What Causes Autism? A Look at Genetic and Environmental Factors

Autism Spectrum Disorder (ASD) is a complex neurodevelopmental condition that affects how a person perceives the world and interacts with others. But what causes it? The answer isn’t simple—and researchers still don’t have all the pieces of the puzzle—but we now know that both genetics and environmental factors play a role in its development.

The Genetic Side of Autism

The strongest risk factor for ASD is genetic. Studies show that autism tends to run in families, and having a sibling with autism significantly increases the risk. But it’s not usually due to a single gene.

1. Multiple Genes Are Involved

ASD is what scientists call “polygenic,” meaning that many different genes—possibly hundreds—can contribute to risk. Some genes involved are linked to how brain cells develop, communicate, and organize.

• Certain rare gene mutations, like those in CHD8, SHANK3, and SCN2A, have strong associations with autism.

• Copy number variants (CNVs)—sections of DNA that are missing or duplicated—are also linked to higher ASD risk.

2. Genetic Conditions Associated with ASD

Some genetic syndromes have a high co-occurrence with autism. These include:

• Fragile X Syndrome

• Tuberous Sclerosis Complex

• Rett Syndrome

These conditions affect brain development and often include features of autism.

3. Heritability Estimates

Twin studies suggest that up to 80% of the risk for ASD can be attributed to inherited genetic factors. But having a genetic predisposition doesn’t mean a child will develop autism—it just raises the likelihood.

Environmental Factors: The Context That Shapes Risk

While genes lay the foundation, environmental factors may influence how—or whether—autism traits develop. These are not causes in the sense of blame, but rather elements that may interact with genetic susceptibility.

1. Prenatal Influences

• Parental age: Advanced maternal and especially paternal age at conception is associated with increased ASD risk.

• Maternal health: Conditions like gestational diabetes, obesity, and immune activation during pregnancy (such as infections or autoimmune conditions) have been linked to a higher risk.

• Exposure to toxins: Prenatal exposure to air pollution, pesticides, or heavy metals may increase ASD risk, though findings are mixed and often small in effect size.

2. Perinatal and Neonatal Factors

• Premature birth or low birth weight

• Birth complications such as lack of oxygen (hypoxia) or neonatal jaundice These factors may increase vulnerability in a brain already genetically predisposed to ASD.

3. Medications During Pregnancy

Some medications, such as valproic acid (used for epilepsy or bipolar disorder), have been associated with higher ASD risk when taken during pregnancy. However, it’s crucial to weigh these risks against the benefits of maternal treatment.

What Doesn’t Cause Autism

Just as important as knowing the risk factors is dispelling myths.

• Vaccines do not cause autism. This claim has been thoroughly debunked by multiple large-scale studies.

• Parenting style or attachment is not a cause. While support and responsiveness matter for all children, autism is not caused by “refrigerator parents”—a damaging theory from the mid-20th century.

A Complex Interplay

Autism is the result of a complex interplay between genetic vulnerability and environmental exposures—not a single cause or event. Each person with autism has a unique developmental path, shaped by biology and life experiences.

Understanding these causes is not about finding someone to blame, but about early identification, better support, and more compassionate care. As research continues, the hope is to uncover not just the "why" of autism, but also the "how"—how we can support individuals and families to thrive.