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Causes Of Autism

The bottom line is, no one really knows for sure what causes autism. Most experts will say that autism is probably caused by a combination of genetic and environmental factors. Even those experts, though, do not have a definite answer. For many people, this uncertainty is terribly frustrating. On the plus side, interest in and funding for autism research is on the rise, so new and better information should be forthcoming in the next months and years.

A Controversial Topic: The question of what causes autism is highly controversial. Many people are passionate about the issue, and hold strong beliefs. Recent books, articles and TV programs have raised awareness -- and heated up the argument. This article provides an overview of different theories, all of which have strong supporters.

Do Vaccines Cause Autism?: There are two theories that link autism and vaccines. The first theory suggests that the MMR (Mumps-Measles-Rubella) vaccine may cause intestinal problems leading to the development of autism. The second theory suggests that a mercury-based preservative called thimerosal, used in some vaccines, could be connected to autism. To find out more, read Do Vaccines Cause Autism? and Is Autism on the Rise?.

Is Autism Genetic?: It is very likely that autism has a genetic basis of some sort. Many studies have shown that parents from families with autistic members are more likely to have autistic children. It is also the case that many families with one autistic child are at increased risk of having more than one autistic child. To find out more about autism and genetics, read Is Autism Genetic?

Is Autism Caused by Bad Parenting?: No. Dr. Kanner, the man who first identified autism as a unique condition, had the idea that cold “refrigerator” mothers caused autism. He was wrong. Dr. Kanner's misinterpretation of autism created a generation of parents carrying the guilt for their child's disability. Fortunately, our generation is spared that burden!

Is Autism Caused By Atypical Brain Development?: Some researchers have found differences between the autistic brain and the typical brain. Autistic individuals seem to have larger brains. They also seem to process information differently; in other words, their brains are "wired" differently. Research on this issue is ongoing at The University of Pittsburgh.

Is Autism an Immune Deficiency Problem?: There is some evidence that autism is linked to problems in the immune system. Autistic individuals often have other physical issues related to immune deficiency. Some researchers say they have developed effective treatments based on boosting the immune system. The NIH, however, states that the evidence is not yet strong enough to show a causal relationship.

Is Autism Caused By Food Allergies?: There is some evidence that allergies to certain foods could contribute to autistic symptoms. Most people who hold to this theory feel that gluten (a wheat product) and cassein (a dairy product) are the most signficant culprits. Explore the Autism Institute's website for more on this theory.

Is Autism Caused By Poor Nutrition?: It seems unlikely that malnutrition, per se, can cause autism. But megavitamin therapies have been used for many years to treat autistic symptoms. Dr. Bernard Rimland, of the Autism Institute, has been a leader in this area.

So...What DOES Cause Autism?: It seems likely, given the research so far, that several factors combine to cause autism. For example, it may be that certain children are genetically more susceptible to certain types of food allergies, or more likely to react badly to certain environmental toxins. Until we have more definitive answers, though, it seems to make sense to focus more on treatments -- and to support researchers as they learn more about causes.

About.com: Autism

Causes of Autism: Recent Findings

Growing Consensus Among Scientists: "In autism there are lots of opinions and very little data," says Lisa Croen, Ph.D. Research Scientist in the Division of Research at Kaiser Permanente in Northern California. In the past few years, however, some consensus has emerged on at least a few new pieces of the puzzle. As the research progresses, it will be easier to see the relationships among findings -- and to tease out the appropriate treatments for each individual on the autism spectrum.

There Is More Than One "Autism": About 25% of autistic people have digestive issues; 25% have seizure disorders; many have sleep problems. Recent findings suggest that the many different symptoms may actually indicate many different causes -- and thus many different "autisms." A massive study now underway at UC Davis's M.I.N.D. Institute is in the process of separating out different autistic phenotypes with the hope that this information will speed better understanding of causes and treatments.

Autism has a genetic component: Autism is hereditary, in that children with autistic people in their family are more likely than other children to be autistic. Researchers are well on the way to finding genes that relate to autism -- but the jury is still out regarding exactly how such genes might function to create autistic symptoms. Sophia Colamarino, Science Program Director at Cure Autism Now, explains,"We’re talking about genes because they allow us to understand the biological origins of the problem."

There is a Relationship Between Autism and Brain Structure: Recent brain studies show that autistic brains grow at an unusual rate between age 1 and 2, and then slow again to a normal rate of growth. Some imaging studies suggest that certain areas of the brain are larger than is typical. Research is ongoing to determine whether these differences in brain structure cause autism, are caused by autism, or are co morbid with autism and caused by something else.

There Is a Relationship Between Autism and Brain Activity: Recent brain imaging studies show that autistic people and typically developing people do not use their brains in the same way. Autistic people do not use their brains to "daydream" in the same way as most people, nor do they process information about faces in the same way. So far, while we know that this information is true, we don't know what causes these differences -- or whether these differences somehow cause autistic symptoms.

There Is a Relationship Between Autism and Brain Chemicals: Chemicals in the brain transmit signals which allow the brain to function normally. Sophia Colamarino explains: "Nerve cells communicate using electrochemical signals; there is evidence from many different domains that the ability of the brain to transfer information may be defective." An understanding of which transmitters are problemmatic may lead to effective treatments.

Genes Probably Interact with Environmental Factors: It is likely that genetics and environmental factors interact to cause autism. As yet, there is no proof of which environmental or genetic factors are to blame. Says Dr. Croen, autism "You need some kind of genetic susceptibility; then you have to be exposed to something which is elusive at the moment. This would be the impetus that sends you into autism."

No One Factor Causes Autism: It is unlikely that any one factor -- vaccines, foods, or environmental toxins -- is the cause of autism. "To find clues about the cause," says Dr. Croen, "we have to do really large studies to look at different configurations of co-morbidities… see what’s unique about each separate group." New research will address the questions "How do these circles overlap? What is the common thread?"

Ahead With Autism

Causes Of Autism

There is no known single cause for autism, but it is generally accepted that it is caused by abnormalities in brain structure or function. Brain scans show differences in the shape and structure of the brain in autistic versus non-autistic children. Researchers are investigating a number of theories, including the link between heredity, genetics and medical problems. In many families, there appears to be a pattern of autism or related disabilities, further supporting a genetic basis to the disorder. While no one gene has been identified as causing autism, researchers are searching for irregular segments of genetic code that autistic children may have inherited. It also appears that some children are born with a susceptibility to autism, but researchers have not yet identified a single "trigger" that causes autism to develop.

Other researchers are investigating the possibility that under certain conditions, a cluster of unstable genes may interfere with brain development resulting in autism. Still other researchers are investigating problems during pregnancy or delivery as well as environmental factors such as viral infections, metabolic imbalances, and exposure to environmental chemicals.

Autism tends to occur more frequently than expected among individuals who have certain medical conditions, including Fragile X syndrome, tuberous sclerosis, congenital rubella syndrome, and untreated phenylketonuria (PKU). Some harmful substances ingested during pregnancy also have been associated with an increased risk of autism. Early in 2002, The Agency for Toxic Substances and Disease Registry (ATSDR) prepared a literature review of hazardous chemical exposures and autism and found no compelling evidence for an association; however, there was very limited research and more needs to be done.

The question of a relationship between vaccines and autism continues to be debated. In a 2001 investigation by the Institute of Medicine, a committee concluded that the "evidence favors rejection of a causal relationship.... between MMR vaccines and autistic spectrum disorders (ASD)." The committee acknowledged, however, that "they could not rule out" the possibility that the MMR vaccine could contribute to ASD in a small number of children. While other researchers agree the data does not support a link between the MMR and autism, more research is clearly needed.

Whatever the cause, it is clear that children with autism and PDD are born with the disorder or born with the potential to develop it. Bad parenting does not cause it. Autism is not a mental illness. Children with autism are not unruly kids who choose not to behave. Furthermore, no known psychological factors in the development of the child have been shown to cause autism.

Autism Society Of America (ASA)
What Causes Autism?
There is no known single cause for autism, but it is generally accepted that it is caused by abnormalities in brain structure or function. Brain scans show differences in the shape and structure of the brain in autistic versus non-autistic children. Researchers are investigating a number of theories, including the link between heredity, genetics and medical problems. In many families, there appears to be a pattern of autism or related disabilities, further supporting a genetic basis to the disorder. While no one gene has been identified as causing autism, researchers are searching for irregular segments of genetic code that autistic children may have inherited. It also appears that some children are born with a susceptibility to autism, but researchers have not yet identified a single "trigger" that causes autism to develop.

Whatever the cause, it is clear that children with autism and PDD are born with the disorder or born with the potential to develop it. It is not caused by bad parenting. Autism is not a mental illness. Children with autism are not unruly kids who choose not to behave. Furthermore, no known psychological factors in the development of the child have been shown to cause autism.

Autism Treatment Center Of America
Autism: Causes

The exact cause of Autism is still unknown although many theories have surfaced.

According to the Autism Society of America, it has recently been accepted in the medical community that the disorder is caused by “abnormalities in the brain structure or function.”

Brain scans have shown differences in the shape and structure of autistic and non-autistic brains. It has been discovered that the brain size in autistic children under 12 was five percent larger than in typically developed children of the same age.

As technology has made study of brain tissue more accessible, researchers are able to look more closely at the structure and workings of the brain and brain tissue. According to researchers, the brain stem, which serves as a kind of relay station for messages going out of the brain to the rest of the body, can be affected by Autism. The cerebellum, at the back of the brain, which helps to fine tune motor skills and speaking is affected along with the corpus callosum, which allows for communication between the two hemispheres work together can also be affected by Autism.

This past August however, a study partially funded by the National Institute of Health among other federal organizations found that Autism affects “virtually the entire brain.”

The study concludes that Autism touches and affects more than particular parts of the brain, rather it affects the entire brain, making complex tasks impossible for most.

Another revelation made possible by recent research is the link between Autism and genetic or hereditary factors which has been shown to play an important role in the cause of Autism.

A study looked at the development of Autism between fraternal twins versus identical twins. In the fraternal twins, the development of Autism in both children occurred 5 to 8 percent of the time, which is the normal sibling rate, whereas identical twins, who share 100 percent of the same genes, the rate hovered around 60 percent, although other studies have concluded the rate to be as high as 95 percent.

While the study fails to show a concrete link between Autism and the role of genetics, the study does suggest a strong connection and offers researchers another glimpse into the disorder.

Recently, concern has surfaced that a link between thimerosal, a mercury based preservative found in the measles-mumps-rubella vaccine and Autism existed. According to the National Institute of Mental Health, no such link exists as the conclusion of several large studies has stated, however the Autism Society of America said research is still ongoing and the link cannot be ruled out.

A persistent and damaging rumor still exists, which was born with Kanner in 1943. Kanner blamed parents for children with Autism, referring to them as “refrigerator mothers.” Kanner claimed that mothers failed to properly bond with their children, hence the development of Autism, and their aversion to physical contact. According to British Autism activist, author and the father of an autistic boy, Adam Feinstein, Kanner later recanted his claim, asking parents for their forgiveness in a 1969 speech.

Most recently a study from the Mount Sinai School of Medicine in New York has concluded that the as the age of the father increased so did the risk of a child born with Autism.

The study looked at 300,000 children born in Israel and found that fathers who were over 40 were six times more likely to have their newborn develop Autism.

While the cause of Autism still remains a mystery, some things have become clear since its first diagnosis more than 60 years ago. The disorder is not a mental illness. It appears to be something a child is born with, not something they can “pick up” and it is not caused by any action of parenting skills or by a child who simply chooses to misbehave.

According to some though, the onset of Autism can be affected by the introduction of outside stimulus. There is a raging debate regarding the use of vaccinations, as earlier stated, as well as evidence pointing to gluten and casein in diets contributing to Autism.

Evidence in both cases is still being gathered, and time will tell if a “smoking gun” cause of Autism and Autism spectrum disorders will be found.

Behavioural Neurotherapy Clinic (Austrailia)

Causes Of Autism

The causes (aetiology) of Autism are still unclear. However, recent research suggests that Autism may result when a child with a genetic susceptibility is exposed to one or more of a number of environmental insults resulting in a series of dysfunctional interactions between Genes and Nutrients (Nutrigenomic interactions). This can happen “in utero” (during pregnancy) or after birth.

Eventually these dysfunctional interactions can affect body systems, most obviously the gastrointestinal, endocrine, immune system and the central nervous system. The complexity of the thousands if not millions of possible interactions may well defy description. We can however, build a basic biomedical model of autism, based on the published scientific evidence to-date.

The model that we developed and continuously refine reflects the way that a human being is built: Genes and nutrients interact to enable a single cell (fertilised egg) to multiply and differentiate to form multiple cell systems and organs, each with their own specific functions. It is the continuous interaction between the genetic code and nutrients (Nutrigenomics) that grows a human being in the womb and in the early years, as well as maintains optimum health and function throughout the lifespan. The following is an outline of some of the most common mechanisms that have been implicated in the aetiology of Autism Spectrum Disorder:

Genetic Evidence In Autism
A recent study has demonstrated a link between between autism and the Engrailed 2 (EN2) gene, which may contribute to up to 40% of autism cases in the general population. EN2 is involved in normal neural development. The study provides further genetic evidence that EN2 might predispose to ASD, and the authors suggest that disruptions in the expression of EN2 gene could significantly alter normal brain development."

Whilst there are genes that predispose to Autism, and we know that there is a genetic component to Autism, genetics alone cannot explain the recent rise in Autism in industrialised nations. This leaves nutrients and toxins interacting with these genes as the most likely candidates as causal factors for Autism.
The integrity of cell membranes that protects each cell can become impaired, leaving the cells vulnerable. There is much support in animal and human studies that a reduced intake in Omega 3 fatty acids results in impaired cell membranes and Neurodevelopmental disorders.
Deficiencies in Key Nutrients in our modern diet, such as Zinc, Vit B12, Vit 6 and Folate, may affect neural development and result in weaknesses in cellular membranes and internal processes of cells. These weaknesses predispose to the often devastating effect of environmental antigens.

Antigens (foreign toxic matter, heavy metals, viruses and bacteria) attack vulnerable cells and damage them, resulting in cells that cannot carry out their function normally.
Antigens can damage or change the expression of the genetic code in the cell, causing the cell to malfunction.
There is emerging evidence that a dysfunctional methylation system may result in abnormal genetic expression leading to dysfunctional cells. Vit B12, folate, B6 and Magnesium play a central role in regulating Methylation.
Abnormal methylation can interfere with metallothionein protein which regulates zinc/copper ratios and other metals in cell membranes.
Impaired cellular structures can result in multi-systemic disorders, affecting gastrointestinal, Immune, endocrine and central nervous system.
The delicate balance between beneficial and detrimental bacteria in the gut can be upset by antibiotics, food preservatives, toxic additives and environmental toxins, leading to a condition known as intestinal dysbiosis (a condition whereby gut bacteria species are abnormally distributed ).
Opportunistic bacteria in the gastrointestinal tract may irritate a vulnerable gut wall resulting in Irritable Bowel Syndrome, leaky gut and food sensitivities.
Some bacteria produce amines. When these bacteria are overgrown, they can produce large amounts of amines. A recent discovery in Neuroscience is that there are receptor sites in the same areas in the brain as neurotransmitter (messenger chemical) receptors. This finding suggests that excess amines can result in a scrambling of brain signals and affect learning, attention and mood. In a developing brain this can have serious implications.

Autism and Folate Metabolism
Research over the last 20 years has suggested a relationship between maternal diet and the birth of an affected infant, and recent evidence has confirmed that folic acid, a water soluble vitamin, found in many fruits (particularly oranges, berries and bananas), leafy green vegetables, cereals and legumes, may prevent the majority of neural tube defects.

Women who have one infant with a neural tube defect have a significantly increased risk of recurrence (40-50 per thousand compared with 2 per thousand for all births).
A randomised controlled trial conducted by the Medical Research Council of the United Kingdom demonstrated a 72% reduction in risk of recurrence by periconceptional (i.e. before and after conception) folic acid supplementation (4mg daily).
Other epidemiological research, including work done in Australia, suggests that primary occurrences of neural tube defects may also be prevented by folic acid either as a supplement or in the diet.
This has been confirmed in a randomised controlled trial from Hungary, which found that a multivitamin supplement containing 800mcg folic acid was effective in reducing the occurrence of neural tube defects in first births.

Extract from NHMRC Publication

Normal brain development and function depend on the active transport of folates across the blood-brain barrier. A study reported on the clinical and metabolic findings among five children with normal neurodevelopmental progress during the first four to six months followed by the acquisition of marked irritability, decelerating head growth, psychomotor retardation, cerebellar ataxia, dyskinesias, pyramidal signs in the lower limbs and occasional seizures. After the age of six years the two oldest patients also manifested a central visual disorder. Known disorders had been ruled out by extensive investigations. Despite normal folate levels in serum and red blood cells with normal homocysteine, analysis of Cerebro Spinal Fluid suggested disturbed transport of folates across the blood-brain barrier.

Oral treatment with folinic acid resulted in clinical amelioration.
Little, J., Epidemiology of neurodevelopmental disorders in children. Prostaglandins Leukot Essent Fatty Acids, 2000. 63(1-2): p. 11-20.
This study and others like it supports our contention that normal methylation is vitally important for the prevention and treatment of neurodevelopmental disorders including Autism.

Why the diversity of behaviours in Autism
This model goes a long way to explaining why there is such diversity in Autistic behaviours. It also explains why some interventions work for some children and not others, highlighting the need for conducting Biomedical testing as a basis for formulating hypotheses and to justify treatment approaches..

There is emerging evidence that Autism is a multifaceted Nutrigenomic disorder (interaction between genes and nutrients). Since our genetic code is unlikely to have changed or mutated in the last 100 years, genetics cannot account for the rise in all modern diseases, including Autism, in that short period. According to the US Bureau of Statistics, the incidence of Autism has increased 870% in ten years. However, while genetics have not changed, our diet and food chain has changed dramatically in that time and may be responsible for the observed increase in Autism Spectrum Disorder.

The introduction of toxic chemicals (antibiotics, heavy metals, pesticides, additives and preservatives) in our food chain.
The depletion of nutrients in our food due to high intensity farming of the same soil year after year for decades.
Farming methods that substitute chemical fertilisers for the natural decomposition of organic matter by bacteria.
Food processing methods that destroy nutrients.
The excessive use of antibiotics in babies and children.
The 20-40X increase of Omega 6 oils in our diet, and a reduction in Omega 3 consumption from fish. and Heating of these oils producing trans-fatty acids that damage cell membrane integrity
Slow prolonged cooking methods that destroy vitamins.
Microwave cooking that destroys some vitamins and bioflavanoids.
A change in eating habits in the last 50 years: We have replaced natural nutrient-rich foods, such as organically grown fruit and vegetables, with nutrient-poor and processed foods.
Chickens that are grown several times faster on a diet laced with antibiotics.

Nutritional deficiencies interacting with pre-existing genetic flaws result in cellular structures and internal cellular apparatus that are less than optimum. When these cells are later exposed to toxic chemicals, heavy metals and antigens such as bacteria and viruses they can be easily damaged and consequently fail to perform their normal functions.

These cells are distributed throughout the body's systems, such as: Immune System, Central Nervous System, Gastrointestinal system, Neuroendocrine and musculoskeletal systems and others. Consequently Autism has been described as a multi-systemic disorder.

Centers For Disease Control and Prevention (CDC)

Measles, Mumps, and Rubella (MMR) Vaccine and Autism Fact Sheet

Basic Information

The MMR vaccine protects children against dangerous, even deadly, diseases.
Because signs of autism may appear at around the same time children receive the MMR vaccine, some parents may worry that the vaccine causes autism.
Carefully performed scientific studies have found no relationship between MMR vaccine and autism.
CDC continues to recommend two doses of MMR vaccine for all children.

Additional Facts

MMR is a combination vaccine that protects children from measles, mumps, and rubella (also known as German measles). The first dose of the vaccine is usually given to children 12 to 15 months old. The second dose is usually given between 4 and 6 years of age.
In 1998, a study of autistic children raised the question of a connection between MMR vaccine and autism.
The 1998 study has a number of limitations. For example, the study was very small, involving only 12 children. This is too few cases to make any generalizations about the causes of autism. In addition, the researchers suggested that MMR vaccination caused bowel problems in the children, which then led to autism. However, in some of the children studied, symptoms of autism appeared before symptoms of bowel disease.
In 2004, 10 of the 13 authors of the 1998 study retracted the study's interpretation. The authors stated that the data were not able to establish a causal link between MMR vaccine and autism.
Other larger studies have found no relationship between MMR vaccine and autism. For example, researchers in the UK studied the records of 498 children with autism born between 1979 and 1998. They found:

The percentage of children with autism who received MMR vaccine was the same as the percentage of unaffected children in the region who received MMR vaccine.
There was no difference in the age of diagnosis of autism in vaccinated and unvaccinated children.
The onset of "regressive" symptoms of autism did not occur within 2, 4, or 6 months of receiving the MMR vaccine.

Groups of experts, including the American Academy of Pediatrics, agree that MMR vaccine is not responsible for recent increases in the number of children with autism. In 2004, a report by the Institute of Medicine (IOM) concluded that there is no association between autism and MMR vaccine, or vaccines that contain thimerosal as a preservative.
There is no published scientific evidence showing that there is any benefit to separating the combination MMR vaccine into three individual shots.

Even Better Now

Causes

Many experts say the multiple, wide-ranging vaccines now given to children before they enter kindergarten play a role in: withdrawn behavior, lack of emotion, speech delay or absence, learning difficulties, extreme sensitivity to sound and touch, repetitive movements, impairment in social interaction, and a delay in fine and gross motor skills.
And most doctors and scientists agree - the rise in brain abnormalities among children are likely caused by an interplay between genes and environmental factors. Our children are exposed to more and more toxic metals and chemicals every day, such as mercury, lead, food additives, pesticides, vaccination toxins, and so on.

In some children, especially those on the spectrum, these toxins build up in their systems due to a “metal-metabolism” disorder.

In other words, they are genetically unable to detoxify sufficiently. This causes neurological as well as digestive problems. As a result, chelation therapies to remove toxic metals including thimerosal (mercury from vaccinations) from the body are now a cornerstone with DAN doctors and other experts treating childhood neurological and behavioral conditions.

Although thimerosal has been removed from most childhood vaccinations (except the flu shot), vaccinations today still contain such toxins as aluminum, formaldehyde, and animal viral antibodies.

Many experts agree that when you remove these chemicals and metals and balance the child biochemically, all other behavioral and integrative therapies such as music therapy, applied behavioral analysis (ABA) and so on, work much more easily and effectively.

Chelators are substances that bind to heavy metals. The two most common chelators used by medical professionals are transdermal DMPS and oral DMSA. As excellent as these chelation therapies are, they are not without problems: They are expensive, they require visits to a doctor, and there is a greater risk of side effects, especially for children.

There are more natural ways to detoxify the body of heavy metals and chemicals that have less risk of reactions and work just as well. Two of these are the Kids Clear™ Detoxifying Clay Baths and Kids Chelat™, a liquid internal chelating dietary supplement.

Maryland Chapters of the Autism Society of America
What Causes Autism?
Although it is generally accepted that autism has neurobiologic causes, the precise nature of those causes is as yet unknown. There is an increased incidence of autism with certain disorders, such as fragile X syndrome, tuberous sclerosis, encephalitis, or untreated PKU.

Autism may co-exist with other syndromes or symptoms, such as severe impairments of vision or hearing. Researchers are investigating a number of theories, including the roles of heredity, genetics, and medical problems. Some families appear to demonstrate a pattern of autism or related disabilities, further supporting the theory of a genetic basis for the disorder in these cases. The present state of research hypothesizes that autism is not caused by a single gene, but more likely by a combination of several contributing genes. Several researchers have proposed a relationship between vaccines, particularly the measles, mumps, rubella (MMR) vaccine, and autism. Research is continuing to address these issues.

What we do know is that autism spectrum disorders are not caused by bad parenting. No known psychological factors in the development of the child have been shown to cause autism.

Mayo Clinic

Causes

Autism has no single, identifiable cause. The disorder seems to be related to abnormalities in several regions of the brain. Researchers have identified a number of gene defects associated with autism.

Families with one autistic child have a one in 20 chance of having a second child with the disorder. In some cases, relatives of autistic children show mild impairments in social and communication skills or engage in repetitive behaviors.

Children with symptoms of autism have a higher than normal risk of also having:

Fragile X syndrome, which causes mental retardation
Tuberous sclerosis, in which tumors grow in the brain
Tourette's syndrome
Epilepsy

Some people believe autism is caused by vaccines — particularly the measles-mumps-rubella vaccine (MMR), as well as vaccines containing thimerosal, a preservative that contains a very small amount of mercury. But extensive studies have shown no link between vaccines and autism.

Wikipedia, The Free Encyclopedia

Causes Of Autism

Autism and autism spectrum disorders are complex neurodevelopmental disorders. Many causes of autism have been proposed, but its theory of causation is still incomplete.[1] Heritability contributes about 90% of the risk of a child developing autism, but the genetics of autism are complex and typically it is unclear which genes are responsible.[2] In rare cases, autism is strongly associated with agents that cause birth defects.[3] Many other causes have been proposed, such as exposure of children to vaccines; these proposals are controversial and the vaccine hypotheses have no convincing scientific evidence.[4]

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