Abstract
The heterogeneous nature of mental retardation, as well as certain common practices of workers in the area, has resulted in a variety of conceptual am biguities. Considerable order could be brought to the area if, instead of viewing all retardates as a homogeneous group arbitrarily defined by some I.Q. score, workers would clearly distinguish between the group of retardates known to suffer from some organic defect and the larger group of retardates referred to as familial retardates. It is the etiology of familial retardation that currently constitutes the greatest mystery.