TABLE 111 Multiple regression analysis of predictor variables and accuracy reading-age discrepancy scores

Multiple Initial r Equarion variable Step variable R R' with criterion F df F df

Language status 0.58 0.33 **-0,58 ***16.96 1.34 ***16.96 1,34 Birth status 0.59 0.35 -0. 14 ***8.91 2,33 0.91 1.33 SES scores 0.61 0.38 0.22 ***6.44 3,32 1.32 1,32 PPVT IQ score 0.62 0.39 0.01 ***4.89 4,31 0.54 1,31

Performance IQ score 0.63 0.39 0.13 ***3.90 5.30 0.33 1.30

WISC-R

*p<O. 10; **p<O.05; ***p<0.01

TABLE IV Multiple regression analysis of predictor variables and comprehension reading-age discrepancy scores

Multiple Iniiial r Equarion variable Step variable R R ' wirh criterion F df F df

Language status 0.48 0.23 "-0.48 ***lo. 14 1,34 ***lO. 14 1.34 WISC-R Performance IQ score 0.50 0.25 0.22 ***5.61 2,33 1.06 1,33 SES score 0.52 0.27 0.13 **3.89 3'32 0.60 1,32 Birth score 0.52 0.27 -0. 16 **2.84 4,31 0.05 1.31 PPVT IQ score 0.52 0.27 0.10 '2.21 5,30 0.04 1,30

*p<O. 10; **p<0.05; ***p<0.01

TABLE V Twins' preschool laneuaae status in relation to accuracy and comprehension reading problems

No. twins in each Twins with accuracy Twins with comprehension No. lanfiuage language siaius reading reading problems

problems group No. 9t No. %

<2 24 10 42 6 25 3 7 7 I00 6 86 4 3 3 100 2 67 5 2 2 I00 I 50

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examine the relationship between reading level and the other hypothesized predictor variables. Results from these analyses are presented in summary form in Tables I11 and IV.

Preschool language status emerged as the single significant predictor variable in both cases, accounting for 33 per cent of the variance in accuracy reading-age discrepancy and 23 per cent in compre-

hension reading-age discrepancy. In both cases, once preschool languages status was entered into the equation, there was no significant increase in any subsequent steps to the amount of variance accounted for in the equation. However, in the prediction equation for comprehension reading- scores, a lower relationship with preschool language status and a stronger relationship with WISC-R Performance tQ was shown 591

* TABLE VI s 2 i Australian sample Twin sample 2. .- Toral Male Female Total Male Female

Percentage of 10- and 14-year-old students tested by Australian Council for Educational Research achieving mastery

L3

a. - 0 ba 10-vear-olds

NO students 6628 3328 3300 168 101 67 % 53 46 59 40 36 48

i

.E No. students 6247 3150 3097 129 67 62 2 % 12 70 74 54 42 68

VI 14-year-olds - i:

B m c than in the accuracy equation. This ’$ I- suggests that reading comprehension .+ a involves somewhat different activities .- - from reading accuracy.

However, in further step-wise analyses B I

Ei with language status excluded, no variable 04 made a significant independent contri-

bution to prediction of either reading d accuracy o r comprehension, confirming

preschool language status as the only significant predictor for later reading abilitieddisabili ties.

Table V shows the relationship between language problems and reading disabilities, and indicates clearly that ifa twin has three or more language problems in the preschool years he is at extremely high risk for later reading problems. All of the children in the sample who had three or more language problems in the preschool years have problems with reading accuracy. (Reading problems for this Table referred to any reading age more than six months behind the child’s chronological age.)

As our sample was small, and only included boys, we were concerned to look at twins in relation to a larger sample of schoolchildren. We were able to examine the results of an Australian survey carried out in 1975 to measure competence in basic literacy and numeracy skills (Bourke and Keeves 1977).

One measure employed in this survey was termed ‘Mastery of Reading’, and a child was said to have achieved this if 80 per cent of items were read correctly. We calculated the percentage of both male and female twins achieving this mastery criterion and compared this to the survey data for the two age-groups tested, i.e.

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10-year-olds and 14-year-olds. It can be seen from Table VI that both twin boys and twin girls are behind singletons in reading mastery, a discrepancy which increases between 10 and 14 years among boys (Hay and O’Brien 1983).

At age 10 the interaction of twin- singleton differences with sex was not significant (F, 1, 6392= 1.63) whereas it was at age 14 (F, 1,6372= 16. 19), showing in general that boys at the age of 10 are far behind girls in reading, but the single boys catch u p almost completely by the age of 14. The twin boys d o not, in that only 42 per cent of them reach mastery, compared with 70 per cent of the single-born boys. These data are being analysed further but initial examination indicates clearly that in an Australian sample, twin boys are underachieving markedly in reading compared with singletons.

Discussion The results of this study seem unequivocal. Twin boys show an incidence of reading disability well above that expected in an unselected population (Bourke and Keeves 1977). The degree of underachievement ranges from six months to more than two years for both reading accuracy and comprehension.

Yule (1967) has argued that estimates of reading disability may be defined in terms of a regression equation which takes into account the level of intelligence of the child. Using this method with a random and representative sample of nine- to 12- year-old children of normal IQ, Yule reported that 10 per cent of children can be expected to he 18 to 20 months behind their predicted accuracy and comprehension

reading ages; and 5 per cent 24 months or more behind. Yule's estimates were based on the Neale Analysis and a short form of the WISC and thus permit some comparison with the present results. Within the twin sample for the accuracy measure, 33 per cent were more than 18 months behind their age level, and 17 per cent were more than 24 months behind. For compre- hension, 18 per cent were 18 or more months behind, and 12 per cent were 24 months or more behind. Thus the incidence of specific reading retardation here is very high. We did not use the Yule regression equation to provide precise comparisons but considering the very high performance IQs of the sample, the figures presented here for this twin sample must be cause for concern. Conservatively, we might claim that twins are two or three times more likely to be specifically reading retarded according to Yule's criteria (1967). However, this estimate is for a relatively high socio-economic group, and Gibson and Levin (1976) have noted that reading failure is more common in less- advantaged groups, so it is likely to be a conservative figure. It is also likely to be conservative since estimates were un- corrected for IQ. Even allowing for the fact that inclusion of the WISC Verbal subtests might well have lowered the IQ, a mean Performance IQ for the group of 120 indicates a highly intelligent group who might be expected to have a lower incidence of reading problems.

The very strong relatiqnship found between early language difficulties and later reading disability provides support for those who maintain that language abnormality is the major influence on the development of reading disability (e .g . Vellutino 1979). Rutter and Yule (1975) have also noted this relationship in their study of specifically retarded readers. One third of the twins in our sample had had three or more language problems in the preschool years, i.e. had severe difficulties. Of this group 100 per cent showed accuracy reading problems and 68 per cent showed comprehension reading problems.

So it seems that although early language problems in twins may appear to be transi- tory, they continue during school age (in handicapped twin boys at least) in the form of reading difficulties. These data are in

accordance with those of Matheny et al. (1976), who reported that 67 per cent of a sample of reading-disabled twins had a history of speech-articulation problems.

We noted in the results that although preschool language status was over- whelmingly the best predictor of reading disability in the regression analyses, when we considered the results for Neale reading comprehension specifically, WISC-R Per- formance IQ appeared to make some contribution. It is likely that the above- average IQ of the reading-disabled twins meant that they were better at under- standing written material, enabling them to comprehend better than they can read, at least on the Neale Test. Above-average intelligence then may compensate to some extent for difficulties relating to poor language skills.

It was perhaps not surprising to find that the variables, socio-economic status, gestationlbirth status, and WISC-R Per- formance IQ alone did not show any significant, independent contribution to prediction of reading. The literature suggests that such factors might affect language development rather than reading, so that effects on the latter would be secondary or derived.

However, it might have been predicted that the verbal comprehension measure PPVT IQ should have shown a stronger relationship to reading level. Two possible explanations for its failure as a predictor may be offered: first, since PPVT reflects vocabulary comprehension (Mittler 1970) and twins in this study seem less handicapped on comprehension of written language, they may be relatively stronger on this measure; second, if PPVT is testing a more general ability, as suggested by Waugh (1975), its moderate correlation with WISC-R Performance IQ (0.33) and lower correlation with language status score (0.23) may explain its lack of predictive power.

With all of these results, it must be conceded that the sample was a small one and that more extensive investigation of these issues is desirable. Nevertheless, both from our own data and from the Australian survey results, it appears that twin boys are at very high risk for the development of reading disability. Our second major finding of the relationship

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between preschool language problems and Authors’ A~pointments Carol Johnston, Postgraduate Student, Department

later reading problems in twins must be of Genetics and Human Variat ion; seen as preliminary results only, needing ‘Margot Prior, M.D., Senior Lecturer, Department further investigation. Even so, the presence $ $ ~ ~ ~ , g ~ h , D , , Senior Lecturer, Department of of language problems in the early years ~~~~~i~~ and H~~~~ Variation; should perhaps alert parents and pro- La Trobe University, Bundoora, Victoria 3083, fessionals t o those risks, and to the need for Aus t ra l i a .

preventive and protective measures. *Correspw~dence to second author.

SUMMARY Thirty-six twin boys aged between nine and 13 were assessed for intelligence and reading disability. Records of their birth status, early language development and social demographic characteristics were also available. 72 per cent of the sample had an accuracy reading age below their chronological age, and 59 per cent had a comprehension reading age below their chronological age, despite above-normal IQ. The best predictor of reading disability was preschool language status, accounting for 33 per cent of the variance in accuracy reading age discrepancy, and 23 per cent in comprehension reading age discrepancy in regression equations. This study shows that male twins are at very high risk for reading disability, and also offers support for the claim that language disability is basic to the development of reading problems.

RESUME PrPdicrion des difficuliis de lecture chez des jumeaux L’intelligence et les difficultis de lecture ont kt6 appreciees chez 36 garqons jumeaux igCs de neuf a 13 ans. Les donnees concernant les conditions de naissance, le developpement prkcoce du langage et les caractkristiques socio-dbmographiques Ctaient aussi disponibles. 72 pour cent de I’echantillon prksentaient un Age de capacite de lecture au-dessous de I’Ige chronologique et 59 pour cent avaient un Ige de comprehension de lecture audessous de I’ige chronologique en dkpit d’un Q.I. au-dessus de la moyenne. Le meilleur prkdicteur de difficult6 de lecture 6tait le niveau de langage en periode pre-scolaire, responsable de 33 pour cent de la variance sur la dispersion des lges de lecture, et 23 pour cent de la variation dans I’Ige de comprehension de lecture aux equations de regression. Cette etude montre que les garcons jumeaux presentent un risque trks &lev6 de difficulte de lecture et apporte un 61Cment supplkmentaire A I’affirmation que Ie trouble du langage joue un rBle essentiel dans le dkveloppement des problkmes de lecture.

ZUSAMMENFASSUNG Prognostische Aussage zur Lesefahigkeir bei Zwillingsjungen 36 Zwillingsjungen im Alter zwischen neun und 13 Jahren wurden hinsichtlich ihrer Intelligenz und Lesefahigkeit untersucht. Anamnestische Erhebungen uber Geburtsverlauf, friihe Sprachentwicklung und sozialstatistische Charakteristika lagen vor. 72 Prozent der Jungen hatten ein Genauigkeits und 59 Prozent ein Verstandnislesealter, das unter ihrem chronologischen Alter lag, trotz iibernormalem IQ. Der beste Indikator fur eine Leseschwache war der Sprachentwicklungsstand im Vorschulalter und zwar in 33 Prozent fur das Genauigkeitslesealter und in 23 Prozent fur das Verstandnislesealter. Diese Untersuchung zeigt, dap mannliche Zwillinge sehr zur Leseschwache neigen und stutzt die Behauptung, dap eine Spachentwicklungsstorung die Grundlage fur Leseprobleme ist.

RESUMEN Prediccidn de la dificuliad de lectura en muchachos gemelos Treinta y seis rnuchachos gemelos fueron evaluados en su inteligencia y su dificultad para la lectura. TambiCn se obtuvieron sus historias anteriores de nacirniento, desarrollo precoz del lenguaje y caracteristicas demogrificas sociales. El 72 por ciento de la muestra tenia una edad de perfeccion en el lenguaje por debajo de su edad cronologica, y el 59 por ciento, una edad de comprension del lenguaje por debajo de su edad cronologici a pesar de tener un CI por encima del normal. La mejor prediccion de la dificultad de lectura estaba en la s i tuac ih del lenguaje preescolar, contano en un 33 por ciento la varianza en discrepancia de la edad de perfeccion en el leguaje, y 23 por ciento en la discrepancia en la edad de comprension de la lectura en ecuaciones de regresion. Este estudio muestra que 10s muchachos varones tienen u n alto riesgo para la dificultad en la lectura y ademis, apoya la idea de que la dificultad en el lenguaje es un hecho blisico en el desarrollo de probremas de lectura.

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