How Often Does Autism Have No Known Cause?

While autism is increasingly common, its cause is usually unknown on an individual level. 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 the condition; 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.

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 cases are idiopathic.

In other words, in the vast majority of cases:

• Neither of a child's biological parents are not on the autism spectrum
• Autism is not a known part of the child's family history
• The child was not born prematurely
• The biological father was under 40 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 that is idiopathic may mean there is a genetic component; if so, the chances that a sibling may also be on the autism spectrum can be higher than someone who does not have a brother or sister on the autism spectrum.

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%, in addition to an already 4% to 6% risk for a milder condition that includes language, social or behavioral symptoms. Brothers have a higher risk (about 7%) of developing autism, plus the additional 7% 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, we don't know exactly how or why. Dozens of genes seem to be involved in autism, and research is ongoing.

There are screening tests for phenotypic traits, but 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 have don't have all the details down about genetics and autism, a diagnostician can only attempt to draw a direct line between a particular genetic anomaly and a particular person's autism.

Theories About Environmental Exposures

There has been a rise in cases of autism, and theories about why abound. While there are certainly people who believe there is a connection between some of the following and autism, there is no solid evidence to support this:

• Ultrasound used to monitor fetal growth
• Vaccines given to young children. This theory has been disproven in numerous studies.
• Cell phone usage among parents
• Allergies to peanuts and gluten
• Prevalence of Lyme disease

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 situations the better route is to spend that time, money, and energy on helping your child reach their potential.