The Stability of Individual Differences in Mental Ability from Childhood to Old Age: Follow-up of the 1932 Scottish Mental Survey
Introduction
The stability of individual differences in human mental abilities is of scientific and popular interest (Jensen, 1980). In childhood, it is of interest to discover whether educational initiatives can boost ability levels and whether environmental insults—such as poor nutrition or lead pollution—can lower cognitive functions. In old age, there is intense interest in whether mental ability differences earlier in life contribute to the risk of dementia and other syndromes of cognitive decline (Snowdon et al., 1996).
There are reports of the stability of measures of mental ability: (a) within childhood (Humphreys, 1989); (b) from childhood to mid-adulthood (Kangas & Bradway, 1971); (c) across young- to mid-adulthood Eichorn et al., 1981, Nisbet, 1957, Owens, 1966, Plassman et al., 1995, Schwartzman et al., 1987, Tuddenham et al., 1968; and (d) in old age (Mortensen & Kleven, 1993). Table 1 summarizes these studies, showing the duration across which stability of individual differences was assessed and the stability coefficients obtained. The studies collected in Table 1 show that intellectual ability differences become increasingly stable throughout childhood, and have high stability across many years of adulthood. Both the Concordia study (Schwartzman et al., 1987) and the Intergenerational Studies (Eichorn et al., 1981) found higher stability across adulthood for verbal abilities than for non-verbal/performance IQ-type abilities.
The value of knowing the stability of mental ability differences from early adulthood to old age was emphasized in two long-term follow-up studies. In the “Nun Study,” the linguistic complexity of hand-written autobiographies in early adulthood correlated with the incidence of dementia and mental ability level in late life (Snowdon et al., 1996). In a separate study, recruits from the American armed forces in the early 1940s were administered the Army General Classification Test and followed up 50 years later using a brief telephone-administered cognitive interview (Plassman et al., 1995). This latter study made particular mention of the necessity yet rarity of having early life cognitive estimates in the interpretation of cognitive scores in old age. Therefore, though it is a research priority, we do not know the stability of psychometric intelligence differences from early to late life. The principal reason for this gap in our knowledge is the rarity of samples of the population who were tested in youth and then followed up in old age.
We now report the first follow-up study of human cognitive ability that extends from childhood (mean age 11 years) to old age (mean age 77 years), and is thus informative about the stability of mental functions across most of the human lifespan. Further improvements upon the best currently available studies Plassman et al., 1995, Schwartzman et al., 1987, Snowdon et al., 1996 include: (i) the use of the same validated mental test at baseline and follow-up using identical instructions; (ii) characterization of the follow-up sample in terms of age, sex, and initial IQ with respect to the entire relevant Scottish population; and (iii) concurrent validation of the mental test at first testing (age 11 years) and follow-up (age 77).
Section snippets
Background to the Present Study
We used data from the 1932 Scottish Mental Survey to investigate the stability of psychometric intelligence differences across a gap of 66 years. The Scottish Mental Survey 1932, under the auspices of the Scottish Council for Research in Education (SCRE), sought to quantify the number of people in Scotland who were “mentally deficient.” It was broadened to “obtain data about the whole distribution of the intelligence of Scottish pupils from one end of the scale to the other” (Scottish Council
Subjects
The Scottish Council for Research in Education made the complete data set for the 1932 Scottish Mental Survey available to the authors. From January to May 1998, we traced local (North–East Scotland) survivors of the 1932 Scottish Mental Survey. With the approval of the Grampian Ethics of Research Committee and family doctors, we contacted 199 survivors randomly selected from the Community Health Index (the local register of people's allocations to family physicians in the UK's National Health
Results
The mean score on the Moray House Test for the 101 subjects in 1932 was 43.3 (SD=11.9), and for the same subjects in 1998 was 54.2 (11.8) (Table 2). Mean scores for men and women were very similar at age 11 but, at age 77, men scored higher than women by almost three points. A mixed model analysis of variance of Moray House Test scores was carried out with time as a repeated measure (1932 score vs. 1998 score) and sex as a between subjects factor. The effect of time was highly significant, with
Lifetime Stability of Mental Ability Differences
The Pearson r-correlation between the Moray House Test scores in 1932 and 1998 was 0.63 (p<.001). The 95% confidence limits on this correlation are from 0.50 to 0.74. This raw correlation is an underestimate of the true correlation in the population because of the attenuation of the re-tested sample with respect to variance on the 1932 Moray House Test scores. The disattenuated correlation across the 66-year gap, allowing for the restricted range of the sample, is 0.73. This corrected
Discussion
To our knowledge, this is the longest follow-up study of human psychometric intelligence differences reported to date. The interval between the two testing sessions comprises most of the normal human lifespan. The present study has design features which rarely occur together in other studies: the same test was used at first test and follow-up; the test had concurrent validation at both test sessions; the original and follow-up samples can be compared quantitatively with the entire age-relevant
Acknowledgements
The study was supported by a grant to LJW from Henry Smith's Charities. Patricia Whalley and Mariesha Struth assisted in collection and collation of data. We are indebted to the Scottish Council for Research in Education—especially Graham Thorpe, Rosemary Wake and Professor Wynne Harlen—for providing data from the 1932 Scottish Mental Survey.
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