The symptoms of autism and their severity can vary widely among autistic individuals. Nonetheless, there are three core areas which characterize the disorder: difficulties in social interaction, communication deficiencies, and the tendency to engage in repetitive behaviors with or without preoccupations. In addition, these abnormalities may be accompanied by other conditions such as sensory processing abnormalities, cognitive deficits, irritability and aggressive/destructive behaviors, hyperactivity, and medical conditions such as gastrointestinal disorders or seizures.

Communication

There are developmental milestones that non-autistic children typically pass in learning to communicate. Babbling begins in infancy, and by age 10-14 months most toddlers begin to say a word or two, respond to their names by looking, point to objects they want, and shake their head or make a negative sound when offered something distasteful. By contrast, a child with autism typically has delayed babbling and speech, and does not use gestures as expected. Some autistic children initially develop speech patterns normally before losing the ability to communicate. By the age of 24 months, when most other children are putting together simple sentences, the autistic child is usually unable to use 2-word phrases. They may speak only single words or may repeat the same short phrase over and over. Occasionally, they may echo all the words they hear, without using them appropriately. Non-verbal communication such as gestures, facial expressions and body language are typically limited and may not match the contents of the verbal speech. Tone of voice is often flat and robotic but occasionally inappropriately high-pitched and sing-song. These communication deficits are often the first abnormalities to be noticed in the autistic child and are a common rationale for bringing the child to seek medical attention.

Repetitive Behaviors

One of the more striking features of autism is the tendency to engage in bizarre repetitive behaviors, such as spinning, arm flapping, rocking or jumping, sometimes for hours at a time. Other common repetitive actions include repeating sounds or words, rearranging objects, and unusual motions of the fingers. Occasionally, these behaviors can take the form of intense preoccupations or obsessions, such as an extreme interest in a household item or a need to know every fact and figure concerning a television show or a scientific topic.
Another form of this characteristic is the tendency to engage in a restricted range of activities, such as a single television show or game. Compulsive behavior is also common, and manifests in a need to maintain an extremely orderly environment. Examples include lining up toys in a specific place and order, or placing household items in a precise location. A variation of this is ritualistic behavior, which involves keeping an unvarying pattern of daily activity such as an unchanging menu or dressing ritual. Any change in the order or location of items in the home or of the pattern of daily activities can be extremely emotionally distressful and can lead to emotional outbursts.

Sensory Processing

Individuals with autism often have difficulty processing and integrating sensory information, such as touch, lights, smells or movements. They may also be hyper- or hypo-sensitive to external stimuli. Ordinary sensory experiences may feel extreme and painful in nature, or may be not felt at all. Because of this, it is not unusual to see an autistic individual react in an extreme manner to a simple touch or to the flipping on of a light switch.

Others

There are other conditions that are commonly associated with autism as well. Greater than 50% of autistic individuals have some degree of cognitive deficits, and up to 85% have some form of gastrointestinal (GI) distress, such as constipation, diarrhea, abdominal pain, or irritable bowel. Seizure disorders are common in autism, especially in those with some intellectual impairment, and can take the form of generalized shaking seizures, periods of unresponsiveness and staring absently, and clinically asymptomatic seizures that can only be seen with specific testing.