IJE Advance Access originally published online on December 14, 2006
International Journal of Epidemiology 2007 36(1):212-219; doi:10.1093/ije/dyl261
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Cognitive function in childhood and early adulthood and injuries later in life: the Metropolit 1953 male birth cohort
1 Department of Epidemiology, University of Southern Denmark, Denmark.
2 Department of Social Medicine, University of Copenhagen, Denmark.
3 National institute of Public Health, Denmark.
4 Department of Social Medicine, University of Bristol, UK.
* Corresponding author. Department of Epidemiology, Institute of Public Health, University of Southern Denmark, JB Winsløwsvej 9b, 5000 Odense, Denmark. E-mail: m.osler{at}health.sdu.dk
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Abstract |
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Background It has been suggested that cognitive function in childhood is a modifiable risk factor for adult injury. This study examines the relationship between cognitive function measured at the age of 12 and 18 years and fatal and non-fatal injuries later in adult life.
Methods A total of 11 532 males born in Copenhagen, Denmark in 1953 were followed from 1978 until 2001 with outcomes (death from and hospital admission for unintentional injury) obtained from national registers. At the age of 12 years, 7987 of these cohort members had completed a questionnaire, which included information on cognitive performance. In addition, cognitive test scores measured on most (90%) cohort members were retrieved from the conscription board records (18 years).
Results During follow-up, 100 of the men died as a result of and 2123 had been admitted to hospital at least once for injury. Cognitive function measured at both the age of 12 and 18 years was inversely associated with any form of unintentional injury. Adjustment for educational attainment at the age of 18 years attenuated these associations but did not remove them completely. The association was most evident for falls and poisoning, while associations with other injury types were weaker and disappeared after adjustment for educational status. Cognitive function was associated with repeated hospital admissions for injuries as well as length of hospital stay.
Conclusions We found marked inverse associations between cognitive function measured in ages 12 and 18 years and adult risk of fatal or non-fatal unintentional injury. An overall increase in educational level may result in a reduction in adult injury risk.
Keywords Injury, cohort study, intelligence, educational status
Accepted 24 October 2006
Adult injuries are common in most countries, with a large impact on public health and economic expenditure.1,2 In addition to a number of environmental conditions, childhood cognitive function has been suggested as a modifiable risk factor for adult injury. Thus, in the Australian Veterans Health Study there was a strong inverse linear association between psychometric intelligence test scores assessed in early adulthood (at entry into military conscription) and deaths from motor vehicle accidents.3 Associations with other forms of injury or with non-fatal injury were not assessed. Further, in a previous analysis we found an inverse relation between psychometric intelligence at the age of 12 years and mortality from external causes.4 Whilst death from injury is clearly an important clinical and public health outcome, injuries are also an important cause of morbidity and disability. The varying rates of injury by gender, socioeconomic position and geography suggest that many injuries are preventable. Understanding the range of factors from across a person's life course that influence injury risk is therefore important. A recent analysis from the Aberdeen children of the 1950s study has shown a clear inverse relationship between childhood psychometric intelligence and non-fatal unintentional injuries, with this association being partly (though not fully) explained by educational attainment.5 However, to our knowledge, no other study to date has assessed the association of early life cognitive function with non-fatal injuries in adulthood.
The purpose of the present analysis is to: (i) add to the sparse literature on intelligence and disease-specific outcomes by examining the relationship of cognitive function measured at childhood (age of 12 years) and early adulthood (around the age of 18 years) with death and hospital admissions from injuries later in adult life; (ii) assess the role of the potential confounding variables of socioeconomic position and birth weight in these relationships in a middle-aged Danish male population.
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Methods |
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Study population
Boys born within the metropolitan area of Copenhagen during 1953 formed the population of the Danish longitudinal study (Project Metropolit) which has been described in detail elsewhere.6 Briefly, 11 532 of this population, who were alive and living in Denmark in 1968 were registered with a unique person identification number when the Civil Registration System (CRS) was established. Data from birth certificates, including information on date and place of birth, mother's marital status and father's occupational status at time of delivery, on the study population were abstracted in 1965. In the same year, 7987 (69.2%) the cohort members took part in a school-based survey, which included a questionnaire administered by their class-teachers. The questionnaire included tests of cognition and enquiries regarding social aspirations and leisure time activities. The main reasons for not being in the school survey were: dead or moved out of area between the ages 0 and 12 years (9%) or lack of willingness of school or class to participate in survey (8%). Those who did not attend the school survey were more likely to be born to single mothers, were less likely to have fathers from a high social class and had lower educational attainment at the time of their conscript.6
Conscription is still maintained in Denmark and on attaining the age of 18 years all men are required to appear before the draft board, which assesses suitability for military services. Some 10% of men are exempted from appearing in person, these being predominantly men who can medically document a disqualifying disorder such as asthma or extreme myopia. Those who do appear undergo a group administered cognitive test, the Børge Priens Prøve, and a health examination by a medical doctor.7 In 2004, this information was collected from all Danish conscript district registers for the cohort members who were still alive in 1971.
Assessment of cognitive function
From the school questionnaire in 1965 we used the information from the Härnquist test. This consisted of spatial, arithmetic and verbal sub-tests. In the spatial test, students were asked to choose one out of four alternative three-dimensional figures, which corresponded to a flat, two-dimensional template. In the arithmetic test the students had to work out the logical order of series of six numbers. In the verbal test the students had to identify one word out of four alternatives that had the opposite meaning to a given word. Each sub-test included 40 problems and a correct answer counted one point, so the sub-test scores ranged from 0 to 40 points and the overall test ranged from 0 to 120 points (higher score indicating greater psychometric intelligence).
The 45 min validated cognitive test, taken at the conscript board, which most cohort members (90%) attended 1972–73, comprises four categories: letter matrices, verbal analogies, number series and geometric figures. The score is the total number of correct answers for 78 questions and this tests has been shown to be correlated well with the Wechler Adult Intelligence Scale.8
Assessment of covariates
From the birth records we used information on birth weight, father's occupational social class, mother's marital status and age at the time of delivery. Birthweight was recorded in 100 g groups and was analysed in z-scores. Father's occupation at participant's birth was recorded in 23 categories and re-coded into 5 categories: self-employed, salaried employed, unskilled, skilled workers and unknown. The marital status of the mother at time of delivery was treated in three categories: married, unmarried (i.e. single, divorced, widowed) and unknown, while maternal age at birth was recorded in 5-year age categories. Educational attainment was obtained from information collected at the conscript board examination. Since in most cases conscription occurs before entry to higher education, educational attainment was categorized into four groups from basic school (up to 9 years of schooling) to at least ‘studenter’ exam (equivalent to the British Advanced (A) level qualifications).9 Height and weight (recorded in centimetres and kilograms, respectively) were measured at conscription and analysed in z-scores.
Assessment of death or hospital admission for unintentional injuries
In August 2004, the Metropolit cohort was followed-up regarding the vital status by a record-linkage with the CRS Registry and if the person was not alive or no-longer living in Denmark we got information on date of death or date of emigration/disappearance. Information on time of admission to hospital wards and diagnosis on discharge was obtained from the National Patient Registry from 1978, when this registry was established, until August 2005.10 Causes of death were obtained from 1978 to December 2001 by record linkage with the Cause of Death Registry.11
Table 1 shows the codes used to define unintentional injuries in the two registers, together with the number and percentage of participants with at least one event for each injury type.
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Statistical analyses
Associations between cognitive function and unintentional injury were analysed using Cox's proportional hazards regression models with age as the underlying time scale. Entry time was the age on January 1, 1978 and follow-up ended at the age of first admission, death, emigration or January 1, 2002, whatever came first. Participants were omitted from the analyses if they died (n = 118; of which 51 were for unintentional injury), emigrated (n = 74) or had a hospital admission record (n =1) before the register became nationwide in 1978. This left 11 339 men for analysis. Proportionality assumptions were assessed by inspection of cumulative incident plots and there was no evidence of any violation.
For variables collected from birth registries and conscript boards there were small amounts of missing data as shown in Table 2. Childhood intelligence scores were obtained from the questionnaire survey in school in 1965 and 30% of the cohort members did not have these data. We used multivariate multiple imputation, using all other covariates, the log of survival time and the censoring indicator, to impute a distribution of missing values for those variables with some missing data (imputations were made for all variables that had some missing data, irrespective of the amount of missingness for a single variable).12 We used switching regression in Stata as described by Royston,12 and carried out 20 cycles of regression switching and generated five imputation data sets. We also undertook all analyses on the sub-sample with complete data on intelligence test scores at both ages of 12 and 18 years and complete data on all other covariables: (n = 7543 minus the 129 who were censored before 1978). The estimates from this complete data sub-set analyses were essentially the same as those presented here using the multiple imputation databases, but these were less precisely estimated.
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All analyses were conducted using Stata version 8.0 (Stata corporation, TX, USA, 2002).
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Results |
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Table 2 shows the baseline characteristics of the cohort. The study participants contributed 288 900 and 230 390 person-years of risk over the follow-up period for death and hospital admissions, respectively. During the follow-up there were 100 deaths and 2223 subjects with at least one hospital admission for an unintentional injury giving rates of 3.4/10 000 person-years (95% CI: 2.8–4.2) and 92.2/10 000 person-years (95% CI: 88.4–96.2) for fatal and non-fatal unintentional injury, respectively. The injury rates were similar among the 3481 men who did not complete the childhood cognitive function test [4.5 (95% CI: 3.3–6.1) rate for fatal and 96.7 (95% CI: 89.8–10.4) for non-fatal unintentional injury] The correlation between cognitive function at age 12 and at
18 years was 0.73 in the sub-set with actual measures of cognitive function at both ages and was 0.75 when estimated using the multiple imputation data sets.
The effects of cognitive function on either fatal or non-fatal unintentional injury were similar, and therefore all further analyses are for these two outcomes combined. Table 3 shows the unadjusted associations of cognitive function and early life characteristics with death from or hospital admissions for all forms of unintentional injury in adulthood, with these analyses conducted on the multiple imputation data set. Intelligence test scores at the age of 12 and 18 years were inversely associated with death or hospital admission for injury. There was a linear association across the intelligence score distribution assessed both at the age of 12 and 18 years (Figure 1). The associations of cognitive function at the age of 18 years with all outcomes were essentially the same for the 4166 men who completed cognitive function tests at the conscription examination, but who did not have childhood cognitive function tests, as for the 7173 participants included in the main analyses on whom we had cognitive function tests available at both time points. Children of skilled and unskilled workers, compared with those who were self-employed, and those whose mothers were single at the time of their birth were also at greater risk of injuries in adulthood, whereas maternal age at birth and birth weight were not associated with adult injury risk. There was an association between weight at conscription and adult injury, though this did not reach conventional 5% levels of statistical significance. Individuals who were shorter at the time of the conscript board examination were at greater risk of injury in adulthood. Educational attainment at the age of 18 years was inversely associated with injury risk later in adult life.
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Tables 4 and 5 show the multivariable associations between cognitive test results measured at the age of 12 and 18 years and death from or hospital admissions for all forms of unintentional injury and different types of injury in adulthood. With no adjustment (Model 1) there were inverse linear associations between intelligence measured at the age of 18 years and any form of injury, road traffic injuries, falls, unintentional poisoning and unintentional medical or surgical injuries. Cognitive function measured at the age of 12 years was also inversely associated with most types of unintentional injury, though the association with medical and surgical injuries did not reach conventional (5%) levels of statistical significance. Adjustment for childhood socioeconomic position resulted in some attenuation of these associations, though most remained (Model 2). Birth weight had very little impact on the inverse associations (Model 3), but adjustment for height and weight and educational attainment at conscription resulted in marked attenuation of the associations (Models 4, 5 and 6). With adjustment for all covariates considered in this study (Model 6) cognitive function measured at both ages remained inversely associated with any form of unintentional injury and with falls; cognitive function measured at the age of 18 years also remained inversely associated with unintentional poisoning. For each of the models used in the analyses presented in Tables 4 and 5, there was statistical evidence supporting a linear association across the cognitive test score distribution when models with the score categories entered as a score were compared with those with the score entered as a series of indicator variables (all P-values <0.001). Similar patterns of associations were found when analyses were restricted to all fatal injuries only, though due to smaller numbers of events these estimates were less precise. For example, the hazard ratio (HR) of death from an injury with each standard deviation increase in intelligence test score at the age of 12 years was 0.71 (95% CI: 0.56–0.91) and that at the age of 18 years was 0.60 (95% CI: 0.49–0.74). The fully-adjusted (equivalent to Model 6) results were 0.96 (95% CI:0.69–1.33) and 0.96 (95% CI:0.71–1.28). Within each sub-type of injury there were too few deaths for meaningful analyses of mortality.
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Of the 2123 individuals with at least one hospital admission for any form of injury 692 (33%) had two or more admissions for an injury over the follow-up period and 288 had three or more admissions. Cognitive function measured both at 12 and 18 years were inversely associated with repeated admissions for injury. Thus, among those with any hospital admission a 1 SD increase in cognitive function at the age of 12 years was associated with a HR of 0.83 (95% CI: 0.76–0.92) for two or more hospital admissions (compared with just one admission) for any form of injury in unadjusted models. This attenuated to 0.89 (95% CI: 0.80–0.99) with further adjustment for childhood socioeconomic position and to 0.91 (95% CI: 0.81–1.01) with additional adjustment for educational attainment. Similar results were found for these associations with intelligence measured at 18 years.
For the 2123 participants with at least one admission for any form of injury the median length of hospital stay was 2 days with a range from 0 to 148 days. The associations of cognitive function at the ages of 12 and 18 years with injury admissions of <3 days were similar to those of 
3 days (all P-values for evidence of difference in effect estimates > 0.4), suggesting that intelligence was associated both with minor injuries requiring short-stay admissions and more serious injuries requiring longer length admissions. For example, a 1 SD increase in intelligence at the age of 18 years was associated with an HR for an admission for 
3 days (n = 1393) compared with no admission for any form of injury of 0.80 (95% CI: 0.75–0.85) in crude models and the equivalent HR for an admission of 3 days (n = 831) compared with none was 0.86 (95% CI: 0.79–0.93).
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Discussion |
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This study adds to scant evidence of the association of cognitive function measured in childhood and early adulthood with the risk of both fatal and non-fatal injury in adulthood. We found inverse linear associations with any form of unintentional injury that were graded across the cognitive test score distribution. Of the specific injury forms the association was most evident for falls and poisoning. Adjustment for educational status at the age of 18 years attenuated the effects for road traffic accidents and medical or surgical injuries. Cognitive function in early life was associated with repeated hospital admissions and the magnitude of the association was similar for injuries requiring a short or long hospital stay, suggesting that cognitive function is associated with the occurrence of both severe and less severe (requiring shorter lengths of stay in hospital) injuries.
Study strengths and limitations
The present study includes all males born in a well-defined area (covering one-third of the Danish population), and who survived to the age of 25 years. We had prospectively collected information on early life conditions and measures of cognitive function at the age of 12 years and at the conscript board examination (mean age 18 years). By using the population-covering registers we managed to get complete follow-up information. However, information on intelligence at the age of 12 years was only available for 69% of the cohort members. We used multiple imputation in order to produce more precise estimates and to examine possible selection bias resulting from missing data. The similarity of results from the analyses using the multiple imputation data sets and the sub-set without missing data suggests that for the associations examined here there was no important selection bias resulting from some participants not completing the cognitive function test at the age of 12 years. Further, we had near complete data for cognitive measures at conscript (18 years of age) and the results for the associations with cognitive function at the age of 18 years, were very similar when examined separately in those who had cognitive function scores at the age of 12 years and those who did not, providing further evidence that missing data did not bias our findings. Our cohort consists of men only and therefore, results are not necessarily generalizable to women.
Since information on hospital admissions was only available from 1978, we were able to examine associations with injuries occurring between the age of 25 and 50 years. In Denmark, admission to hospital is free and it is therefore likely that our study will have captured all moderate to severe injuries requiring hospital admission. However, we are unable in this study to examine the association of childhood cognition with minor injuries that do not require hospital admission since the National Patient Register only began to include injuries that were treated on the emergency ward only (and not requiring admission) from 1994 onwards.
There may be some inaccuracy in the diagnostic codes for the hospital admissions and to a lesser extent death certificates. For example, there could be overlap between categories of poisoning other than with medications and medical/surgical injury, and there is often concern that some suicides or attempted suicides are coded as unintentional injuries. This is a problem that affects all research using routine data sources including those related to death certificates and hospital admissions. In general, broad disease categories (for example in this case all unintentional injury) are less prone to misclassification than subcategories. As such we would emphasis the associations with all injury as our main result. However, the analyses by sub-type of injury do not suggest that this is driven by any particular specific type of injury. Specifically, there is no evidence that the association is primarily driven by an association of cognitive function with poisoning, which would be the sub-group most likely to include some suicide attempts. There is some information on ‘intent’ for records of poisoning in the national patient registry. However, this is considered to be of too poor quality to be useful for research or audit.
The codes for registration of injuries have changed over time in Denmark. This mainly affects the registration of falls for which only data from 1987 to 2002 are most valid. However, the restriction of the analyses to this period did not change the estimates. In order to explore the mechanisms underlying the association between intelligence scores and the relative high rates of fall among these young men, we would have liked more information on the circumstances underlying the injuries, for example whether the incident arose from high risk occupations or related to leisure or sporting activities, and more information on the nature of the injuries, for example, extent and type of injuries. Unfortunately, systematic information in this detail is not available in the registers that were used in this study. Further, the study is based on exposure information collected in the past and for some covariables we would also have liked more detailed data. Thus, we had no information on gestational age in the birth register data and no information on work-related and tertiary education in the conscript files. The latter characteristics may provide an important mediating pathway for the associations that we have observed.
Possible mechanisms and implication of our findings
The association between childhood psychometric intelligence and adult injury could be explained by several mechanisms. First, lower childhood intelligence may result in a reduced ability to process and use information that could protect oneself from environmental risks.13,14 This might be reflected in a more impulsive and risk taking behaviour style, including for example, misuse of alcohol and other drugs. Childhood cognitive function has been related to a later problem of alcohol use,15 which is itself associated with greater injury risk. Secondly, low cognitive test scores and low educational attainment are interrelated. Both factors may influence one's ability to process information and assess risk, the type of occupation and physical environment that one lives in and the society and culture in which one lives. These factors may all in turn influence injury risk. The attenuating effect of education on the association suggests that this may be an important pathway, but interventional studies would be required to determine whether general educational interventions reduced injury risk.
It could be argued that cognitive function is not modifiable, and therefore assessment of its association with adult injury is not useful for developing prevention interventions. Support for the importance of environmental factors in determining childhood cognitive function can be found in trials of early learning and school readiness programmes. In two recent systematic reviews of such interventions,16,17 one of which focused on randomized trials only,16 the conclusion of both was that these programmes had important effects on cognitive function that extended to secondary school ages.16,17 Further, the observation of secular increases in childhood intelligence across a range of populations is strong evidence for an important environmental effect. These increases—referred to as the Flynn effect—have occurred far too quickly for them to be explained purely by changes in the gene pool or other biological17 effects. Consequently, childhood cognitive function may provide a further modifiable risk factor for the prevention of adult injury risk.
There is also evidence that providing safe environments and restricting access to toxic substances are important ways of reducing injuries18 and it is likely that for those with lower cognitive function, they will work and live in less safe environments, and therefore these may also be an important means of reducing the association between childhood cognitive function and injury.
In conclusion, we have found marked inverse associations between cognitive function measured at the age of 12 and 18 years and adult risk of fatal and non-fatal unintentional injury. Interventions that are successful at improving childhood cognitive function may result in reductions in adult injury risk. The attenuation towards the null of this association with adjustment for educational attainment suggests that an overall increase in educational level may result in a reduction in adult injury risk. For Danish men, among whom injury is particularly common, this would be an important contribution to improved public health.
KEY MESSAGES
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We thank K Svalastoga, E Høgh, P Wolf, T Rishøj, G Strande-Sørensen, E Manniche, B Holten, IA Weibull and A Ortman, who initiated and/or continued the Metropolit study. The study was funded by Danish Heart Association, the Lundbeck Foundation, the Danish Health Insurance Funds.
Conflicts of interest: None declared.
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