Organic and
Familial Causes
Workers in the field of mental
retardation have found it useful to divide the retarded
population into two groups, organic and familial. An IQ of 50 is set as a rough dividing line between the two groups, with the organic lying below and the familial above this line.
The organically, or pathologically, retarded, who comprise 15% to 25% of the total retarded population, suffer from
retardation of known organic causes. The causes may be genetic in origin, such as Down
syndrome, fragile X syndrome, and phenylketonuria. They may also be prenatal, such as rubella (see German measles); perinatal, such as an incident of asphyxia during birth; or postnatal, such as meningitis or head trauma. The second group, the familially (also called subculturally or physiologically) retarded, comprise 75% to 85% of the retarded population. Genetic causes may be involved, but sociocultural deprivation and economic status are important factors. These individuals are generally the offspring of parents who are themselves of low intelligence. They also tend to have experienced impoverished conditions in their early years. A disproportionate number come from families of the lowest socioeconomic classes.
Efforts have been aimed at the medical prevention or amelioration of various types of organic retardation. Genetic testing is used to tell parents if their unborn offspring will suffer from Down's syndrome or from a variety of other organic disabilities. Genetic counseling informs prospective parents in several high-risk categoriesÑfor example, parents who have already had a retarded child, and older parentsÑof the chances that their next child will be retarded. Special dietary supplements have helped children with phenylketonuria, an inborn chemical imbalance, to avoid the progressive retardation that usually accompanies the syndrome if left untreated. Medical prevention or amelioration, however, does not exist for familial retardation.