Intellectual disability can be caused by a problem that starts any time before a child turns 18 years old – even before birth. It can be caused by injury, disease, or a problem in the brain. For many children, the cause of their intellectual disability is not known. Some of the most common known causes of intellectual disability – like Down syndrome, fetal alcohol syndrome, fragile X syndrome, genetic conditions, birth defects, and infections – happen before birth. Others happen while a baby is being born or soon after birth. Still other causes of intellectual disability do not occur until a child is older; these might include serious head injury, stroke, or certain infections.
Usually, the more severe the degree of intellectual disability, the earlier the signs can be noticed. However, it might still be hard to tell how young children will be affected later in life.
There are many signs of intellectual disability. For example, children with intellectual disability may:
■ sit up, crawl, or walk later than other children
■ learn to talk later, or have trouble speaking
■ find it hard to remember things
■ have trouble understanding social rules
■ have trouble seeing the results of their actions
■ have trouble solving problems