Most people with mental retardation have relatively minor deficits and are classified as having mild retardation. These individuals, who have IQ scores ranging from 55 to 69, constitute some 90 percent of all people with mental retardation. Although their development is typically slower than that of their peers, they can function quite independently by adulthood and are able to hold jobs and have families of their own.
At greater levels of retardation—moderate retardation (IQs of 40 to 54), severe retardation (IQs of 25 to 39), and profound retardation (IQs below 25)—the difficulties are more pronounced. For people with moderate retardation, deficits are obvious early, with language and motor skills lagging behind those of peers. Although these individuals can hold simple jobs, they need to have a moderate degree of supervision throughout their lives. Individuals with severe and profound mental retardation are generally unable to function independently and typically require care for their entire lives.