The child is essentially sound physically, and there is no history of disease of injury that might have produced brain damage. Retardation is usually minimal; the individual suffers from a general deficiency rather than an identifiable defect. Such individuals tend to come from families that are low in intelligent and living under impoverished conditions.
This type of retardation has been called familial-cultural the cause is assumed to be both genetic and environmental. Individuals of low intelligence cannot get jobs that pay well; they marry other low IQ persons and produce children whose intellectual potential is limited.
Environmental circumstances-inadequate infant nutrition and medical care, lack of intellectual stimulation and parental concern-contribute to further intellectual impairment. Thus a vicious cycle continues.