The goal of this observational study is to compare the developmental trajectories of prosocial behaviors and functional network connections in infants and toddlers at high and low risk of autism spectrum disorder (ASD). The main questions it aims to answer are what the differences in prosocial behaviors and related brain network connections between infants/toddlers at high and low risk of ASD are. Participants will receive developmental and social communicational assessments (Griffiths Mental Developmental Scales, Peabody Developmental Motor Scales, The Communication and Symbolic Behavior Scales Developmental Profile Infant-Toddler Checklist), resting-state EEG and MRI in a natural sleeping state.

The longitude observational study will be performed to compare the developmental trajectories of prosocial behaviors and functional network connections in infants and toddlers at high and low risk of autism spectrum disorder (ASD). We will follow up the subjects at 3-, 6-, 9-, 12-, 18-, 24-, 30-, and 36-month-old. Clinical diagnosis will be made on 24-month-old or after, with the DSM-5 as the diagnostic criterion.

Inclusion Criteria:

• Subjects were enrolled as high-risk if they had an older sibling with a clinical diagnosis of ASD confirmed on the Autism Diagnostic Observation Schedule, second edition (ADOS-2) or Autism Diagnostic Interview (ADI-R).
• Subjects were enrolled in the low-risk group if they had an older sibling without evidence of ASD and no family history of a first or second-degree relative with ASD.

Exclusion Criteria:

1. diagnosis or physical signs strongly suggestive of a genetic condition or syndrome (e.g., fragile X syndrome) reported to be associated with ASDs;
2. a significant medical or neurological condition affecting growth, development or cognition (e.g., CNS infection, seizure disorder, congenital heart disease);
3. sensory impairment such as vision or hearing loss;
4. low birth weight (<2000 grams) or prematurity (<37 weeks gestation);
5. possible perinatal brain injury from exposure to in-utero exogenous compounds reported to likely affect the brain adversely in at least some individuals (e.g., alcohol, selected prescription medications);
6. contraindication for MRI (e.g., metal implants);
7. a family history of intellectual disability, psychosis, schizophrenia or bipolar disorder in a first-degree relative.

The longitude study will be performed in high-risk infants of ASD who have at least one older sibling with a clinical diagnosis of ASD. The low-risk group will also be recruited from healthy infants as control group if they had an older sibling without evidence of ASD and no family history of a first or second-degree relative with ASD.