Mental retardation is characterized by subaverage intellectual functioning and deficits in adaptive behaviors that manifest before age 18. It is diagnosed based on IQ scores and impairments in areas such as communication, self-care, social skills, and more. The levels of mental retardation range from mild to profound based on IQ scores. The causes include genetic syndromes like Down syndrome and Fragile X syndrome, as well as various prenatal, perinatal, and postnatal factors. Current theories view mental retardation as resulting from an interaction between biological and environmental influences.