected differently by autism.

Attwood (2000), Ehlers and Gillberg (1993) and Wing (1981) have all speculated that many girls with Asperger syndrome are never referred for diagnosis, and so are simply missing from statistics. This might be because the diagnostic criteria for Asperger syndrome are based on the behavioural characteristics of boys, who are often more noticeably "different" or disruptive than girls with the same underlying deficits. Girls with Asperger syndrome may be better at masking their difficulties in order to fit in with their peers, and in general have a more even profile of social skills.

Another hypothesis (Wing 1981) is based on evidence that, in the general population, females have better verbal skills, while males excel in visuo-spatial tasks. There may be a neurological basis for this, so that autism can be interpreted as exaggeration of "normal" sex differences. But environmental and social factors may also play a part in sex differences in ability, which means that no direct analogy can be drawn between the poorer verbal skills of boys and the higher incidence of autism in males.

In 1964 Bernard Rimland pointed out that, overall, males tend to be more susceptible to organic damage than girls, whether through hereditary disease, acquired infection or other conditions. Since it is now almost universally accepted that there is an organic cause for autism, it should not be surprising that boys are more vulnerable to it than girls.

In recent years researchers have put forward a genetic explanation for the differences. Skuse (2000) has suggested that the gene or genes for autism are located on the X chromosome. Girls inherit X chromosomes from both parents, but boys only inherit one, from their mothers. Skuses hypothesis is that the X chromosome which girls inherit from their fathers contains an imprinted gene which "protects" the carrier from autism, thus making girls less likely to develop the condition than boys.

This theory has been used to support Aspergers view that autism and Asperger syndrome are at the extreme end of a spectrum of behaviours normally associated with "maleness". Such behaviours can be extremely useful in areas of life such as engineering and science, where attention to detail and single-mindedness may be more valuable than social skills, for example.

Lord and Schopler (1987) have outlined several possible mechanisms for the transmission of autism on the sex-linked X chromosome, and also for autosomal transmission (ie involving non-sex chromosomes. However, these are merely theoretical models and in fact researchers are still a long way from identifying a simple genetic cause for autism. It is likely that several genes on different chromosomes will be found to be associated with autism. This means that Skuses theory, based on the X chromosome alone, may not represent the full picture.

Various theories have been put forward for the excess of males with autism and Asperger syndrome, but the picture is far from complete and until we have a fuller understanding of the causes of autism, it is unlikely that a proper explanation can be reached.


References

Asperger, H. (1944) Die autistischen Psychopathen im Kindesalter. Archiv fur Psychiatrie und Nervenkrankheiten, 117 , pp. 76-136.

Attwood, T. (2000) Asperger syndrome: Some common questions: Do girls have a different expression of the syndrome?. Online. Available from: www.asperger.org/asperger/asperger_questions.htm#girls

Ehlers, S. & Gillberg, C. (1993): The Epidemiology of Asperger syndrome. A total population study. Journal of Child Psychology and Psychiatry, 34 (8), pp. 1327-1350.

Kanner, L. (1943) Autistic disturbances of affective contact. Nervous child, 2 , pp. 217-50.

Lord, C. & Schopler, E. (1987) Neurobiological implications of sex differences in autism. In: Schopler, E. & Mesibov, G.M. (Eds.), Neurobiological issues in autism. New York: Plenum Press, pp. 191-211.

Rimland, B. (1964) The etiology of infantile autism. In: Infantile autism: the syndrome and its implications for a neural theory of behaviour. New York: Appleton-Century-Crofts. p. 54

Skuse, D.H. (2000) Imprinting the X-chromosome, and the male brain: explaining sex differences in the liability to autism. Pediatric Research, 47 (1), pp. 9-16.

Wing, L. (1981) Sex ratios in early childhood autism and related conditions. Psychiatry Research, 5, pp. 129-37.

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