Intellectual Disability 231Table 28.1Characteristics of mild and marked intellectual disability
Mild intellectual Marked intellectual
disability disabilityMajor CNS disorder 14% 72%
Prevalence in families oflowsocio-economic status 3.3% 0.8%
Prevalence in families ofhighsocio-economic status 0.3% 0.4%
Male 46% 63%
Mean IQ of siblings 85 103
Source:Data from Bromanet al.(1987).
be taken too literally: organic and normal-variant causes of intellectual
disability may co-exist, with additive or synergistic effects.
Causes of intellectual disability
1 Mild intellectual disability. Most mild intellectual disability is assumed to
be due to the same sorts of polygenic and environmental factors that
determine IQ within the normal range. Just as the polygenic component
is assumed to be due to many genes, each of which has a small but addi-
tive effect on IQ, so the psychosocial component seems to involve many
factors, each of which has a small additive effect on IQ. Examples of
adverse psychosocial factors include: lack of early stimulation; reduced
access to books; and parental indifference to educational achievements.
Adverse factors in the physical environment, such as exposure to low-
level lead, may also add to the effects of genetic and psychosocial factors.
2 Marked intellectual disability. The organic causes that account for the
majority of marked intellectual disability (and for some instances of mild
intellectual disability) are conventionally subdivided according to their
time of onset:
(a)Prenatal: for example, chromosome abnormalities, single gene de-
fects, congenital infections, fetal alcohol syndrome.
(b)Perinatal: for example, intraventricular haemorrhage in premature
neonates, severe neonatal jaundice. Though the role of obstetric
complications was emphasised in old textbooks, it now seems
unlikely that these are common causes of intellectual disability.
If a child has a difficult delivery and subsequently turns out to
have marked intellectual disability, was the delivery to blame? Not
usually. More often, the obstetric complications were either irrele-
vant or a consequence of pre-existing abnormalities in the unborn
child. Thus, children with chromosomal problems or prenatal brain
damage are at greater risk of an abnormal delivery.
(c)Post-natal: for example, encephalitis and meningitis, trauma due to
child abuse and accidents, severe lead poisoning.