IAN Assistant Editor
Connie Anderson, Ph.D.
IAN Community Scientific Liaison
Kennedy Krieger Institute
Date First Published: February 23, 2010
Why are autism spectrum disorders (ASDs) so much more common in boys than in girls? To answer this question, researchers are exploring how gender-based differences might contribute to a vulnerability to ASD in boys -- or to a buffer providing some degree of protection in girls. What factors make boys more likely to have an ASD? What takes place during development, perhaps even in the womb, which could result in more males than females later exhibiting autistic traits? One theory is that people with ASD have "an extreme male brain," and that this may be the result of exposure to high levels of fetal testosterone.
The Autism Spectrum: A Preponderance of Males
In many ways, males appear to be more vulnerable to life's hazards than females. For a variety of reasons, both biological and social, they are more prone to death or damage before birth, more likely to suffer accidents or fall victim to violence, and have a shorter average lifespan than females. 1 2 They are also more likely than females to have autism, pervasive developmental disorder not otherwise specified (PDD-NOS), Asperger's syndrome, or any other type of ASD. 3
In the first clinical account of autism in 1943, psychiatrist Leo Kanner noted that boys with the condition outnumbered girls by a ratio of 4 to 1. 4 Similarly, in 1944, when Hans Asperger described the syndrome that would come to bear his name, he initially found it only in boys. 5 The impression that there are far more boys with autism than girls, and that this is even more striking in Asperger syndrome, has not changed much in the years since. Recent studies continue to show that the ratio of boys to girls with autism is about 4 or 5 to 1, 6 7 while the ratio of boys to girls with Asperger's syndrome is between 6 and 11 to 1. 8 9
What no one has been able to discover yet is why.
Autistic Traits: Male Plus?
It is not just that there are more boys than girls with ASDs. Researchers have also noticed that there is something "male and then some" about the way individuals with ASD think, act, and cope. Asperger himself speculated that the characteristics he saw in his young patients could represent an "extreme variant of male intelligence." 10 More recently, Simon Baron-Cohen, a prominent autism researcher at Cambridge University, has proposed the extreme male brain theory of autism, which attempts to explain the remarkable similarities between traits generally associated with human "maleness" and traits associated with the autism spectrum. 11
What are these traits? For one thing, typically developing males tend to show strengths in mathematical and spatial reasoning and the ability to discriminate details from a complex whole. Compared with typically developing females, however, males tend to be at higher risk for language impairment and at a disadvantage on social-judgment tasks, measures of empathy and cooperation, and imaginary play during childhood. 12 Many of the traits associated with ASDs could be thought of as an extreme profile of "typical male" strengths and challenges. Where, on average, typical men may be good at detail-oriented processing, people with ASD may be incredibly good at perceiving detail...and impaired when it comes to seeing "the big picture." Where typical men may be less able than women to make social judgments or empathize with others, people with ASDs are literally disabled in these areas.
Baron-Cohen and colleagues have come up with a model for this idea which divides the way the brain works into two major dimensions: systemizing and empathizing.
Systemizing is defined as "the drive to analyze or construct systems" that "follow rules." 13 It also involves being able to predict the behavior of a system (as opposed to predicting or understanding the behavior of other people). 11 Males are, on average, more skilled at "systemizing" than females are. Think of mathematicians and engineers as good systemizers.
Empathizing is defined as "the drive to identify another person's emotions and thoughts, and to respond to these with appropriate emotion." It also involves being able to predict the behavior of people. Females are, on average, more skilled at "empathizing" than males are. Think of therapists and teachers as good empathizers.
Please note: The gender differences being discussed are "overall average" differences. In other words, it is understood that there are individual men who are wonderful at empathizing and women who are fantastic at systemizing; there are intuitive male therapists and brilliant female mathematicians.
The extreme male brain theory, meanwhile, views people on the autism spectrum as hyper-systemizers: people who are extremely interested in and engaged with rule-bound non-human systems, whatever their level of functioning. 14 For someone with less cognitive ability, "hyper-systemizing" might translate into collecting buttons, stones, or some other objects and organizing them by type. For someone with a higher IQ, it might translate into a huge catalog of knowledge on a particular subject, like insects, astronomy, or history -- a subject that is endlessly thought about, talked about, and expanded upon. Routine would be one form of system, and disruptions to routine would be upsetting because they interfere with expectations of a rule-bound system. Rigidity, not flexibility, would be the rule.
Empathizing, on the other hand, would be impaired in individuals with ASD. Reading social cues, noticing what others are feeling (let alone figuring out how to respond appropriately), and making sense of social hierarchies would be very challenging -- even more challenging than it might be for the average male.
Fetal Testosterone: Support for the Extreme Male Brain Theory?
What might explain this tendency of people with ASD towards an extremely male style of thinking and coping with the world? Could factors that contribute to human "maleness," especially if intensified, be somehow associated with the development of ASD?
Baron-Cohen and colleagues hypothesized that high levels of fetal testosterone, a hormone that drives the development of male physical and cognitive characteristics, were involved. 15 After all, the differentiation of male vs. female begins at conception and continues through prenatal development and beyond. Could a high dose of fetal testosterone explain the "very male" cognitive and emotional profile of people with ASD, as well as the fact that so many more males have ASD compared with females?
Some key studies have been carried out to try to answer this question. In one, researchers found that girls who had been exposed to high levels of fetal testosterone in the womb due to a genetic disorder had a more male-typical play style. 16 In another, using amniotic fluid that had been collected during pregnancy, researchers were able to demonstrate that the lower the level of fetal testosterone in the womb, the higher a child's ability to empathize as measured by eye contact, quality of peer relationships, and other factors 6 to 8 years later. 17
Finally, researchers explored whether there was any association between high fetal testosterone levels during pregnancy and autistic traits in children. Over 200 mothers who had had amniocentesis during pregnancy completed questionnaires about their child's behavior, including social, language, and repetitive-behavior traits common to ASDs. The researchers found that the higher the level of fetal testosterone during the pregnancy, the more likely the now 6-to-10-year-old child was to have some autistic characteristics. 15
The idea that people with ASD have an "extreme male brain," and that high fetal testosterone levels can cause this, makes an intriguing theory. This theory provides a framework for thinking about ASD in terms of "systemizing," which people with ASD excel at, and "empathizing," which they find challenging. It also provides an explanation for the fact that boys with ASD outnumber girls with ASD.
Still, so far researchers have not been able to test this theory working with people who actually have a diagnosed ASD, only those who have ASD-like traits. Baron-Cohen and his team are now following up with a new study which will test whether high levels of fetal testosterone are associated with an actual diagnosis of ASD. 18 Since ASD occurs only in about 1% of the population, the researchers will need a huge number of samples to test their idea, something they hope to achieve by using the Danish Biobank – a massive storehouse of biological samples. 19
Researchers are also interested in finding out if current levels of testosterone, as opposed to prenatal levels, play any role in ASDs. Is there some critical window of time when testosterone levels matter, or do they always matter? If there is a window, when is it? 15
At the same time, interest in oxytocin, another hormone, is growing. Some are theorizing that this hormone, which is associated with social memory and attachment, maternal behavior, and human bonding, may be associated with ASDs somehow. 20 There have even been some preliminary studies which show that oxytocin nasal spray may improve social communication and interaction in people with ASD. 2122
There is reason to hope that an understanding of how specific hormones contribute to ASD may lead, with additional research and testing, to promising new treatments that address the core social deficits of ASD.
- Read more about gender differences in Not Just for Boys: When Autism Spectrum Disorders Affect Girls.
- For more on the extreme male brain theory, see the IAN page Cognitive Theories Explaining ASDs.
- Wells, J. C. (2000). Natural selection and sex differences in morbidity and mortality in early life. Journal of Theoretical Biology, 202(1), 65-76. View Abstract
- Kraemer, S. (2000). The fragile male. British Medical Journal, 321(7276), 1609-1612. View Abstract
- Interactive Autism Network. (2007). IAN Research Report #2 - July 2007. Retrieved February 22, 2010
- Kanner, L. (1943). Autistic disturbances of affective contact. Nervous Child, 2, 217-250.
- The National Autistic Society. (2003). Autism: Why do more boys than girls develop it? Retrieved February 19, 2010.
- Fombonne, E. (2005). Epidemiological studies of pervasive developmental disorders. In F. R. Volkmar, R. Paul, A. Klin, & D. Cohen (Eds.), Handbook of autism and pervasive developmental disorders (3rd ed., Vol. 1, pp. 42-69). Hoboken, NJ: John Wiley & Sons, Inc.
- Scott, F. J., Baron-Cohen, S., Bolton, P., & Brayne, C. (2002). Brief report: Prevalence of autism spectrum conditions in children aged 5-11 years in Cambridgeshire, UK. Autism: The International Journal of Research and Practice, 6(3), 231-237. View Abstract
- Gillberg, C., Cederlund, M., Lamberg, K., & Zeijlon, L. (2006). Brief report: "The autism epidemic": The registered prevalence of autism in a Swedish urban area. Journal of Autism and Developmental Disorders, 36(3), 429-429-435. View Abstract
- Tantam, D. (1991). Asperger syndrome in adulthood. In Uta Frith (Ed.), Autism and Asperger syndrome, (pp. 147-183). Cambridge, UK: University Press.
- Wing, L. (1981). Sex ratios in early childhood autism and related conditions. Psychiatry Research, 5, 129-137. View Abstract
- Baron-Cohen, S. (2002). The extreme male brain theory of autism. Trends in Cognitive Sciences, 6(6), 248-254. View Abstract
- Baron-Cohen, S. (1999). The extreme-male-brain theory of autism. In H. Tager-Flusberg (Ed.), Neurodevelopmental disorders. Cambridge, MA: The MIT Press.
- Baron-Cohen, S. (2009). Autism: The empathizing-systemizing (E-S) theory. The Year in Cognitive Neuroscience, 1156, 68-80. View Abstract
- Baron-Cohen, S. (2006). Two new theories of autism: Hyper-systemizing and assortative mating. Archives of Disease in Childhood, 91(1), 2-5. View Abstract
- Auyeung, B., Baron-Cohen, S., Ashwin, E., Knickmeyer, R., Taylor, K., & Hackett, G. (2009). Fetal testosterone and autistic traits. British Journal of Psychology, 100, 1-22. View Abstract
- Auyeung B, Baron-Cohen S., Ashwin E., Knickmeyer R., Taylor K., Hackett G., & Hines M. (2009). Fetal testosterone predicts sexually differentiated childhood behavior in girls and boys. Psychological Science, 20(2), 144-148. View Abstract
- Chapman, E., Baron-Cohen, S., Auyeung, B., Knickmeyer, R., Taylor, K., & Hackett, G. (2006). Fetal testosterone and empathy: Evidence from the empathy quotient (EQ) and the "reading the mind in the eyes" test. Social Neuroscience, 1(2), 135-148. View Abstract
- Baron-Cohen, S., Auyeung, B., Ashwin, E., & Knickmeyer, R. (2009). Author response: Fetal testosterone and autistic traits: A response to three fascinating commentaries. British Journal of Psychology, 100, 39-47.
- Tachibana, C. (2010, January 15). Career advice: Denmark: Making global connections. Retrieved from the American Association for the Advancement of Science February 19, 2010.
- Lee, H. J., Macbeth, A. H., Pagani, J. H., & Young, W. S. (2009). Oxytocin: The great facilitator of life. Progress in Neurobiology, 88(2), 127-151. View Abstract
- Guastella, A. J., Einfeld, S. L., Gray, K. M., Rinehart, N. J., Tonge, B. J., Lambert, T. J., & Hickie I. B. (in press). Intranasal oxytocin improves emotion recognition for youth with autism spectrum disorders. Biological Psychiatry.
- Andaria, E., Duhamela, J. R., Zallab, T., Herbrechtb, E., Leboyerb, M., & Sirigua, A. (in press). Promoting social behavior with oxytocin in high functioning autism spectrum disorders.