230 Chapter 28
deviation of 15. Many of these individuals are never identified by medi-
cal, educational or social services. Sometimes this is because their social
functioning is adequate and they are coping well enough in mainstream
schools, but in other instances they are drowning quietly without the
extra input that might have helped them had their intellectual disability
been recognised.
2 Marked intellectual disabilityis used in this chapter to refer to an IQ
under 50, sometimes banded into three levels: moderate (IQ 35–49),
severe (IQ 20–34) and profound IQ 0–20. Marked intellectual disability
affects about 0.4% of the population, which is some ten times higher
than would have been expected if IQ were normally distributed with
a mean of 100 and a standard deviation of 15. In other words, there
is a small extra ‘hump’ at the bottom of the normal IQ distribution.
Individuals with marked intellectual disability are nearly always known
to health, education or social services, either because of the severity of
their educational difficulties or because of co-existing physical features
such as cerebral palsy or epilepsy.
The two-population model
It is useful for some purposes to distinguish between two sorts of in-
tellectual disability:organicandnormal variant(sometimes described as
‘subcultural’). The distinction can be clarified by an analogy. The genetic
and environmental factors that account for the normal variation in adult
height will inevitably result in some adults being at the lower end of the
height distribution. In addition to these individuals with normal-variant
short stature, there are other individuals with short stature due to organic
conditions, for example, genetic syndromes such as achondroplasia. The
organic group will tend to be shorter and to have more medical problems.
The normal-variant group will have many relatives of below-average
height (due to shared environment and polygenes), whereas most of the
relatives of the organic group will be of around average height because
they do not have the same organic syndrome. Using a specific height cut-
off to determine short stature, it would be possible to define a very short
stature group (which is mostly organic), as opposed to a moderately short
stature group (which is mostly a normal variant), but no height cut-off
would distinguish perfectly between organic and normal-variant groups.
For intellectual disability, the equivalent of a height cut-off is an IQ cut-
off of around 50. As shown in Table 28.1, this approach does identify
two relatively distinct populations. By comparison with mild intellectual
disability, marked intellectual disability is more often associated with
neurological disorder and less often associated with social disadvantage.
Only mild intellectual disability is associated with a below-average IQ in
relatives. Not surprisingly, an IQ cut-off of 50 cannot distinguish perfectly
between organic and normal-variant cases. Though useful as a conceptual
model, the two-population model of intellectual disability should not