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The “Female Protective Effect”

February 26, 2013

A new study provides insights into why girls are much less likely than boys to develop autism

Boys are up to five times more likely to be diagnosed with autism than girls are. Researchers have proposed theories to explain why. It may be that girls are less likely to be diagnosed because their symptoms are less obvious. Or, something about a girl’s biology may protect her from developing autism.

A new study lead by researchers at Harvard Medical School supports the idea of a “female protective effect.” If girls do have some degree of protection, they proposed, then it may take more autism risk factors to tip their brain development into the realm of autism. This is because many if not most cases of autism appear to result not from a single risk factor – such as one gene mutation – but from a combination of risk factors.  Some are inherited, or genetic, and others may be “environmental.” By environmental, scientists mean a broad range of nongenetic factors – not just toxic chemicals.  Examples include a maternal infection during pregnancy and age of parents. (For more perspective on additive risk, please see “Risk vs. Cause” in the science blog.) 

The researchers looked at nearly 10,000 sets of 12-year-old fraternal (nonidentical) twins. In particular, they looked at the siblings of boys and girls who scored high on a scale of autistic traits - high enough to be considered on the autism spectrum. Overall the twin brothers or sisters of the highly impaired girls themselves had greater autism impairments than did the siblings of the boys. This suggested that the girls were more likely to come from families with a strong inherited risk for autism and that this higher risk was necessary to overcome a possible female protective effect.

“Understanding what protects certain people from developing autism is just as important as is pinpointing the various causes,” comments Alycia Halladay, Ph.D., Autism Speaks senior director of environmental and clinical sciences. “We don’t know what it is,” she says of the female protective effect. “It could be a gene that girls have and boys don’t. It could also have something to do with the different hormones present during early brain development, or both.” What’s particularly useful about this study, Dr. Halladay adds, is that it helps guide researchers in their search for other protective factors for autism.

Autism Speaks has organized a scientific symposium exploring gender differences in autism, to be held April 25th, at the annual meeting of the Organization for the Study of Sex Differences. Registration information here. Autism Speaks also continues to fund a wealth of research on factors that increase or decrease autism risk, including the influence of gender. You can explore these and other studies using this website’s Grant Search.