Gender ratio in dyslexia

This paper is based on a study carried out in Great Britain on a national sample of 11,804 ten-year olds. The first section describes an attempt to pick out cases of "specific developmental dyslexia" (Critchley 1970), a constellation or syndrome of difficulties which some believe to be recognizable clinically. When specified criteria for dyslexia were used, 269 children qualified as dyslexic (2.28 percent of the sample). These included 223 boys and 46 girls, for a ratio of 4.51 to 1. Two possible difficulties in interpreting these data are discussed, and a defense is offered of the criteria used.

Since some recent research papers report a gender ratio much nearer 1:1 (Shaywitz et al. 1990; Wadsworth et al. 1992; Lubs et al. 1993), those papers were examined for possible differences in procedure; it was found that the definition of dyslexia they used was "poor reading in relation to intelligence." We carried out a further analysis on our own data using the same criterion. Of the 494 children who qualified as dyslexic on the

basis of discrepancy criteria alone (4.19 percent of the sample), 314 were boys and 180 were girls for a ratio of 1.69 to 1. It seems, therefore, that the apparent differences in gender ratio reported in the literature have arisen because different criteria for dyslexia have been used.

We argue that the definition based on clinical criteria leads to a more powerful taxonomy and that the widespread equation of "dyslexia" with "poor reading" is a hindrance to progress.

INTRODUCTION

It has traditionally been supposed that there are many more dyslexic males than females (Hinshelwood 1917; Hallgren 1950; Hermann 1959; Critchley 1970; Goldberg and Schiffman 1972; Hier 1980; Finucci and Childs 1981). More recently, however, this imbalance has been called into question (Shaywitz et al. 1990; Wadsworth et al. 1992; Lubs et al. 1993).

The data presented in this paper are in agreement with traditional accounts in finding a preponderance of males when clinically based criteria for dyslexia are applied. These criteria include not only poor reading and spelling in relation to general intelligence, but also such clinical indices as uncertainty over left and right (a phenomenon first noted by Dr. Samuel Orton) or difficulty in the recall of auditorily presented digits. On the other hand, if a diagnosis of dyslexia is based solely on measures of reading and intelligence, making what we argue to be an unwarranted equation of dyslexia with specific reading retardation, we also found the gender ratio to be closer to 1:1.

For ease of communication, we shall distinguish between "specific developmental dyslexia" (SDD), a condition diagnosed on multiple criteria of a clinical kind (Critchley 1970), and "specific reading retardation" (SRR) (Rutter and Yule 1975), which refers to poor reading in relation to intelligence.l

The data which form the basis of the present paper were collected in 1980 and relate to 11,804 ten-year olds. This body of data will be referred to in what follows as the "British Births Survey." The size of the sample and the fact that it targets a complete population has provided a unique opportunity for an investigation of dyslexia. The following publications have already appeared: Miles and Haslum (1986); Haslum (1989); Miles (1991); Miles, Haslum, and Wheeler (1993, 1996, 1997); Miles, Wheeler, and Haslum (1994). Analyses of the data are ongoing and further papers are in preparation.

The primary objective of the dyslexia component of the research was to find a formula which, if consistently applied across the cohort, would pick out those children showing strong evidence of SDD. The underlying assumption was that children with dyslexia can be recognized by persons with relevant clinical experience including doctors, teachers, psychologists, parents, and many individuals who are themselves dyslexic. We also assumed that the early pioneers-Hinshelwood (1917), Orton (1937/1989), Hallgren (1950), Hermann (1959), and Critchley (1970)-were talking about the same group of children. Our research goal, therefore, was to sharpen the definition of the concept that these early researchers referred to without radically changing it. To adapt some wise words used in a different context by the philosopher, Sidgwick (1922), "A definition may be given . . . which will be accepted by all competent judges as presenting, in a clear and explicit form, what they have always meant by the term, though perhaps implicitly and vaguely. In seeking such a definition we may, so to speak, clip the ragged edge of common usage, but we must not make excision of any considerable portion" (p. 264).

To put the matter another way, our intention was to operationalize clinical judgments of SDD. We hoped not only to specify operations which would distinguish children with SDD from others with literacy problems, but to show that the differences between them were important and could be used as the basis for a meaningful taxonomy (classificatory principle).