What constitutes mental retardation?

Mental retardation i: Mental retardation is used to describe fundtioning that is significantly below someones chronologic age; e.g., someone who is 4 years old but can only function like someone age 2 years. It usually refers to cognitive and communication skills but can also involve motor, emotional and behavioral skills. Official definition is iq and adaptive behavior score < or = 70 (90-110 is considered normal).

Legal definition: According to the individuals with disabilities education act, intellectual disability (formerly mr) is "significant subaverage general intellectual functioning, existing concurrently with deficits in adaptive behavior and manifested during the developmental period, that adversely affects a child's educational performance." idea 2004, public law 108-446.