Intellectual disability can be divided into 4 grades based on intelligence quotients (or developmental quotients):

• Mild disability (IQ 50–74). Common in disadvantaged communities with poverty, malnutrition, neglect and few educational opportunities. These children can usually remain in mainstream education.
• Moderate disability (IQ 35–49). They need special schooling.
• Severe (IQ 20–34) and profound disability (IQ less than 20). They require constant supervision and are dependent on others for their daily needs.

Moderate and severe intellectual disability are usually due to brain damage.