It had been widely supposed from the time of Hinshelwood and Orton that there were more dyslexic males than females. However, quite a stir was created in the 1990s when some researchers in the USA challenged this view (see, in particular, Shaywitz et al., 1990). Head teachers of schools for dyslexic pupils in Britain had always planned for an excess of boys, and if they were now to have an increased intake of girls this would have all kinds of planning implications - more female staff, building of fresh toilets etc.
I looked forward eagerly to reading the paper by Shaywitz et al. (1990), but with some scepticism, I must admit. The argument used by Dr Shaywitz and colleagues is an interesting one. They argue that the alleged excess of boys was a consequence of referral bias. This was because boys with educational difficulties were more trouble to teachers than were girls with similar difficulties. The true proportion, they argue, could be determined only by a proper sampling of school populations.
In my own study (Miles, 1993a) there were 182 boys and 41 girls. I therefore had to ask myself if, in accordance with the argument of Shaywitz et al., the excess of males who had come to me for assessment was due to referral bias. These were not children who had been picked out in class because they were a nuisance; they were children who were causing particular concern to their parents because of their literacy problems. Was it conceivable that in these circumstances parents were more than four times as likely to refer their sons rather than their daughters? I did not think so.
I was surprised to find, however, that the definition of dyslexia in the Shaywitz et al. paper was simply poor reading in relation to intelligence; there was no reference to poor spelling. They also found, not surprisingly, that children who came out as dyslexic on one occasion and had then improved their reading had thereby ceased to be dyslexic on a later testing. Clearly the authors of the paper were not talking about 'specific developmental dyslexia' in the sense that Dr Macdonald Critchley had promoted, and which was the familiar sense of the word 'dyslexia' in Britain at that time.
When Mary Haslum, Tim Wheeler and I considered how best to present our own data, we thought it tactful to suggest that there could be two different definitions of dyslexia: 'specific reading retardation' (SRR), which our American colleagues had used, and 'specific developmental dyslexia' (SDD), which we ourselves had been using.
In the British Births Cohort Study we found a total of 269 children who satisfied our criteria for being dyslexic - for the most part severe underachievers at spelling who also showed two or more positive indicators on the supplementary items. Of these, 223 were boys and 46 were girls - a ratio of about 4.5 : 1 (a slight adjustment being made to allow for the greater number of boys overall - 5995 as against 5809 girls). In the case of the 417 severe underachievers who showed few or no indicators of dyslexia there were 243 boys and 174 girls, which gave an adjusted ratio of 1.3 : 1. Whatever else we had established, it was clear that the four supplementary items were having an effect on the outcome.
There was one particular issue, however, which nearly landed us with a serious problem. We needed to be sure that there was no gender bias in any of the supplementary items. However, we found that many more boys than girls had had difficulty in saying the months of the year correctly. It seemed, therefore, that we had used an item for picking out our dyslexics in which there was an in-built gender bias. However, there was no gender bias in the other three supplementary items, and when the Months Forwards item was omitted the adjusted boy : girl ratio still came out as 4.5 : 1.
We also decided to check what would happen if we used the same criteria for dyslexia as had our American colleagues. We therefore defined dyslexia as 'poor reading in relation to intelligence' and took the children's results from the Edinburgh Reading Test, which we had not so far used in our other analyses. We obtained a boy : girl ratio of about 1.6 : 1.
'Poor reading in relation to intelligence' does not seem to me to provide a taxonomy of any strength, whereas what we called in this paper 'SDD', in my view, provides a far more powerful one.
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