MONOGRAPHS OF THE SOCIETY FOR RESEARCH IN CHILD DEVELOPMENT
MCCALL, ROBERT B. APPELBAUM, MARK I. AND HOGARTY, PAMELA S. Developmental Changes in Mental Performance. Monographs of the Society for Research in Child Development, 1973, 38 (3, Serial No. 150) A distinction was made between the general level of a person's IQ (i.e. the average of his scores across age) and his profile contour (i.e. the developmental pattern of inflections in performance) Most research has focused on the general level of IQ and its correlates; this project systematically investigated patterns of IQ change with data from the Fels Longitudinal Study. Normal middle class children changed an average of 28.5 IQ points between 21/2 and 17 years of age, and one in seven displayed shifts of more than 40 points. While siblings were more similar in their general level of IQ than unrelated children matched or unmatched for social class, the pattern of developmental changes in childhood IQs or infant DQs was not more similar among siblings than among unrelated children. Twins and triplets scored lower in general level and were less variable in IQ over age than singletons, thus potentially qualifying attempts to generalize IQ change results based upon multiple birth samples to singletons. Parent child IQ correlations were higher when the parent was assessed as an adult than if both parent and child were tested at the same chronological ages. A sample of 80 subjects who had relatively complete IQ data (maximum of 17 tests) between 2/2 and 17 years were clustered into five groups which represented different patterns of IQ change over age. These profiles were relatively simple linear or quadratic trends and not random fluctuations about a constant value. Major inflection points occurred at 6 and 10 years of age. It was not obvious that these patterns were simple products of repeated testing or the changing nature of the IQ test. The children in the five IQ profile clusters had parents who were different in the extent to which they attempted to accelerate their child's development and the severity of punishments they administered. These parental correlates appeared to hold up even when parental education and IQ as well as the general level of the child's IQ were statistically controlled. Given the assumptions prompted by these data, the changing nature of environmental circumstances across the childhood years for a given individual may be as potent in changing IQ as the differences between family environments. This content downloaded from 131.232.13.6 on Sat, 25 Jun 2016 06:09:01 UTC All use subject to http:/about.jstor.org/terms I. THE LONGITUDINAL STUDY OF DEVELOPMENTAL CHANGES IN IQ Several reviews of the literature on the development of mental performance are available (Bayley 1970; Bloom 1964; Pinneau 1961) However, most of the research surveyed involves correlations between mental test performance at several different ages and correlations between personality and social variables with IQ at one age versus another. Such research strategies are valuable and have yielded important results, but they are "developmental" only in the sense that data are available at several ages on the same subjects and correlational relationships are derived within and across age periods. Because the correlation coefficient is independent of the means of the distributions, it is possible to obtain high correlational stability of IQ across age but marked fluctuations in the group mean. In short, cross age correlations reveal little about the nature of absolute changes in level of performance, and they provide only partial information about individual differences in developmental pattern. This project focuses on developmental patterns of change in the level of mental performance as assessed by traditional intelligence tests.