Nonverbal autism, also called nonspeaking autism, is a subset of autism spectrum disorder (ASD) where the person does not learn how to speak.^{[failed verification]}

Early intervention in nonspeaking autism emphasizes the critical role of language acquisition before the age of five in predicting positive developmental outcomes; acquiring language before age five is a good indicator of positive child development, that early language development is crucial to educational achievement, employment, independence during adulthood, and social relationships. The likelihood of acquiring functional language in the future past this age is minimal.

The biological basis of autism is unknown. However, the most obvious signs of autism, such as atypical social and language development, and restricted or repetitive behaviors and interests, often present themselves between the ages of two and three, and most children with ASD can be diagnosed in early childhood as a result. Other disorders such as epilepsy, ADHD, gastrointestinal problems, sleep disorders, hypermobility, depression and anxiety often accompany ASD.

The causes of nonspeaking autism are unknown. However, there appears to be a relationship between joint attention and verbal communication. Joint attention occurs between two individuals when one draws the other's attention to an object through gesturing (i.e. eye gazing, pointing). The ability to achieve joint attention at an early age plays a significant role in language development, and studies indicate severe lapses in joint attention in autistic children. Research findings indicate that young children diagnosed with autism exhibit challenges in both initiating and responding to joint attention. Notably, while the difficulties in responding to joint attention may improve with development, it has been observed that autistic children who have higher IQs and lower mental ages may display distinct joint attention deficits. These deficits in joint attention are observed consistently across various age groups, spanning from infancy to adolescence within the autism spectrum. Moreover, there is a concurrent relationship between joint attention skills and language development, with joint attention abilities serving as predictors for long-term gains in expressive language among individuals with autism. In one study, researchers suggest that a displayed pattern of delays, absences, or a general impaired response to stimuli (hyporesponsiveness) and a fascination with intense or repetitive stimulation (sensory seeking) is more likely in nonspeaking autistic children, suggesting that both hyporesponsiveness and sensory seeking is related to poor communication outcomes in children with ASD.

Research also suggests that the differences in nonverbal communication are evident during early childhood. In a comparison of autistic children aged two to three years old with children who had other developmental delays and typically developing children, autistic children demonstrated significantly greater impairments in joint attention, eye contact, pointing, and gesture use. These early nonverbal communication differences may help distinguish autism from other developmental disorders.

There is a growing body of tentative evidence indicating the amygdala's involvement in the development of autism. The amygdala theory of autism focuses on the importance of the amygdala in relation to social functioning and observes that autism is largely a severe impairment of social functioning. The amygdala is thought to be associated with the fight-or-flight response in animals and its activity is heavily correlated with fear in humans. Additionally, it has been heavily implicated in relation to social functioning in various animal studies. Evidence suggests an amygdala hyperactivity model may be more accurate than one comparing it to a lesion.

Lesion studies have shown that amygdala damage results in severe social impairment among animal models. Vervet monkey mothers with amygdala lesions were shown to be much less caring with their young, neglecting and even abusing them. Rats with amygdala ablations become much more docile. Monkeys with lesions to the anterior temporal lobe develop a disorder known as Klüver–Bucy syndrome, characterized by loss of fear, hypersexuality, hyperorality, and an inability to recognize visual objects (often, but not always).

Evidence shows the amygdala accounts for the emotional, oral, and sexual abnormalities listed above. These abnormalities coincide with several characteristics of the diagnostic guidelines for autism, at least passably for an animal model.