252 Chapter 31
Conversely, being markedly behind in reading attainments (reading back-
wardness) does not necessarily imply SRD because poor reading skills may
be in line with the low intelligence (for example, subject A in Box 31.1).
Box 31.1A schematic representation of the relationship between
reading age and IQ in 10-year-olds6068
Reading age10141280 100
IQ120 140A
BC–2SD+2SDMean predictedAlthough it is somewhat arbitrary to define SRD in terms of reading
attainments that are at least two SDs below the predicted level, this cut-
off does identify a group who have a substantial and persistent disability.
These individuals may represent an extra ‘hump’ at the bottom of the
normal distribution curve of reading ability. Whether such individuals are
qualitatively or just quantitatively different from individuals who are less
far behind with their reading (for example, only 1 or 1^12 SDs behind) has
not yet been resolved.
Epidemiology
SRD affects 3–10% of children and adolescents. Most studies show it to
be two to three times more common in males than females. SRD is more
common among the children of parents with manual rather than non-
manual occupations. The prevalence of SRD at 10 years of age was 4%
on the Isle of Wight and 10% in inner London; a difference that could be
explained by social, school and family factors.
Associated features
1 Spelling is often more severely affected than reading itself, and problems
with spelling may persist even if reading becomes reasonably fluent.
Arithmetical skills are not typically as far behind as reading skills,
though some delay is usual.
2 Spelling errors are often severe and bizarre. They are commonly non-
phonetic (for example, ‘umderlee’ for umbrella) rather than phonetic
(for example, ‘mite’ for might). Reading errors are often based on