The Diagnosis of Mental Retardation in Infancy

There seems to be an increasingly firm conviction among psychiatrists and psychologists that developmental tests in infancy have little or no predictive value. For instance, Bowlby (1951) wrote in his famous monograph that 'mental tests have no predictive value in the first 18 months of life'. Kirman (1953) wrote that 'few people are so bold as to claim to be able to measure intelligence in babies or to predict the future course of their mental development'. Eysenck (1953) wrote that 'intelligence testing before the age of 6, or 5 at the earliest, should be discouraged', because, he said, 'testing before the age of 2 has no predictive accuracy at all for adult intelligence'. Apgar, Girdany, McIntosh and Taylor (1955) wrote of the meagre value of Gesell rating of adaptive behaviour at 2 years as compared with the Stanford Binet test at 5 years. Wittenborn (1957) concluded that developmental tests were of very little value in determining the intelligence of babies for the purposes of adoption. Sontag, Baker and Nelson (1958) wrote that levels of I.Q. at pre-school age have hardly any predictive value. Karelitz (1958) wrote that ' it is now recognized that psychological tests of infants cannot be used to predict the child's future development'. Kirman (1958) wrote 'in regard to mental deficiency it is possible to recognize only severe mental defect at an early stage', and 'the predictive value of tests under the age of 5 years is very low'. Many of these statements are incontrovertible. Correlation coefficients calculated between the results of assessments of ability made first in infancy and later in childhood are known to be very low. When the first assessment is made before the age of 2 years they commonly approximate to zero and may even be negative (Bayley, 1940). Until a correlation coefficient rises to the order of 0 9 prediction of one variable from another is very little superior to a prediction based upon chance. It is frequently forgotten, however, that a low correlation between two sets of scores may not be incompatible with reasonably accurate prediction within certain narrow limits within the total range of the two measures. If the two measures follow an approximately Gaussian distribution and if the particular narrow limits, within which accurate prediction is possible, lie at either extreme of the distributions, then the contribution of the relatively few scores involved may be negligible in the calculation of a product moment correlation coefficient. It seems that such is the case in the prognosis of mental retardation, for it has long been known that the I.Q. of dull children is much more constant than it is in normal or super-normal children (Terman and Merrill, 1937). For this reason psychologists (e.g. Valentine, 1950), who would not subscribe to the general usefulness of early testing, have advocated procedures, including tests, for early diagnosis of mental defect, and Goodenough (1949) commented, 'in as much as radical changes in the treatment of backward and feeble minded children are more likely to depend upon the results of mental tests than is the case with children of normal or superior intelligence, the greater dependability of the intelligence quotient at the lower levels is a matter of considerable importance for those actively concerned with the welfare of children'. Vernon did not rule out early testing for the detection of mentally defective children when he wrote 'indeed the various tests of developmental level sometimes applied to babies from 0 to 21 years provide practically no indication of later intelligence, except perhaps in cases of severe mental defect' (Vernon, 1957). We imagine that any paediatrician would agree that in the majority of cases the diagnosis of severe mental retardation can readily be made in 269 coright.