Autism spectrum disorder is a developmental condition that affects how people communicate, interact socially, and experience the world around them, with symptoms typically appearing before age three and lasting throughout a person’s life.
Epidemiology
The number of people identified with autism spectrum disorder has grown significantly over recent decades. According to global estimates, in 2021 about one in 127 people worldwide had autism. However, this figure represents an average, and the actual prevalence can vary substantially across different studies and regions.[7] In the United States, the numbers are higher, with about one in 31 children diagnosed with autism spectrum disorder.[6] In the United Kingdom, more than one in 100 people are estimated to be autistic, meaning there are at least 700,000 autistic adults and children across the country.[5]
The diagnosis of autism has become more common over the past several decades. It remains unclear whether this increase reflects a true rise in the number of autistic people or simply better recognition of milder forms of the condition among medical providers and increased awareness in society. Many low- and middle-income countries still lack comprehensive data on autism prevalence, making it difficult to understand the true global picture.[7]
Autism can affect people of all ages, genders, and ethnicities. However, people from marginalized groups may face greater barriers to receiving a diagnosis. This disparity often stems from a lack of awareness about the true diversity of autistic people and how autism can present differently across various populations.[5]
Causes
The exact causes of autism spectrum disorder are not fully understood, but research suggests that multiple factors work together to influence brain development. Scientists believe that both genetic and environmental factors play important roles in causing autism, though no single cause has been identified.[2]
Autism is highly heritable, meaning it tends to run in families. Some people with autism have known genetic differences, such as specific genetic conditions. However, many other causes remain unknown. Research indicates that there are probably many different factors that make a child more likely to have autism, and these factors can include environmental, biological, and genetic influences.[2][7]
Scientists continue to investigate how these various factors interact to change the most common ways people develop. The brain differences that characterize autism begin before birth and affect how a person’s brain functions throughout their life. Some children with autism show symptoms within the first 12 months of life, while in others, symptoms may not appear until 24 months of age or later. In some cases, children gain new skills and meet developmental milestones until around 18 to 24 months of age, and then they stop gaining new skills or lose the skills they once had.[2]
Risk Factors
While the precise causes of autism remain under investigation, researchers have identified several factors that may increase the likelihood of a child developing autism spectrum disorder. It is important to understand that having one or more risk factors does not mean a child will definitely develop autism, as the condition results from complex interactions between multiple influences.
Available scientific evidence suggests that both genetic predisposition and environmental factors contribute to autism risk. Although scientists know little about specific causes, they recognize that autism has a strong hereditary component. Families with one autistic child may have a higher chance of having another child with autism. Some genetic conditions, such as fragile X syndrome or Rett syndrome, are associated with autism spectrum disorder.[2][9]
Environmental factors during pregnancy and early development may also play a role, though research in this area is ongoing. Scientists believe there are multiple causes of autism that act together to change typical developmental patterns. The interaction between genetic susceptibility and environmental influences appears to be particularly important in understanding who develops autism.[2]
Symptoms
People with autism spectrum disorder experience a wide range of symptoms that affect how they communicate, interact socially, and behave. The abilities of people with autism can vary significantly, which is why the term “spectrum” is used. Some autistic people may have advanced conversation skills, while others may be nonverbal. Some need a lot of help in their daily lives, while others can work and live with little to no support.[2]
One core area of difficulty involves social communication and interaction. Autistic individuals often have problems with social communication, which can include difficulty understanding the feelings and intentions of others, maintaining eye contact and facial expressions, and adapting to social norms and expectations. They may find it challenging to establish and maintain friendships and might not understand the two-way nature of social interactions. Some autistic people may not respond to their names or may seem uninterested in sharing interests and activities with others.[2][5]
Communication deficits represent another core feature of autism. This may include delays in language development, such as delays in uttering first words or simple sentences. Some autistic individuals may not use language to communicate at all. Even among those with normal language skills, there may be difficulty using language in conversations to express thoughts, feelings, or needs. Understanding and using nonverbal communication, such as body language and facial expressions, may also be affected.[3]
Restricted and repetitive patterns of behavior and interests are characteristic of autism. Autistic people often display a strong fixation on specific topics or activities, repetitive body movements such as rocking or hand-flapping, and an overreliance on daily routines. These repetitive behaviors are sometimes seen as a way of self-soothing or as an attempt to control an environment that might otherwise feel unpredictable and overwhelming. Changes to established routines can cause significant distress.[2][5]
Many autistic individuals have differences in how they process sensory information. They may have very strong or delayed responses to sound, light, touch, taste, or smell. For example, some may find everyday background noises unusually harsh or overwhelming, while others may not notice pain or other bodily sensations as quickly. Some autistic people may seek out certain sensory experiences, while others may avoid them.[5][6]
As children with autism become adolescents and young adults, they may experience difficulties developing and maintaining friendships, communicating with peers and adults, or understanding what behaviors are expected in school or at work. They may also develop conditions such as anxiety, depression, or attention-deficit/hyperactivity disorder, which occur more often in people with autism than in people without autism.[2]
Prevention
Currently, there is no known way to prevent autism spectrum disorder, as the condition results from complex interactions between genetic and environmental factors that are not fully understood. The exact causes of autism are still being researched, and scientists have not identified specific preventable risk factors that would allow for primary prevention strategies.[2]
However, early identification and intervention can significantly improve outcomes for children with autism. Routine developmental screening during well-child checkups is essential for detecting early signs of autism. Healthcare providers should check a child’s development at regular intervals, and if there are any signs of autism spectrum disorder, the child should receive a comprehensive evaluation as soon as possible.[2]
Early intervention services, ideally beginning in infancy or as soon as symptoms appear, can greatly improve the development of a child with autism. Research shows that in an appropriate educational setting, early intervention for at least two years prior to the start of school can result in significant improvements. Some children may gain enough skills to successfully enter a regular education elementary school setting. Getting treatment early, during the preschool years, can make a big difference in the lives of many children with the condition.[2][3][13]
Parents, teachers, and healthcare providers should be aware of developmental milestones and act early if there are concerns. The earlier autism is identified and appropriate support is provided, the better the long-term outcomes tend to be. Programs that promote early childhood developmental monitoring by families, child care providers, and healthcare providers can help improve early identification of developmental delays and disabilities, including autism.[2]
Pathophysiology
Autism spectrum disorder is fundamentally a condition related to brain development that affects how the brain functions throughout a person’s life. The brain differences associated with autism begin before birth and continue to influence a person’s experiences and abilities as they grow and develop. These differences affect how autistic people see others and socialize with them, as well as how they process information and respond to their environment.[3]
The brain of an autistic person develops and functions differently from the brain of a neurotypical person. These differences are not defects, but rather variations in brain structure and function that lead to unique patterns of thinking, learning, moving, and paying attention. The specific brain changes vary from person to person, which explains why autism is described as a spectrum condition with such wide variation in how it affects different individuals.[6]
Research indicates that autism involves changes in how different regions of the brain communicate with each other. These alterations in brain connectivity can affect multiple systems simultaneously, including those responsible for social cognition, language processing, sensory integration, and motor control. The brain differences associated with autism influence how information is processed and integrated, which in turn affects behavior, communication, and social interaction.[2]
The sensory processing differences commonly seen in autism reflect how the autistic brain receives and interprets sensory information. Many autistic individuals have abnormalities in sensory processing, which can lead to either heightened sensitivity or reduced responsiveness to various stimuli. This can affect how they experience everyday environments and may contribute to some of the behavioral patterns associated with autism, such as seeking out or avoiding certain sensory experiences.[5]
While some people with autism have known genetic differences that affect brain development, many cases involve multiple genetic and environmental factors acting together in ways that are not yet fully understood. The highly heritable nature of autism suggests that genetic factors play a significant role in determining how the brain develops, but environmental influences during critical periods of development may also contribute to the final outcome.[2]
