Causes of Autism Spectrum Disorder
Autism spectrum disorder is caused by a complex interplay of genetic factors (the primary driver) and environmental exposures during critical prenatal and early developmental periods, with rare genetic mutations having large effects and common genetic variants contributing polygenic risk. 1, 2
Genetic Factors (Primary Cause)
The most robustly identified genetic risks for ASD are rare mutations with large effect, though common genetic variation also contributes through polygenic mechanisms 1. The genetic architecture includes:
- Rare de novo and inherited mutations affecting synaptic function, chromatin regulation, and neurodevelopmental pathways 2, 3
- Copy number variations (e.g., 15q duplication) that disrupt brain development 1
- Polygenic risk from common variants that overlap with other psychiatric and cardiometabolic disorders 4
- Hundreds of candidate genes identified, converging on pathways involving synaptic signaling, gene regulation, immune processes, and excitatory-inhibitory balance 2
The heritability is substantial—genetic testing including chromosomal microarray analysis is warranted and clinically indicated for all suspected cases of ASD 1.
Environmental Factors (Modifying Risk)
Environmental exposures interact with genetic susceptibility, particularly during the prenatal period when the developing brain is most vulnerable 5, 6. Key environmental contributors include:
Maternal Factors During Pregnancy
- Maternal immune activation (infections, inflammatory conditions) 2, 4
- Maternal metabolic disorders (diabetes, obesity) 4
- Maternal nutritional deficiencies 4
- Air pollution exposure 3
Chemical Exposures
- Pesticides, heavy metals, bisphenol A (BPA), and phthalates may target autism-related genes 5
- Selected teratogens interacting through inflammatory, oxidative, and epigenetic mechanisms 3
Perinatal Factors
Mechanisms of Action
Genetic mutations and environmental exposures converge on:
- Aberrant brain development with altered cortical trajectories and atypical connectivity 1, 3
- Disrupted synaptic function and neuronal connectivity 1, 2
- Immune dysregulation and inflammatory pathways 3
- Epigenetic modifications affecting gene expression 3
Sex Differences
The male predominance (3-4.5 times higher diagnosis rate) likely involves protective factors that reduce ASD risk in females 1, 5. Evidence suggests:
- Increased fetal testosterone exposure (measured by digit ratio) associates with ASD diagnosis 5
- Sex-specific genetic architecture contributes to differential risk 2
Critical Caveats
Genetics alone cannot explain the quadrupling of ASD prevalence over recent decades 4. The increase reflects:
- Broadening diagnostic criteria
- Increased awareness and screening
- Possibly increased environmental exposures 4
Over 100 environmental exposures have been associated with ASD, but only about 40 have been confirmed in meta-analyses 4. Many proposed associations may be markers rather than causal factors, and shared genetic risk between mother and child can confound apparent environmental effects 4.
The early developmental period, especially prenatal, represents the critical window when disruptions have the greatest impact, with subtle signs detectable as early as 9 months of age 5.