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The unsatisfying answer to this question is that scientists still don’t fully grasp what leads to autism. Rather than a single gene or lifestyle trend, autism is the result of complex interactions between many different factors.

Genetics contribute more to the condition than do environmental factors. Autism is highly heritable, and people who have a sibling with autism are more likely to have autism themselves. People are more likely to have autism if they have related genetic disorders, such as fragile X syndrome, Rett syndrome, and tuberous sclerosis.

Autism is also more likely due to a number of environmental factors, such as maternal infection, diabetes, high blood pressure, and older paternal age at conception. Pinning down the connection to other elements of the environment, such as air pollution, has proven more difficult.

The number of children diagnosed with the disorder has risen dramatically since the 1960s. Experts believe that much of the change can be attributed to changes in diagnostic criteria and greater awareness among clinicians and the general public.

Is autism genetic?

Yes, genes play an important role in autism. The heritability of autism is estimated to be around 80 percent. This means that if one identical twin has autism, the other twin would have an 80 percent chance of developing autism. In the case of fraternal twins, who share half rather than all of their DNA, the second twin has a 40 percent chance of developing autism. People whose siblings have the condition are also more likely to develop it. People with genetic disorders such as fragile X syndrome, Rett syndrome, or tuberculosis sclerosis are also more likely to have autism, according to the CDC.

What are genetic risk factors for autism?

Dozens of genes are now known to correlate strongly with autism, and hundreds more have smaller connections. Genetic differences can be inherited through a parent’s DNA or they could spontaneously occur in the sperm, egg, or embryo. A person who is said to have risk factors often has a first degree relative with an autism spectrum disorder, but it is impossible to quantify the risk at this time.

It is important to understand that having genetic risk factors for autism does not mean that a person will necessarily go on to develop the condition.

Do environmental factors cause autism?

Several environmental factors increase the risk of autism, and they center around experiences in the pregnancy or infancy that may influence brain development. Premature birth and low birth weight raise the risk of autism, as do diabetes, high blood pressure, and infection during pregnancy. Having an older father is also a risk factor, likely because the chance of spontaneous genetic mutations increases with age.

There is some evidence of a correlation between exposure to air pollution during pregnancy or early childhood and the onset of autism. However, researchers are still working to understand this relationship. Pinning down the connection between autism and any single environmental factor is difficult because it’s nearly impossible to isolate—there are so many variables at play.

Do vaccines cause autism?

No. There is no connection between vaccines and autism. Vaccines are powerful tools that doctors and parents rely on to protect children from dangerous illnesses.

The specious claim emerged from a paper published in the journal The Lancet in 2013 by disgraced physician Andrew Wakefield, who claimed that a preservative in vaccines called Thimerosal caused autism. The paper was deeply flawed and later retracted by the journal that published it. Rigorous research has continued to find no connection between the two.

What prenatal factors increase the risk of autism?

Taking the medications thalidomide or valproic acid during pregnancy can raise the risk of autism, according to the CDC. There is some research that antidepressant use during pregnancy increases the risk of autism, but that research is not yet definitive.

Other elements linked to a higher likelihood of developing autism include having an infection, autoimmune disease, diabetes, or high blood pressure during pregnancy. This may be due to the manner in which inflammation and the immune response influence the developing fetus. There also seem to be correlations between autism risk and pregnancies that are less than a year apart.

Some evidence indicates taking prenatal vitamins containing folic acid, vitamin B-9, and vitamin D may offer some protection and reduce the likelihood of autism.

What is causing rates of autism to increase?

The prevalence of autism has continued to rise over the past few decades. Today, 1 in 54 children are estimated to have autism, according to the CDC. Research suggests that this surge is not due to changes in the genetic and environmental factors that contribute to the condition. The rising prevalence is instead likely due to changes in the diagnostic criteria and greater awareness of autism and its symptoms.

Clinicians diagnose mental health conditions based on criteria delineated in the Diagnostic and Statistical Manual of Mental Disorders (DSM). The pre-2013 version, the DSM-IV, contained three categories: autistic disorder, Asperger’s disorder, and pervasive developmental disorder not otherwise specified. The current iteration, the DSM-5, replaces those categories with one overarching diagnosis: autism spectrum disorder. The current definition of autism is no longer reserved for the most extreme cases; it embraces subtler ones as well.

Are boys more likely than girls to have autism?

Autism occurs 4 times more often in boys than in girls. This asymmetry may be due to a combination of biological and diagnostic forces.

From a biological perspective, women with autism have more family members with the condition than men and have more genetic mutations. These findings and others form the basis for the “female protective effect,” a theory that women possess some form of biological protection against autism.

Why are boys more likely to have autism?

The “extreme male brain,” was proposed by British scientist Simon-Baron Cohen. People with autism have an extreme form of the male brain, the theory goes, as men tend to be drawn to systems and efficiency while women excel at empathy and reading emotions. Others have pushed back that underlying biological differences between the sexes could not be large enough to account for the condition and the idea hasn’t been replicated by other scholars.

From a diagnostic perspective, clinicians may be more aware of the condition in boys and therefore identify it more readily. Additionally, the instruments to screen and diagnose the conditions were created and validated on males.

Although autism occurs more frequently in boys, many girls and women have the condition. The symptoms may present differently, and they may learn to camouflage the condition. Women may be overlooked or misdiagnosed as a result, but finally receiving a diagnosis has the potential to be life-changing.

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