Since autism was first added to the psychiatric literature fifty years ago, there have been numerous studies and theories about its cause. Researchers still have not reached agreement regarding its specific causes.

First, it must be recognized that autism is a set of symptoms and may have many causes. This concept is not unusual in medicine. For instance, the set of symptoms that we perceive of as a "cold" can be caused by literally hundreds of different viruses, bacteria, and even our own immune system.

Autism is, undoubtedly, a biologically based disorder. In the past, some researchers had suggested that autism was the result of poor attachment skills on the part of the mother. This belief has caused a great deal of pain and guilt on the part of the parents of autistic children.

In support of a biological theory of autism, several known neurological disorders are associated with autistic features. Autism is one of the symptoms of these disorders. These conditions include tuberous sclerosis (an inherited disorder), the fragile X syndrome, cerebral dysgenesis (abnormal development of the brain), Rett syndrome, and some of the inborn errors of metabolism (biochemical defects). Autism, in short, seems to be the "final common pathway" of numerous disorders that affect brain development. There is a strong association between autism and seizures. This association works in two ways: First, many patients (20 to 30%) with autism develop seizures. Second, patients with seizures, which are probably due to other causes, may develop autistic-like behaviors.

Brain Abnormalities

Good time to review the basic parts of the brain and their function. Neuropsychological tests demonstrate frontal lobe impairments, and impairments with the temporal lobe. Brain scans have consistently demonstrated abnormalities in the following areas:

  • Cerebellum (motor movement--vocalization)
  • Medial temporal lobe (memory)
  • Limbic system (emotion, learning)
  • Elevated rates of epilepsy and abnormal EEG patterns are also common in autism.
  • Decreased blood flow to the frontal and temporal lobes is noted.
  • In terms of neurotransmitters, autistic children have elevated levels of blood serotonin (upper 5% of population)

It is generally accepted that autism is caused by abnormalities in brain structures or functions. Using a variety of new research tools to study human and animal brain growth, scientists are discovering more about normal development and how abnormalities occur.

Scientists now know that a number of problems may interfere with normal brain development. Cells may migrate to the wrong place in the brain. Or, due to problems with the neural pathways or the neurotransmitters, some parts of the communication network may fail to perform. A problem with the communication network may interfere with the overall task of coordinating sensory information, thoughts, feelings, and actions

Researchers supported by NIMH and other National Institute of Health (NIH) Institutes are scrutinizing the structures and functions of the brain for clues as to how a brain with autism differs from the normal brain. In one line of study, researchers are investigating potential defects that occur during initial brain development. Other researchers are looking for defects in the brains of people already known to have autism. Scientists are also looking for abnormalities in the brain structures that make up the limbic system. Inside the limbic system, an area called the amygdala is known to help regulate aspects of social and emotional behavior. One study of high-functioning children with autism found that the amygdala was indeed impaired but that another area of the brain, the hippocampus, was not.

Differences in neurotransmitters, the chemical messengers of the nervous system, are also being explored. For example, high levels of the neurotransmitter serotonin have been found in a number of people with autism. Since neurotransmitters are responsible for passing nerve impulses in the brain and nervous system, it is possible that they are involved in the distortion of sensations that accompanies autism.

The NIMH is also exploring differences in overall brain function, using a technology called magnetic resonance imaging (MRI) to identify which parts of the brain are energized during specific mental tasks. In a study of adolescent boys, NIMH researchers observed that during problem-solving and language tasks, teenagers with autism were not only less successful than peers without autism, but the MRI images of their brains showed less activity. In a study of younger children, researchers observed low levels of activity in the parietal areas and the corpus callosum. Such research may help scientists determine whether autism reflects a problem with specific areas of the brain or with the transmission of signals from one part of the brain to another.

Each of these differences has been seen in some but not all the people with autism who were tested. What could this mean? Perhaps the term autism actually covers several different disorders, each caused by a different problem in the brain. Or perhaps the various brain differences are themselves caused by a single underlying disorder that scientists have not yet identified. Discovering the physical basis of autism should someday allow us to better identify, treat, and possibly prevent it.

But what causes normal brain development to go awry? Some NIMH researchers are investigating genetic causes-the role that heredity and genes play in passing the disorder from one generation to the next. Others are looking at medical problems related to pregnancy and other factors.

Genetic Contributions

Is autism genetic? Because many different disorders can result in autism, this question is complex. Certainly, disorders such as the fragile X syndrome and tuberous sclerosis, which are both associated with autism, are inherited. There are many families with more than one autistic child where the autism is not clearly due to another cause.

Recent studies have found that the gene for at least one kind of familial autism may be on chromosome 13.

In some families, autism seems to be passed from generation to generation. In other families, autism is not found in prior generations, but affects multiple siblings (brothers or sisters). The results of this research make it likely that at least one "autism gene" will be found in the next few years.

Evidence from a variety of sources indicates that autism may be a genetic condition. About 3% of Fragile-X children have in the past been mistaken as autistic. Family studies show that about 6% of siblings and extended family members of autistic children have the disorder (this is much higher, about 75 times, than the general population), and many others display autistic-like features (social deficits).

Several studies of twins suggest that autism- or at least a higher likelihood of some brain dysfunction-can be inherited. For example, identical twins are far more likely than fraternal twins to both have autism. Unlike fraternal twins, which develop from two separate eggs, identical twins develop from a single egg and have the same genetic makeup.

It appears that parents who have one child with autism are at slightly increased risk for having more than one child with autism. This also suggests a genetic link. However, autism does not appear to be due to one particular gene. If autism, like eye color, were passed along by a single gene, more family members would inherit the disorder. The NIMH, using state-of-the-art gene splicing techniques, is searching for irregular segments of genetic code that the autistic members of a family may have inherited.

Some scientists believe that what is inherited is an irregular segment of genetic code or a small cluster of three to six unstable genes. In most people, the faulty code may cause only minor problems. But under certain conditions, the unstable genes may interact and seriously interfere with the brain development of the unborn child.

Problems In Pregnancy and Birth:

Autistic children do experience more health problems during pregnancy, at birth, or immediately following birth than normal kids. In 25% of cases the following risk factors have been identified:,

  • Mother's age (more likely with increased age)
  • Prematurity
  • Bleeding during pregnancy
  • Toxemia (blood poisoning)
  • Exposure to a virus

Note: These factors although identified are not consistent. Thus, it is thought that problems associated with pregnancy are not the primary cause of autism, but suggest that fetal development may have been altered in some fashion.

Throughout pregnancy, the fetal brain is growing larger and more complex, as new cells, specialized regions, and communication networks form. During this time, anything that disrupts normal brain development may have lifelong effects on the child's sensory, language, social, and mental functioning.

For this reason, researchers are exploring whether certain conditions, like the mother's health during pregnancy, problems during delivery, or other environmental factors may interfere with normal brain development. Viral infections like rubella (also called German measles), particularly in the first three months of pregnancy, may lead to a variety of problems, possibly including autism and retardation.