Etiology

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 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.