When Autism Has No Known Cause

Causes of autism remain mysterious

While autism is increasingly common, its cause is usually unknown. In fact, only 15-17% of cases result from a clear, well-understood source. In a general way, researchers believe that there is a strong genetic component to autism and that there are environmental "triggers" that may cause certain individuals to develop symptoms; for any individual, however, the precise nature of the genetic and environmental triggers is unknown.

When autism is of known origin (caused by a known genetic anomaly or exposure), it is referred to as "secondary autism." When autism is of unknown origin, it is called "idiopathic autism."

Graphic of question marks

Known and Unknown Causes of Autism

While there are over a dozen established causes of autism, most are very rare genetic disorders or prenatal exposures. As a result, approximately 85% of autism is "idiopathic." In other words, in the vast majority of cases:

  • a child is born to parents who are not autistic;
  • autism is not a known part of the child's family history;
  • the child was not premature;
  • the parents were under 35 years old;
  • tests did not uncover genetic anomalies (such as Fragile X syndrome) that might cause autism in the child;
  • the mother was not exposed to or taking any of the drugs known to increase the risk of autism while she was pregnant (rubella, valproic acid, and thalidomide are known to cause autism in unborn children)

Heredity, Genetics, and Autism Risk

Heredity does play a role in autism: having one child with autism does increase the likelihood that your next child may also be autistic. This is a concern to bear in mind when planning your family's future.

According to the National Human Genome Research Institute, "The risk that a brother or sister of an individual who has idiopathic autism will also develop autism is around 4 percent, plus an additional 4 to 6 percent risk for a milder condition that includes language, social or behavioral symptoms. Brothers have a higher risk (about 7 percent) of developing autism, plus the additional 7 percent risk of milder autism spectrum symptoms, over sisters whose risk is only about 1 to 2 percent."

While we know that heredity plays a role in autism, however, we don't know exactly how or why. Dozens of genes seem to be involved in autism, and research is ongoing. There is no existing genetic test to determine whether a parent "carries" autism or whether a child (or fetus) is likely to develop autism.

Genetic mutation may also cause autism. Genetic mutation can occur for many different reasons and may or may not be related to parents' genetics. Genetic mutation occurs frequently but does not always result in physical or developmental challenges. 

Because we know so little (so far) about genetics and autism, it's rare that a diagnostician can draw a direct line between a particular genetic anomaly and a particular person's autism.

Theories About Environmental Exposures

Theories abound regarding a possible "explosion" in the number of people with autism. After all, the steep increase in diagnoses does coincide with a steep increase in many environmental changes. In fact, autism diagnoses increased at just about the same rate as:

  • cell phone use
  • ultrasound used to monitor fetal growth
  • cable television
  • video games
  • awareness of climate change
  • use of anti-tick and flea medications and shampoos for pets
  • number of vaccines given to young children
  • interest in organic and GMO foods
  • allergies to peanuts and gluten
  • prevalence of Lyme disease

Could any or all of these changes in the world have contributed to or caused 85% of autism? There are certainly people who believe the answer is yes, and most have picked out one or two of these potential causes to focus on.

The reality, however, is that autism presents itself differently in different people. This suggests a variety of causes and, perhaps, a variety of syndromes with some (but not all) symptoms in common. 

A Word From Verywell

The reality is that, for most autism parents, there will never be a clear answer to the question "why did my child develop autism?" While this can be terribly frustrating, the good news is that causes don't really matter when it comes to taking action for your child's future. Whether your child's autism was the result of a genetic difference, a prenatal exposure, a mutation, or heredity, the same therapies and treatments are likely to be helpful. Rather than spending a great deal of time and money seeking reasons, in most situation the better route is to spend that time, money, and energy on helping your child reach his or her potential.

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