A condition of incomplete or halted development of the mind, which is characterized by impairment of skills as manifested during developmental period that contributes to the overall level of intelligence (WHO).

Significantly below average intellectual functioning (IQ is <70) with accompanying impairment in person's effectiveness in meeting the standards expected of one's age, as is expected by social and cultural influences in the following areas:

• · Communication
• · Self-care
• · Home living
• · Social/interpersonal skills
• · Use of community resources
• · Self-direction
• · Functional academic skills
• · Work, leisure, health and safety

The four different degrees of mental retardation are classified into mild, moderate, severe and profound. The four categories are based on the functioning level of the every person.

There are other physical defects associated with MR

A. Genetic factors of mental retardation

The inherited abnormality of the genes like the fragile X syndrome can be the cause of mental retardation. The retardation can be also brought about by an accident or mutation in genetic development.

B. Prenatal illnesses and issues of mental retardation

• The Fetal alcohol syndrome caused by excessive alcohol intake in the first twelve weeks (trimester) of pregnancy.
• The other causes that had been linked to mental retardation are cigarette smoking and drug abuse of women during their pregnancy.
• There are also several maternal infections and illnesses that may cause mental retardation, namely: glandular disorders, rubella, toxoplasmosis, and cytomegalovirus infection.
• The low flow of oxygen to the fetus may cause brain damage and metal retardation if the pregnant mother has high blood pressure (hypertension) or blood poisoning (toxemia).
• The mental retardation is also frequently caused by birth defects which had caused physical deformities of the head, brain, and central nervous system.
• The neural tube defect is another cause of mental retardation. This happens when the birth defect in which the neural tube that forms the spinal cord does not close together completely.
• The mental retardation may result also from defect like the Hydrocephalus. This causes the person’s learning impairment by putting pressure on the brain.

C. Childhood illnesses and injuries of mental retardation

• Hyperthyroidism, whooping cough, chickenpox, measles, and Hib disease (a bacterial infection) may cause mental retardation if they are not treated adequately.
• An infection of the membrane covering the brain (meningitis) or an inflammation of the brain itself (encephalitis) causes swelling that in turn may cause brain damage and mental retardation.
• Traumatic brain injury caused by a blow or a violent shake to the head may also cause brain damage and mental retardation in children

D. Environmental factors of mental retardation

• Ignored or neglected infants who are not provided the mental and physical stimulation required for normal development may suffer irreversible learning impairments.
• Mental retardation can be caused by some environmental factors such as: poverty and suffer from malnutrition, unhealthy living conditions, and improper or inadequate medical care.
• Mental retardation can be the result of too much exposure to the element called “lead".
• Another cause of mental retardation among is through eating the flaking lead-based paint often found in older buildings because of the development of lead poisoning.

A Number of individuals differ in relation to the level of country development.

In developing countries, the percentage of young persons aged 18 and below suffering from severe mental retardation reaches 4.6% percent while in developed countries it is estimated to be between 0.5% percent - 2.5% percent.