Several factors have been identified as being responsible for causing mental retardation. These factors have been broadly classified into these groups.

• Unexplained - This category is the largest and a catchall for undiagnosed incidences of mental retardation.
• Trauma (prenatal and postnatal) and physical agents
  • Intracranial hemorrhage before or after birth
  • Hypoxic injury before, during or after birth
  • Severe head injury
• Infections (congenital and postnatal) and intoxication
  • Congenital rubella
  • Meningitis
  • Congenital CMV
  • Encephalitis
  • Congenital toxoplasmosis
  • Listeriosis
  • HIV infection
  • Drug usage during pregnancy - alcohol, tobacco, illegal substances
  • Chronic maternal illnesses such as diabetes, kidney disease and hypertension
• Chromosomal abnormalities
  • Errors of chromosome numbers (Down’s syndrome)
  • Defects in the chromosome or chromosomal inheritance (Fragile X syndrome, Angelman syndrome, Prader-Willi syndrome)
  • Chromosomal translocations (a gene is located in an unusual spot on a chromosome, or location on a different chromosome than usual) and deletions (Cri du chat syndrome)
• Genetic abnormalities and inherited metabolic disorders
  • Galactosemia
  • Tay-Sachs disease
  • Phenylketonuria
  • Hunter syndrome
  • Hurler syndrome
  • Sanfilippo syndrome
  • Metachromatic leukodystrophy
  • Adrenoleukodystrophy
  • Lesch-Nyhan’s syndrome
  • Rett syndrome
  • Tuberous sclerosis
• Metabolic
  • Reye’s syndrome
  • Hypernatremic dehydration
  • Congenital hypothyroid
  • Hypoglycemia
• Toxic
  • Intrauterine exposure to alcohol, cocaine, amphetamines, and other drugs
  • Methylmercury poisoning
  • Lead poisoning
• Nutritional
  • Kwashiorkor
  • Marasmus
  • Malnutrition
• Environmental and Psycho-social
  • Poverty
  • Low socioeconomic status
  • Deprivation syndrome

Assessment of a child suspected of having a developmental disability, such as mental retardation, may establish whether a diagnosis of mental retardation or some other developmental disability is warranted, assessing eligibility for special educational services, and/or aid in determining the educational or psychological services needed by the child and family. Child assessment is the systematic use of direct as well as indirect procedures to document the characteristics and resources of an individual child. The four components of assessment are norm-referenced tests, interviews, observations, and informal assessment. These components complement each other and form a firm foundation for making decisions about children.

In diagnosing infants or preschoolers, it is important to distinguish between mental retardation and developmental delay. A diagnosis of mental retardation is only appropriate when cognitive ability and adaptive behavior are significantly below average functioning. In the absence of clear-cut evidence of mental retardation, it is more appropriate to use a diagnosis of developmental delay. In practice, children under the age of 2 should not be given a diagnosis of mental retardation unless the deficits are relatively severe and/or the child has a condition that is highly correlated with mental retardation (e.g., Down syndrome).

Assessment tools for children with mental retardation can be further classified into four distinct categories. These are

• Developmental Assessment Tools
• Adaptive Behavior Assessments
• Achievement Tests
• Other Tools

In recent years, identifying functional curriculum goals for students with mental retardation has become a major priority for special educators. The curriculum for such students should focus on functional skills that will help the student be successful in self-care, vocational, domestic, community and leisure domains.

Research has developed an instructional approach that reliably produces improvements in the lives of individuals with mental retardation. This approach is known as applied behavior analysis and it uses a systematic approach to teaching based on scientifically demonstrated principles that describe how the environment affects learning. Most teaching tactics based on behavior analysis share the following six features

1. Task analysis of the new skill or behavior to be learned - This means breaking down complex or multiple-step behaviors or skills into small, easier-to-teach, subtasks. The subtasks are then sequenced, either in the natural order in which they are typically performed or from the easiest to the most difficult. Assessing a student's performance on a sequence if task-analyzed subskills helps pinpoint exactly where instruction should begin.
2. Direct and frequent measurement of the subject's performance of the skill - Measurement is direct when it objectively records the learner's performance of the behavior of interest in the natural environment for that skill. Measurement is frequent when it occurs ona regular basis; ideally, measurement should take place as often as instruction occurs.
3. Frequent opportunities for active student response during instruction - Active student response occurs when a students emits a detectable response to ongoing instruction. Depending on the instructional objective, examples of such student response include words read, problems answered, boards cut, test tubes measured, praise and supportive comments spoken, notes or scales played, sentences written, etc.
4. Immediate and systematic feedback for student performance - Feedback is generally most effective when it is specific, immediate, positive, frequent and differential.
5. Procedures for achieving the transfer of stimulus control from instructional cues or prompts to naturally occuring stimuli - Instead of waiting to see whether a student will make a correct response, the effective teacher of mental retartdation provides a prompt that makes a correct response very probable. The response prompts are then gradually and systematically withdrawn, and the student's behavior comes under the stimulus control of the curriculum content, or things in the natural enviroment that typically serve as cues for that skill.
6. Strategies for promoting the generalization and maintenance of newly learned skills to different situations and enviroments - These refer to the extent to which students extend what they have learned across settings and over time. There are four main strategies used by special educators to promote the generalization and maintenance of new skills and knowledge:
   • Aim for naturally ocurring contingencies of reinforcement
   • Use a general case strategy to select teaching examples
   • Program common stimuli
   • Teach self-management skills