Autism spectrum disorder (ASD) is a neurodevelopmental disorder characterized by deficits in social communication/interaction and restricted, repetitive behaviors, interests, and activities that cause significant impairment in functioning.1,2 The terminology and diagnostic criteria of ASD have changed several times since the disorder was first described in 1943, which has made it challenging to standardize research and identification.3 Yet in recent years, the prevalence of ASD has been markedly increasing, and the demographics of those diagnosed with ASD has been changing.1 Understanding the basic facts about ASD and the latest statistics and trends is essential for identifying patients early and providing them with optimal care.
Causes of Autism Spectrum Disorder
Although researchers have not identified a single, unifying cause of ASD, it is believed to result from a combination of genetic and environmental factors that affect the developing brain.4,5
Limited evidence suggests individuals with ASD have anatomical changes in the layers of their cortex. Patients with ASD exhibit differences in limbic areas involved in fear and emotional regulation, such as the amygdala. The brains of individuals with ASD frequently have “overgrowth” of their cortical areas and increased cerebral spinal fluid. They also have changes in the balance of excitatory and inhibitory neurotransmission and signs of abnormal cellular differentiation.4 Extensive evidence has demonstrated that the measles, mumps and rubella vaccine and other childhood vaccines do not cause ASD.5
Risk Factors for ASD
Many risk factors are associated with developing ASD. Older maternal and paternal age have each been associated with an increased risk of ASD.4
The use of certain medications during pregnancy has also been associated with the risk of having a child with ASD. Maternal antidepressant use — specifically selective serotonin reuptake inhibitors — during the second or third trimester has been associated with an increased risk of ASD, even after adjusting for maternal depression.6 Prenatal use of thalidomide and valproic acid also have been linked to an increased risk of ASD in offspring.4 Conversely, taking prenatal folic acid while also taking an antiepileptic medication might decrease the risk of ASD.4
Genetics may play a role in ASD risk. Siblings of a person with ASD have a higher chance of developing the disorder.4 One monozygotic twin have a higher chance of developing ASD if the other twin has it. Several chromosome-linked disorders, including Fragile-X and Down syndrome, have also been associated with the occurrence of ASD.4,7
What Are the Most Common Signs?
The current American Psychiatric Association diagnostic criteria for ASD are published in the Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition, Text Revision.2 Those criteria include 2 primary categories of symptoms: deficits in social communication/social interaction, and restricted, repetitive patterns of behavior, interests, or activities.2
Many people with ASD have atypical social behaviors. They may have impaired verbal and nonverbal communication, as well as difficulty making friends and maintaining relationships.2,7 Individuals with ASD tend to avoid eye contact and have difficulty interpreting normal social cues.7 They have trouble understanding implications or hidden meanings and engaging in the back-and-forth of a conversation.
Repetitive behaviors commonly exhibited by people with ASD include the following2:
- Repetitive motor movements or speech (such as body rocking, arm or hand flapping, repeating words just spoken by another person); and
- Insisting on sameness, inflexibly adhering to routine, or performing ritualized patterns of behavior (such as having difficulty with transitions, having rigid thinking patterns, needing to eat the same food each day).
Behaviors associated with ASD also may include a strong preoccupation with minute details or obsessions with certain topics or objects, such as a type of toy. Lining up objects in a specific manner or order can be a characteristic behavior of ASD.2,8
Individuals with ASD may be hyperreactive or hyporeactive to sensory input. They may be uncomfortable or upset by certain sounds. They may also display high sensitivity to certain types of visual or tactile stimuli, such a textures, lights, or movement.2,8
For an individual to receive an ASD diagnosis, their symptoms need to significantly impair their functioning. This could include their ability to focus at school, communicate with others, or hold a job and live independently.2,8
Comorbid Conditions
Autism spectrum disorder can co-occur with many other conditions, particularly neurological or psychiatric conditions.4 Approximately 37% of children with ASD also have an intellectual disability.9 Psychiatric disorder such as anxiety disorders, attention deficit/hyperactivity disorder, mood disorders, disruptive behavior disorders, and obsessive-compulsive disorder are also highly comorbid with ASD.4 Most adults with ASD have at least 1 comorbid psychiatric condition.10 Other common comorbidities include seizures, sleep disorders, gastrointestinal problems, and immune dysfunction.4,11
When Is Autism Spectrum Disorder Diagnosed?
Autism spectrum disorder is usually diagnosed during childhood, generally during the first 2 years of life.7 Social deficits typically are noticeable in the toddler years, and parents may recognize that their children miss developmental milestones. Early diagnosis is critical for the implementation of early interventions, including psychological and behavioral therapies.7 Earlier ASD diagnoses are associated with improved quality of life compared to those in whom the diagnosis is delayed.12
A person’s sex may play a role in the age of diagnosis. A study that compared the age at diagnosis of ASD in 208 people found that males were more likely to be diagnosed before they were age 18 years, and females were more likely to be diagnosed in adulthood.12
In the United States, early diagnosis of ASD may be improving. According to the Centers for Disease Control and Prevention (CDC), in 2020, children born in 2016 were 1.6 times as likely as children born in 2012 to be identified as having ASD by age 4.9
Autism Spectrum Disorder Statistics
Autism spectrum disorder is an increasingly common condition. The CDC’s Autism and Developmental Disabilities Monitoring (ADDM) Network published its most recent surveillance report in 2023; it focused on data from 2020.1,13 Th ADDM Network found approximately 1 in 36 children in the United States was estimated to have ASD.1,13 This prevalence has increased steadily over the last 20+ years. The estimated prevalence of ASD was 1 in 150 children in 2000, and 1 in 44 in 2018.1,13
The World Health Organization estimates that worldwide, approximately 1 in 100 people have ASD.5 This is not likely to be accurate, however, because many low-income countries have limitations with consistent reporting methods. In developing countries, there also may be less overall awareness of ASD and access to consistent medical care.5
Autism spectrum disorder affects people of all racial and ethnic groups.13 In the ADDM Network report, the estimated prevalence of ASD in 2020 was highest among Hispanic (3.3%) and Asian/Pacific Islander (3.2%) children, were followed by Black (2.9%), American Indian (2.7%), and White (2.4%) children.1,14 Children of 2 or more races had the lowest incidence of ASD (2.3%).1,14 These data differ from previous estimates, in which the prevalence was highest among White children.9
In the ADDM Network report, the prevalence of ASD also varied by geographic location. Of 11 state sites included in the report (Arizona, Arkansas, California, Georgia, Maryland, Minnesota, Missouri, New Jersey, Tennessee, Utah, and Wisconsin), the overall prevalence was highest in California (4.5%), while the estimated prevalence was lowest in Maryland (2.3%).1,9
The prevalence of ASD varies greatly by sex. In the United States, boys are about 4 times more likely than girls to receive a diagnosis of ASD.1,9 However, 2020 was the first time the ADDM Network estimated that the prevalence in girls was greater than 1%.
Author Bio
Hannah Actor-Engel, PhD, earned a BS in Neural Science at New York University and her PhD in Neuroscience at the University of Colorado. She is a multidisciplinary neuroscientist who is passionate about scientific communication and improving global health through biomedical research.
