Intellectual and developmental disabilities (IDDs) encompass a group of conditions that typically begin before a child turns 18. They can cause lifelong challenges in one or more area of physical, intellectual, or emotional development. The term “IDD” is often used to describe situations where both an intellectual disability (ID) and other developmental disabilities are present.

Signs of an IDD often appear as developmental delays, meaning the child reaches developmental milestones later than their peers. The severity is often classified as mild, moderate, severe, or profound.

2.1 Intellectual Disability (ID)

Intellectual disability is the third leading cause of Disability-Adjusted Life Years among children ages 0 to 14, and the seventh among all ages (Morinaga et al., 2024). Common causes include fetal alcohol spectrum disorders, genetic and chromosomal conditions (such as Down syndrome and fragile X syndrome), and certain infections during pregnancy (CDC, 2025).

Intellectual disability is characterized by significant limitations in two areas:

1. Intellectual functioning: the ability to learn, reason, problem-solve, plan, and make judgments. Standardized tests of intelligence (IQ) often show a score of around 70-75 or below.
2. Adaptive behavior: includes everyday social and practical skills needed to function in daily life, such as communication, social participation, and personal care. Onset typically occurs during the developmental period (childhood and adolescence).

The Diagnostic and Statistical Manual of Mental Disorders Fifth Edition (DSM-5), classifies the severity of intellectual disability by the level of support required:

• mild if a person can live independently with minimal levels of support,
• moderate if a person can live independently with moderate levels of support,
• severe if a person needs daily assistance for self-care activities and safety supervision, and
• profound if a person needs 24-hour care (Morinaga et al., 2024).

People with intellectual disabilities often experience societal stigma, discrimination, and a lack of social support. These factors may be associated with greater risk for victimization by strangers, direct support staff, or family members (Brendli, Broda, and Brown, 2022).

2.2 Developmental Disability

The term “developmental disability” is a broader category of challenges that can be intellectual, physical, or both.

Developmental disabilities affect physical development, learning, language, and behavior. These conditions start during the developmental period, impact daily functioning, and generally last a lifetime. While most begin before birth, some result from injury, infection, or other factors after birth (CDC, 2025).

Developmental disabilities are thought to be caused by a mix of factors including:

• genetics,
• parental health and behaviors (such as smoking and drinking) during pregnancy,
• complications during birth,
• infections the mother might have during pregnancy or the baby might have very early in life, and
• exposure of the mother or child to high levels of environmental toxins, such as lead (CDC, 2025).

Examples of developmental disabilities include:

• Autism spectrum disorder: it affects communication, social interaction, and behavior.
• Cerebral palsy: which affects movement, muscle tone, and posture.
• Down syndrome: a genetic condition that typically includes intellectual disability.
• Fetal alcohol spectrum disorders: caused by prenatal exposure to alcohol.
• Spina bifida: a birth defect where the spinal cord doesn't develop properly.

Attention-Deficit/Hyperactivity Disorder (ADHD) and learning disabilities are sometimes included in the broader category of developmental disabilities.