Although some cases of mental retardation are idiopathic (without a specific known cause), many known causes account for many of the cases of mental retardation. The difficulties in teasing out factors involved in mental and behavioral disorders are seen clearly in the study of children exposed prenatally to radiation following the Chernobyl nuclear plant disaster in 1986, as reported by S. Igumnov and V. Drozdovitch. The children who had been exposed to radiation displayed borderline intellectual functioning and emotional disorders to a greater degree than those in a control group. Other unfavorable social-psychological and sociocultural factors included a low educational level of the parents and problems associated with relocation from the contaminated areas.

Similar complications are seen in the work of M. S. Durkin and colleagues on prenatal and postnatal risk factors among children in Bangladesh. The study screened more than ten thousand children from both rural and urban areas. Significant predictors of serious mental retardation included maternal goiter and postnatal brain infections. Consanguinity (having ancestors who were closely related) also was a significant factor in the rural areas. For less severe mental retardation, maternal illiteracy, maternal history of pregnancy loss, and small size for gestational age at birth were significant independent risk factors.

It is convenient to separate the known causes of mental retardation into

DSM-IV-TR Criteria for Mental Retardation

Significantly subaverage intellectual functioning:

• for children and adults, IQ of approximately 70 or below

• for infants, clinical judgment of significantly subaverage intellectual functioning

Concurrent deficits or impairments in adaptive functioning (effectiveness in meeting standards expected for age and cultural group) in at least two of the following areas:

• communication

• social/interpersonal skills

• use of community resources

• self-direction

• functional academic skills

Onset before age eighteen

DSM code based on degree of severity reflecting level of intellectual impairment:

• Mild Mental Retardation (DSM code 317): IQ level of 50-55 to ap-prox. 70

• Moderate Mental Retardation (DSM code 318.0): IQ level of 35-40 to 50-55

• Severe Mental Retardation (DSM code 318.1): IQ level of 20-25 to 35-40

• Profound Mental Retardation (DSM code 318.2): IQ level below 20 or 25

• Mental Retardation, Severity Unspecified (DSM code 319): IQ level Untestable the two categories of genetic and acquired or environmental. However, many cases of mental retardation may be a result of the interaction of several genes and the environment, in which case the disorder is said to be multifactorial.

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