Why Do More Boys Have Autism?
Photo courtesy of Seattle Children's Research Institute
Autism Spectrum Disorder (ADS) is diagnosed based on the presence of deficits in social communication and interactions and the presence of restricted or repetitive interests and behaviors. It is described as a spectrum because people with ASD range from non-verbal individuals who barely function in society to Nobel Prize winners.
Autism Spectrum Disorder affects one in 68 children in the United States, a number that has stabilized in the past five years.
There are four boys diagnosed for every girl.
Why the gender discrepancy?
Researchers believe that the typical male brain produces more repetitive or restricted behaviors and more narrowly focused interests than the more empathetic and verbally skillful female brain, according to Dr. Sara Jane Webb. One theory describes autism as the “extreme male brain.”
Speaking at a recent Pacific Science Center lecture, Webb, the co-director of the Psychophysiology and Behavioral Systems Lab at the Seattle Children’s Research Institute, shed light on the tangle of genetic and environmental factors that lead to autism. She and her colleagues use electroencephalography (sensors on the scalp to measure electrical impulses from brain activity) and other methods to find links between brain development and autism.
Within two days of conception, brain cells of boys and girls are developing differently, Webb says. By the time autism is usually diagnosed, at 24 to 36 months, autistic and non-autistic girls did better on receptive and expressive language tests and had fewer repetitive motor behaviors than similar boys. “So girls come out ahead,” Webb explains. “Geneticists think that females may be protected genetically from autism.”
Males tend to prefer “systematizing” – figuring out how machines and other things work, solving problems, investigating meteorological patterns and wanting to know lots of information about things. In a typical male brain, these are good things, but boys have more of a platform for these ways of thinking and behaviors to become extreme and clinically impairing.
Girls on the autism spectrum tend to have more of a “male brain” way of thinking. But with their generally better social and verbal skills, they have a lower tendency toward social isolation and difficulty with communication than autistic boys. It’s also possible that the way we nurture girls may gently push them away from typical autistic social deficits, Webb adds. With the disproportionate numbers of boys on the autism spectrum, girls with autism have not been studied as much.
Another possible cause for autism is only beginning to be explored, Webb says. No one gene causes autism; there may be as many as 100 genes involved. She cites recent research on “rare de novo” gene disruptions – “mistakes” or mutations in genes that happen after birth. In a study of families with siblings with and without autism, those with autism had twice as many disruptive mutations. They were especially destructive when they occurred in “hot spots” related to brain development and synaptic processes. “For some reason, it seemed to take more mutations in bigger hot spots to have girls be affected,” Webb says. In other words, boys were much more susceptible to these disruptions that led to autistic behaviors.
Finally autism spectrum disorder may be partially caused by a variety of poorly-understood environmental factors. Inconclusive studies postulate a link between influenza and other viruses during pregnancy, possibly because reactions to viruses create a potential for inflammation in the fetal brain. Ultrasounds that send vibration and heat into tissues may disrupt where neurons end up, Webb says. In fact, boys that already have the genetic mutations that contribute to autism are shown to have lower IQs and more repetitive behaviors if they were exposed to ultrasound in the first trimester. Autism rates are also higher in premature babies.
The reasons for higher incidence of autism in boys are an ongoing area of study for Webb and colleagues around the world. “It may be that boys take more hits – male genetic background, more “rare de novo” genetic disruptions and some kinds of environmental hits in pregnancy,” she concludes.
For more information on Webb’s research and to sign up for studies on gender and autism, visit http://depts.washington.edu/pbslab/Home.html and click on “Research: Current Studies.”
Wenda Reed is an award-winning health writer with an adult son on the Autism Spectrum.