When a person with mental retardation confronts the criminal justice system, they are uniquely unable to take advantage of legal safeguards and to protect their constitutional rights. An estimated 89 percent of all people with retardation have I. Arithmetic abilities were limited to addition and subtraction with the help of concrete aids such as fingers."The threshold I. level for a diagnosis of mental retardation has been progressively lowered over the years, in part because of awareness of the damaging social prejudice suffered by those labeled "retarded." In 1959, the American Association on Mental Deficiency set 85 as the I. below which a person was considered to be retarded. Although there are significant variations among those with mental retardation, in terms of their ability to function and their skill levels, all have significant limitations in their "effectiveness in meeting the standards of maturation, learning, personal independence, and/or social responsibility that are expected for his or her age level and cultural group." For instance, an adult with mental retardation may have trouble driving a car, following directions, participating in hobbies or work of any complexity, or behaving in socially appropriate ways.