Causes of autism

[6] Environmental factors that have been claimed to contribute to autism or exacerbate its symptoms, or that may be important to consider in future research, include certain foods,[7] infectious disease, heavy metals, solvents, diesel exhaust, PCBs, phthalates and phenols used in plastic products, pesticides, brominated flame retardants, alcohol, smoking, and illicit drugs.

First, individuals with autism have significantly reduced fecundity, they are 20 times less likely to have children than average, thus curtailing the persistence of mutations in ASD genes over multiple generations in a family.

[20] Besides these early examples, the role of de novo mutations in autism first became evident when DNA microarray technologies reached sufficient resolution to allow the detection of copy number variation (CNV) in the human genome.

[50] The development of autism is associated with several prenatal risk factors, including advanced age in either parent, diabetes, bleeding, and maternal use of antibiotics and psychiatric drugs during pregnancy.

[63] Children with sleep apnea have "hyperactivity, attention problems, aggressivity, lower social competency, poorer communication, and/or diminished adaptive skills".

[85] Prenatal stress, consisting of exposure to life events or environmental factors that distress an expectant mother, has been hypothesized to contribute to autism, possibly as part of a gene-environment interaction.

This hypothesis is not supported by independently published research, and examination of children whose mothers received an ultrasound has failed to find evidence of harmful effects.

[89] Some research suggests that maternal exposure to selective serotonin reuptake inhibitors during pregnancy is associated with an increased risk of autism, but it remains unclear whether there is a causal link between the two.

[92] A meta-analysis published in 2024 concluded that "evidence does not support that maternal alcohol consumption during pregnancy is independently associated with autism spectrum disorders in offspring".

[94] It has also been proposed that the functions of the hypothalamic-pituitary-adrenal axis and brain connectivity in pre-term infants may be affected by NICU-related stress resulting in deficits in emotional regulation and socio-emotional capabilities.

A wide variety of postnatal contributors to autism have been proposed, including gastrointestinal or immune system abnormalities, allergies, and exposure of children to drugs, infection, certain foods, or heavy metals.

[100] This theory hypothesizes that an early developmental failure involving the amygdala cascades on the development of cortical areas that mediate social perception in the visual domain.

[115] This allows passage of bacterial endotoxins from the gut into the bloodstream, stimulating liver cells to secrete tumor necrosis factor alpha (TNFα), which modulates blood–brain barrier permeability.

Studies on ASD people showed that TNFα cascades produce proinflammatory cytokines, leading to peripheral inflammation and activation of microglia in the brain, which indicates neuroinflammation.

[115] In addition, neuroactive opioid peptides from digested foods have been shown to leak into the bloodstream and permeate the blood–brain barrier, influencing neural cells and causing autistic symptoms.

It has been postulated that the resulting excess of opioids affects brain maturation and causes autistic symptoms including: behavioral difficulties, attention problems, and alterations in communicative capacity and social and cognitive functioning.

[119][120] Although high levels of these opioids are eliminated in the urine, it has been suggested that a small part of them cross into the brain causing interference of signal transmission and disruption of normal activity.

[119] The theory further states that removing opiate precursors from a child's diet may allow time for these behaviors to cease, and neurological development in very young children to resume normally.

Although studies showed significant differences protein intake and calcium in individuals with autism, the results were limited by their imprecision, inconsistency, and indirect nature.

Systematic reviews have been conducted for numerous toxins including air pollution, thimerosal, inorganic mercury, and levels of heavy metals in hair, nails, and bodily fluids.

[123] Although no link was found to exist between the vaccine additive thiomersal and autism risk, this association may hold true for individuals with a hereditary predisposition for autoimmune disorders.

[123] The atypical eating behaviors of autistic children, along with habitual mouthing and pica, make it hard to determine whether increased lead levels are a cause or a consequence of autism.

Evidence includes genetic effects on metabolic pathways, reduced antioxidant capacity, enzyme changes, and enhanced biomarkers for oxidative stress.

[71] Research exploring the evolutionary benefits of autism and associated genes suggests that people with autistic traits may have made facilitated crucial advancements in technology and knowledge of natural systems in the course of human development.

A study conducted by Spikins et al. (2016) examined the role of Asperger syndrome as "an alternative pro-social adaptive strategy", which may have developed as a result of the emergence of "collaborative morality" in the context of small-scale hunter-gathering.

[159] Pre-agricultural Europeans appear to have had similar, or slightly higher,[157] percentages to modern East Asians, and the numbers may have decreased in the former due to dilution with a group of people which had split off before Neanderthal introgression.

[172][a] In addition, Neanderthal genes have also been implicated in the structure and function of the brain,[b] keratin filaments, sugar metabolism, muscle contraction, body fat distribution, enamel thickness and oocyte meiosis.

The study also recommended further research into the link between Neanderthal single nucleotide polymorphisms (SNPs) and neurodevelopmental disorders, including autism, in modern-day humans.

[179] Psychologist Bruno Bettelheim believed that autism was linked to early childhood trauma, and his work was highly influential for decades both in the medical and popular spheres.

[183] The most recent scientific research has determined that changes to brain structures correlated with the development of autism can already be detected while the child is still in the womb, well before any vaccines are administered.