Reading Difficulties 251Most twin studies suggest that genetic variation accounts for about
30–50% of individual differences in reading abilities. Environmental fac-
tors, including the amount of parental input and the quality of schooling,
also have a major impact. Parent–child interaction centred on books
contributes to children’s success in reading. For young children who
cannot yet read, parental storybook reading enhances children’s language
comprehension and expressive language skills which in turn helps learning
to read later.
Specific reading difficulties (SRD)
Some children and adolescents have reading attainments that are sub-
stantially poorer than would be predicted from their age and IQ: these
individuals are said to have SRD. Tests of reading attainment may assess
reading accuracy or reading comprehension. Reading accuracy is typically
tested by asking the individual to read words or passages of increasing
difficulty. In English, it is relatively easy to tell when someone cannot read
a particular word because he or she stumbles over it or mispronounces
it. This is harder to judge in languages such as Spanish that have very
predictable spelling – making it much easier to pronounce unknown
words. Reading comprehension is tested by asking individuals questions
about passages they have read, establishing how well they have taken in
the meaning of the words. People with SRD often have less difficulty on
tests of comprehension as opposed to accuracy since they can use clues
from the general context to guess the overall meaning even when they
have not been able to read some of the words. At the other extreme,
people with hyperlexia do well on tests of reading accuracy, but do not
necessarily understand what they have read.
The relationship at any given chronological age between reading age and
IQ is shown schematically in Box 31.1. The correlation between reading
age and IQ is fairly substantial (with a correlation coefficient of 0.6). Not
surprisingly, brighter individuals are likely to be reading better. It is worth
noting, however, that predicted reading age does not generally equal men-
tal age – there is regression towards the mean. Thus, a 10-year-old with a
mental age of 13 will not, on average, be reading up to 13-year-old level,
while a 10-year-old with a mental age of 7 will, on average, be reading at
better than 7-year-old level. Roughly 95% of children and adolescents fall
within two standard deviations of their predicted reading age. SRD refers
to those individuals, such as subjects B and C in Box 31.1 whose reading
attainments are over two standard deviations (SDs) below their predicted
reading level. This corresponds, at the age of 10, to being about 2^12 years
behind the predicted level. Though most individuals with SRD are reading
at well below the average for their chronological age (for example, subject
B in Box 31.1), some very bright individuals with SRD do have average
reading ability (for example, subject C in Box 28.1). Therefore because a
child or adolescent has a normal reading age does not mean SRD is absent.